Paola Arlotta: Brain Development from Stem Cell to Organoid | Lex Fridman Podcast #32
 The following is a conversation with Paola Arlotta. She's a professor of stem cell and regenerative biology at Harvard University and is interested in understanding the molecular laws that govern the birth, differentiation, and assembly of the human brain's cerebral cortex. She explores the complexity of the brain by studying and engineering elements of how the brain develops. This was a fascinating conversation to me. It's part of the Artificial Intelligence Podcast. If you enjoy it, subscribe on YouTube, give it five stars on iTunes, support it on Patreon, or simply connect with me on Twitter at Lex Friedman, spelled F-R-I-D-M-A-N. And I'd like to give a special thank you to Amy Jefferies for her support of the podcast on Patreon. She's an artist, and you should definitely check out her Instagram at love, truth, good. Three beautiful words. Your support means a lot and inspires me to keep the series going. And now here's my conversation with Paola Arlotta. You studied the development of the human brain for many years, so let me ask you an out of the box question first. How likely is it that there's intelligent life out there in the universe, outside of Earth, with something like the human brain? So I can put it another way, how unlikely is the human brain? How difficult is it to build a thing through the evolutionary process? Well, it has happened here, right, on this planet? Once, yes. Once. So that simply tells you that it could, of course, happen again other places. It's only a matter of probability. What the probability that you would get a brain, like the ones that we have, like the human brain. So how difficult is it to make the human brain? It's pretty difficult, but most importantly, I guess we know very little about how this process really happens, and there is a reason for that, actually multiple reasons for that. Most of what we know about how the mammalian brain, so the brain of mammals, develop, comes from studying in labs other brains, not our own brain, the brain of mice, for example. But if I showed you a picture of a mouse brain, and then you put it next to a picture of a human brain, they don't look at all like each other. So they're very different. And therefore there is a limit to what you can learn about how the human brain is made by studying the mouse brain. There is a huge value in studying the mouse brain. There are many things that we have learned, but it's not the same thing. In having studied the human brain, or through the mouse, and through other methodologies that we'll talk about, do you have a sense, I mean, you're one of the experts in the world, how much do you feel you know about the brain? And how often do you find yourself in awe of this mysterious thing? Yeah, you pretty much find yourself in awe all the time. It's an amazing process. It's a process by which, by means that we don't fully understand, at the very beginning of embryogenesis, the structure called the neural tube literally self-assembles. And it happens in an embryo, and it can happen also from stem cells in a dish. Okay. And then from there, these stem cells that are present within the neural tube give rise to all of the thousands and thousands of different cell types that are present in the brain through time, right? With the interesting, very intriguing, interesting observation is that the time that it takes for the human brain to be made, it's human time. Meaning that for me and you, it took almost nine months of gestation to build the brain, and then another 20 years of learning postnatally to get the brain we have today that allows us to this conversation. A mouse takes 20 days or so for an embryo to be born. And so the brain is built in a much shorter period of time. And the beauty of it is that if you take mouse stem cells and you put them in a culture dish, the brain organoid that you get from a mouse is formed faster than if you took human stem cells and put them in the dish and let them make a human brain organoid. So the very developmental process is... Controlled by the speed of the species. Which means it's on purpose, it's not accidental, or there is something in that temporal dynamic to that development. Exactly, that is very important for us to get the brain. We have, and we can speculate for why that is. You know, it takes us a long time as human beings. After we're born to learn all the things that we have to learn to have the adult brain. It's actually 20 years. Think about it. From when a baby is born to when a teenager goes through puberty to adults, it's a long time. Do you think you can maybe talk through the first few months and then on to the first 20 years and then for the rest of their lives, what is the development of the human brain look like? What are the different stages? Yeah, at the beginning you have to build a brain, right? And the brain is made of cells. What's the very beginning? Which beginning are we talking about? In the embryo. As the embryo is developing in the womb, in addition to making all of the other tissues of the embryo, the muscle, the heart, the blood, the embryo is also building the brain. And it builds from a very simple structure called the neural tube, which is basically nothing but a tube of cells that spans sort of the length of the embryo from the head all the way to the tail, let's say, of the embryo. And then over in human beings, over many months of gestation, from that neural tube, which contains stem cell-like cells of the brain, you will make many, many other building blocks of the brain. So all of the other cell types, because there are many, many different types of cells in the brain that will form specific structures of the brain. So you can think about embryonic development of the brain as just the time in which you are making the building blocks, the cells. Are the stem cells relatively homogeneous, like uniform, or are they all different? That's a very good question. It's exactly how it works. You start with a more homogeneous, perhaps more multipotent type of stem cell. What's multipotent? Multipotent means that it has the potential to make many, many different types of other cells. And then with time, these progenitors become more heterogeneous, which means more diverse. There are gonna be many different types of the stem cells. And also they will give rise to progeny, to other cells that are not stem cells, that are specific cells of the brain that are very different from the mother stem cell. And now you think about this process of making cells from the stem cells over many, many months of development for humans. And what you're doing here, building the cells that physically make the brain, and then you arrange them in specific structures that are present in the final brain. So you can think about the embryonic development of the brain as the time where you're building the bricks. You're putting the bricks together to form buildings, structures, regions of the brain, and where you make the connections between these many different types of cells, especially nerve cells, neurons, right? That transmit action potentials and electricity. I've heard you also say somewhere, I think, correct me if I'm wrong, that the order of the way this builds matters. Oh yes. If you are an engineer and you think about development, you can think of it as, well, I could also take all the cells and print them all together into a brain in the end. But development is much more than that. So the cells are made in a very specific order that subserve the final product that you need to get. And so for example, all of the nerve cells, the neurons are made first, and all of the supportive cells of the neurons, like the glia, is made later. And there is a reason for that, because they have to assemble together in specific ways. But you also may say, well, why don't we just put them all together in the end? It's because as they develop next to each other, they influence their own development. So it's a different thing for a glia to be made alone in a dish, than a glia cell be made in a developing embryo with all these other cells around it that produce all these other signals. First of all, that's mind blowing, this development process. From my perspective in artificial intelligence, you often think of how incredible the final product is, the final product, the brain. But you're making me realize that the final product is just, the beautiful thing is the actual development process. Do we know the code that drives that development? Yeah. Do we have any sense? First of all, thank you for saying that it's really the formation of the brain. It's really its development, it is this incredibly choreographed dance that happens the same way every time each one of us builds the brain, right? And that builds an organ that allows us to do what we're doing today, right? That is mind blowing. And this is why developmental neurobiologists never get tired of studying that. Now you're asking about the code, what drives this? How is this done? Well, it's millions of years of evolution of really fine tuning gene expression programs that allow certain cells to be made at a certain time and to become a certain cell type, but also mechanical forces of pressure, bending. This embryo is not just, it will not stay a tube, this brain for very long, at some point, this tube in the front of the embryo will expand to make the primordium of the brain, right? Now the forces that control that the cells feel, and this is another beautiful thing, the very force that they feel, which is different from a week before, a week ago, will tell the cell, oh, you're being squished in a certain way, begin to produce these new genes, because now you are at the corner or you are in a stretch of cells or whatever it is. And so that mechanical physical force shapes the fate of the cell as well. So it's not only chemical, it's also mechanical. Mechanical. So from my perspective, biology is this incredibly complex mess, gooey mess. So you're saying mechanical forces. How different is like a computer or any kind of mechanical machine that we humans build and the biological systems? Have you been, because you've worked a lot with biological systems, are they as much of a mess as it seems from a perspective of an engineer, a mechanical engineer? Yeah. They are much more prone to taking alternative routes, right? So if you, we go back to printing a brain versus developing a brain, of course, if you've print a brain, given that you start with the same building blocks, the same cells, you could potentially print it the same way every time, but that final brain may not work the same way as a brain built during development does, because the very same building blocks that you're using developed in a completely different environment, right? It was not the environment of the brain, therefore they're gonna be different just by definition. So if you instead use development to build, let's say, a brain organoid, which maybe we will be talking about in a few minutes. For sure, those things are fascinating. Yes, so if you use processes of development, then when you watch it, you can see that sometimes things can go wrong in some organoids, and by wrong I mean different, one organoid from the next, while if you think about that embryo, it always goes right. So this development, for as complex as it is, every time a baby is born has, with very few exceptions, so the brain is like the next baby, but it's not the same if you develop it in a dish. And first of all, we don't even develop a brain, you develop something much simpler in the dish, but there are more options for building things differently, which really tells you that evolution has played a really tight game here for how in the end the brain is built in vivo. So just a quick, maybe dumb question, but it seems like the building process is not a dictatorship. It seems like there's not a centralized, like high-level mechanism that says, okay, this cell built itself the wrong way, I'm gonna kill it. It seems like there's a really strong distributed mechanism. Is that in your sense? There are a lot of possibilities, right? And if you think about, for example, different species building their brain, each brain is a little bit different. So the brain of a lizard is very different from that of a chicken, from that of one of us, and so on and so forth, and still is a brain, but it was built differently, starting from stem cells that pretty much had the same potential, but in the end, evolution builds different brains in different species, because that serves in a way the purpose of that species and the wellbeing of that organism. And so there are many possibilities, but then there is a way, and you were talking about a code. Nobody knows what the entire code of development is. Of course we don't. We know bits and pieces of very specific aspects of development of the brain, what genes are involved to make a certain cell type, how those two cells interact to make the next level structure. That we might know, but the entirety of it, how it's so well controlled, it's really mind blowing. So in the first two months in the embryo, or whatever, the first few weeks, months, so yeah, the building blocks are constructed, the different regions of the brain, I guess, and the nervous system. Well, this continues way longer than just the first few months. So over the very first few months, you build a lot of these cells, but then there is continuous building of new cell types all the way through birth. And then even postnatally, I don't know if you've ever heard of myelin. Myelin is this sort of insulation that is built around the cables of the neurons so that the electricity can go really fast from- The axons, I guess they're called. The axons are called axons, exactly. And so as human beings, we myelinate our cells postnatally. A six-year-old kid has barely started the process of making the mature oligodendrocytes, which are the cells that then eventually will wrap the axons into myelin. And this will continue, believe it or not, until we are about 25, 30 years old. So there is a continuous process of maturation and tweaking and additions, and also in response to what we do. I remember taking AP Biology in high school, and in the textbook, it said that, I'm going by memory here, that scientists disagree on the purpose of myelin in the brain. Is that totally wrong? So I guess it speeds up the, okay, I might be wrong here, but I guess it speeds up the electricity traveling down the axon or something. Yeah, so that's the most sort of canonical, and definitely that's the case. So you have to imagine an axon, and you can think about it as a cable of some type with electricity going through. And what myelin does, by insulating the outside, I should say there are tracts of myelin and pieces of axons that are naked without myelin. And so by having the insulation, the electricity, instead of going straight through the cable, it will jump over a piece of myelin, right, to the next naked little piece and jump again. And therefore, that's the idea that you go faster. And it was always thought that in order to build a big brain, a big nervous system, in order to have a nervous system that can do very complex type of things, then you need a lot of myelin because you wanna go fast with this information from point A to point B. Well, a few years ago, maybe five years ago or so, we discovered that some of the most evolved, which means the newest type of neurons that we have as non-human primates, as human beings in the top of our cerebral cortex, which should be the neurons that do some of the most complex things that we do, well, those have axons that have very little myelin. Wow. And they have very interesting ways in which they put this myelin on their axons, you know, a little piece here, then a long track with no myelin, another chunk there, and some don't have myelin at all. So now you have to explain where we are going with evolution. And if you think about it, perhaps as an electrical engineer, when I looked at it, I initially thought, and I'm a developmental neurobiologist, I thought maybe this is what we see now, but if we give evolution another few million years, we'll see a lot of myelin on these neurons too. But I actually think now that that's instead the future of the brain. Less myelin. Less myelin, and myelow for more flexibility on what you do with your axons, and therefore more complicated and unpredictable type of functions, which is also a bit mind blowing. So it seems like it's controlling the timing of the signal. So in the timing, you can encode a lot of information. Yeah. And so the brain... The timing, the chemistry of that little piece of axon, perhaps it's a dynamic process where the myelin can move. Now you see how many layers of variability you can add, and that's actually really good if you're trying to come up with a new function or a new capability or something unpredictable in a way. So we're going to jump around a little bit, but the old question of how much is nature and how much is nurture in terms of this incredible thing after the development is over, and how much is in the environment and the environment? We seem to be kind of somewhat smart, intelligent. Cognition, consciousness, all of these things are just incredible ability to reason and so on, emerge. In your sense, how much is in the hardware, in the nature, and how much is in the nurtures, learned with our parents, through interacting with the environment and so on. It's really both, right? If you think about it. Most of his cells and most of his structures, and that will take a few years to grow, to add more, to be better. But really, then we have these 20 years of interacting with the environment around us. And so what that brain that was so perfectly built or imperfectly built due to our genetic cues will then be used to incorporate the environment in its further maturation and development. And so your experiences do shape your brain. I mean, we know that. Like if you and I may have had a different childhood or a different, we have been going to different schools, we have been learning different things, and our brain is a little bit different because of that. We behave differently because of that. And so especially postnatally, experience is extremely important. We are born with a plastic brain. What that means is a brain that is able to change in response to stimuli. They can be sensory. So perhaps some of the most illuminating studies that were done were studies in which the sensory organs were not working, right? If you are born with eyes that don't work, then your very brain, piece of the brain that normally would process vision, the visual cortex, develops postnatally differently, and it might be used to do something different, right? So that's the most extreme. The plasticity of the brain, I guess, is the magic hardware that it, and then its flexibility in all forms is what enables the learning postnatally. Can you talk about organoids? What are they? Yes. And how can you use them to help us understand the brain and the development of the brain? This is very, very important. So the first thing I'd like to say, and please keep this in the video. The first thing I'd like to say is that an organoid, a brain organoid is not the same as a brain. Okay? It's a fundamental distinction. It's a system, a cellular system, that one can develop in the culture dish, starting from stem cells, that will mimic some aspects of the development of the brain, but not all of it. They are very small, maximum. They become about four to five millimeters in diameters. They are much simpler than our brain, of course, but yet they are the only system where we can literally watch a process of human brain development unfold. And by watch, I mean study it. Remember when I told you that we can't understand everything about development in our own brain by studying a mouse? Well, we can't study the actual process of development of the human brain because it all happens in utero. So we will never have access to that process, ever. And therefore, this is our next best thing. Like a bunch of stem cells that can be coaxed into starting a process of neural tube formation. Remember that tube that is made by the embryo reon. And from there, a lot of the cell types that are present within the brain. And you can simply watch it and study, but you can also think about diseases where development of the brain does not proceed normally, right, properly. Think about neurodevelopmental diseases that are many, many different types. Think about autism spectrum disorders. There are also many different types of autism. So there you could take a stem cell, which really means either a sample of blood or a sample of skin from the patient, make a stem cell, and then with that stem cell, watch a process of formation of a brain organoid of that person with that genetics, with that genetic code in it. And you can ask, what is this genetic code doing to some aspects of development of the brain? And for the first time, you may come to solutions like what cells are involved in autism, right? So many questions around this. So if you take this human stem cell for that particular person with that genetic code, how, and you try to build an organoid, how often will it look similar? What's the, yeah, so- Reproducibility? Yes, or how much variability is the flip side of that? Yeah, so there is much more variability in building organoids than there is in building brain. It's really true that the majority of us, when we are born as babies, our brains look a lot like each other. This is the magic that the embryo does, where it builds a brain in the context of a body, and there is very little variability there. There is disease, of course, but in general, little variability. When you build an organoid, we don't have the full code for how this is done. And so in part, the organoid somewhat builds itself because there are some structures of the brain that the cells know how to make. And another part comes from the investigator, the scientist, adding to the media factors that we know in the mouse, for example, would foster a certain step of development. But it's very limited. And so as a result, the kind of product you get in the end is much more reductionist, it's much more simple than what you get in vivo. It mimics early events of development as of today, and it doesn't build very complex type of anatomy and structure does not as of today, which happens instead in vivo. And also the variability that you see, one organoid to the next tends to be higher than when you compare an embryo to the next. So, okay, then the next question is, how hard and maybe another flip side of that expensive is it to go from one stem cell to an organoid? How many can you build in life? Because it sounds very complicated. It's work, definitely, and it's money, definitely. But you can really grow a very high number of these organoids, can go perhaps, I told you the maximum they become about five millimeters in diameter. Which is how many cells are in a cell? So this is about the size of a tiny, tiny raisin, or perhaps the seed of an apple. And so you can grow 50 to 100 of those inside one big bioreactors, which are these flasks where the media provides nutrients for the organoids. So the problem is not to grow more or less of them. It's really to figure out how to grow them in a way that they are more and more reproducible. For example, organoid to organoid, so they can be used to study a biological process. Because if you have too much variability, then you never know if what you see is just an exception or really the rule. So what does an organoid look like? Are there different neurons already emerging? Is there, you know, well first, can you tell me what kind of neurons are there? Are they sort of all the same? Are they not all the same? How much do we understand? And how much of that variance, if any, can exist in organoids? Yes. So you could grow, I told you that the brain has different parts. So the cerebral cortex is on the top part of the brain, but there is another region called the striatum that is below the cortex and so on and so forth. All of these regions have different types of cells in the actual brain, okay? And so scientists have been able to grow organoids that may mimic some aspects of development of these different regions of the brain. And so we are very interested in the cerebral cortex. That's the coolest part, right? Very cool. I agree with you. Yeah, sorry. We wouldn't be here talking if we didn't have a cerebral cortex. It's also, I like to think, the part of the brain that really truly makes us human, the most evolved in recent evolution. And so in the attempt to make the cerebral cortex and by figuring out a way to have these organoids continue to grow and develop for extended periods of time, much like it happens in the real embryo, months and months in culture, then you can see that many different types of neurons of the cortex appear. And at some point, also the astrocytes, so the glia cells of the cerebral cortex also appear. What are these? Astrocytes. Astrocytes, yeah. The astrocytes are not neurons, so they're not nerve cells, but they play very important roles. One important role is to support the neuron, but of course they have much more active type of roles. They're very important, for example, to make the synapses, which are the point of contact and communication between two neurons. They do a lot of things. So all that chemistry fun happens in the synapses, happens because of these cells? Are they the medium in which? It happens because of the interactions. It happens because you are making the cells and they have certain properties, including the ability to make neurotransmitters, which are the chemicals that are secreted to the synapses, including the ability of making these axons grow with their growth cones and so on and so forth. And then you have other cells around it that release chemicals or touch the neurons or interact with them in different ways to really foster this perfect process, in this case of synaptogenesis. And this does happen within organoids. Or with organoids. So the mechanical and the chemical stuff happens. Yeah, the connectivity between neurons. This in a way is not surprising because scientists have been culturing neurons forever. And when you take a neuron, even a very young one, and you culture it, eventually finds another cell or another neuron to talk to, it will form a synapse. Are we talking about mice neurons? Are we talking about human neurons? It doesn't matter, both. So you can culture a neuron, like a single neuron, and give it a little friend, and it starts interacting? Yes. So neurons are able to, it sounds, it's more simple than what it may sound to you. Neurons have molecular properties and structural properties that allow them to really communicate with other cells. And so if you put not one neuron, but if you put several neurons together, chances are that they will form synapses with each other. Okay, great. So an organoid is not a brain. No. But there's some, it's able to, especially what you're talking about, mimic some properties of the cerebral cortex, for example. So what can you understand about the brain by studying an organoid of a cerebral cortex? I can literally study all this incredible diversity of cell type, all these many, many different classes of cells, how are they made? How do they look like? What do they need to be made properly? And what goes wrong if now the genetics of that stem cell that I used to make the organoid came from a patient with a neurodevelopmental disease? Can I actually watch for the very first time what may have gone wrong years before in this kid when its own brain was being made? Think about that loop. In a way, it's a little tiny rudimentary window into the past, into the time when that brain in a kid that had this neurodevelopmental disease was being made. And I think that's unbelievably powerful because today we have no idea of what cell types, we barely know what brain regions are affected in these diseases. Now we have an experimental system that we can study in the lab and we can ask, what are the cells affected? When during development things went wrong? What are the molecules among the many, many different molecules that control brain development? Which ones are the ones that are really messed up here and we want perhaps to fix? And what is really the final product? Is it a less strong kind of circuit in brain? Is it a brain that lacks a cell type? Is it a, what is it? Because then we can think about treatment and care for these patients that is informed rather than just based on current diagnostics. So how hard is it to detect through the development of the process, it's a super exciting tool to see how different conditions develop. How hard is it to detect that, wait a minute, this is abnormal development? Yeah. How hard is it? How much signal is there? How much of it is it a mess? Cause things can go wrong at multiple levels, right? You could have a cell that is born and built but then doesn't work properly or a cell that is not even born or a cell that doesn't interact with other cells differently and so on and so forth. So today we have technology that we did not have even five years ago that allows us to look, for example, at the molecular picture of a cell, of a single cell in a sea of cells with high precision. And so that molecular information where you compare many, many single cells for the genes that they produce between a control individual and an individual with a neurodevelopmental disease, that may tell you what is different molecularly. Or you could see that some cells are not even made, for example, or that the process of maturation of the cells may be wrong. There are many different levels here and we can study the cells at the molecular level but also we can use the organoids to ask questions about the properties of the neurons, the functional properties, how they communicate with each other, how they respond to a stimulus and so on and so forth. And we may get abnormalities there, right? Detect those. So how early is this work in a, maybe in the history of science? So- So- What an easy question. I mean, like, so if you were to, if you and I time travel a thousand years into the future, organoids seem to be, maybe I'm romanticizing the notion, but you're building not a brain, but something that has properties of a brain. So it feels like you might be getting close to, in the building process to build this, to understand. So how, how far are we in this understanding process of development? A thousand years from now, it's a long time from now. So if this planet is still gonna be here a thousand years from now. So I mean, if, you know, like they write a book, obviously there'll be a chapter about you. Let's write the science fiction book today. Yeah, today. But I mean, I guess where we really understood very little about the brain a century ago, I was a big fan in high school of reading Freud and so on. I still am of psychiatry. I would say we still understand very little about the functional aspect of just, but how in the history of understanding the biology of the brain, the development, how far are we along? It's a very good question. And so this is just, of course, my opinion. I think that we did not have technology, even 10 years ago or certainly not 20 years ago to even think about experimentally investigating the development of the human brain. So we've done a lot of work in science to study the brain or many other organisms. Now we have some technologies which I'll spell out that allow us to actually look at the real thing and look at the brain, at the human brain. So what are these technologies? There has been huge progress in stem cell biology. The moment someone figured out how to turn a skin cell into an embryonic stem cell, basically, and that how that embryonic stem cell could begin a process of development again, to for example, make a brain, there was a huge advance. And in fact, there was a Nobel Prize for that. That started the field really of using stem cells to build organs. Now we can build on all the knowledge of development that we build over the many, many, many years to say, how do we make the stem cells now make more and more complex aspects of development of the human brain? So this field is young, the field of brain organoids, but it's moving faster. And it's moving fast in a very serious way that is rooted in labs with the right ethical framework and really building on solid science for what reality is and what is not. But it will go faster and it will be more and more powerful. We also have technology that allows us to basically study the properties of single cells across many, many millions of single cells, which we didn't have perhaps five years ago. So now with that, even an organoid that has millions of cells can be profiled in a way looked at with very, very high resolution, the single cell level to really understand what is going on. And you could do it in multiple stages of development and you can build your hypothesis and so on and so forth. So it's not gonna be a thousand years. It's gonna be a shorter amount of time. And I see this as sort of an exponential growth of this field enabled by these technologies that we didn't have before. And so we're gonna see something transformative that we didn't see at all in the prior thousand years. So I apologize for the crazy sci-fi questions, but the developmental process is fascinating to watch and study, but how far are we away from? And maybe how difficult is it to build not just an organoid but a human brain from a stem cell? Yeah, first of all, that's not the goal for the majority of the serial scientists that work on this because you don't have to build the whole human brain to make this model useful for understanding how the brain develops or understanding disease. You don't have to build the whole thing. So let me just comment on this, fascinating. It shows to me the difference between you and I is you're actually trying to understand the beauty of the human brain and to use it to really help thousands or millions of people with disease and so on. From an artificial intelligence perspective, we're trying to build systems that we can put in robots and try to create systems that have echoes of the intelligence about reasoning about the world, navigating the world. It's different objectives, I think. Yeah, that's very much science fiction. Science fiction, but we operate in science fiction a little bit, but so on that point of building a brain, even though that is not the focus or interest perhaps of the community, how difficult is it? Is it truly science fiction at this point? I think the field will progress, like I said, and that the system will be more and more complex in a way, right? But there are properties that emerge from the human brain that have to do with the mind. They may have to do with consciousness, they may have to do with intelligence or whatever. That we really don't understand even how they can emerge from an actual real brain, and therefore we can not measure or study in an organoid. So I think that this field, many, many years from now, may lead to the building of better neural circuits that really are built out of understanding of how this process really works. And it's hard to predict how complex this really will be. I really don't think we're so far from, it makes me laugh really, it's really that far from building the human brain. But you're gonna be building something that is always a bad version of it, but that may have really powerful properties. And might be able to respond to stimuli or be used in certain contexts. And this is why I really think that there is no other way to do this science, but within the right ethical framework. Because where you're going with this is also, we can talk about science fiction and write that book, and we could today. But this work happens in a specific ethical framework that we don't decide just as scientists, but also as a society. So the ethical framework here is a fascinating one, is a complicated one. Yes. Do you have a sense, a grasp of how we think about ethically of building organoids from human stem cells to understand the brain? It seems like a tool for helping potentially millions of people cure diseases, or at least start to cure by understanding it. But is there more, is there gray areas that we have to think about ethically? Absolutely, we must think about that. Every discussion about the ethics of this needs to be based on actual data from the models that we have today, and from the ones that we will have tomorrow. So it's a continuous conversation, it's not something that you decide now. Today, there is no issue, really. Very simple models that clearly can help you in many ways without much to think about. But tomorrow we need to have another conversation and so on and so forth. And so the way we do this is to actually really bring together constantly a group of people that are not only scientists, but also bioethicists, the lawyers, philosophers, psychiatrists, and so on, psychologists, and so on and so forth, to decide as a society, really, what we should and what we should not do. So that's the way to think about the ethics. Now, I also think, though, that as a scientist, I have a moral responsibility. So if you think about how transformative it could be for understanding and curing a neuropsychiatric disease, to be able to actually watch and study and treat with drugs the very brain of the patient that you are trying to study. How transformative at this moment in time this could be. We couldn't do it five years ago, we could do it now. Taking a stem cell of a particular patient. A patient and make an organoid for a simple and different from the human brain, it still is his process of brain development with his or her genetics. And we could understand perhaps what is going wrong. Perhaps we could use it as a platform, as a cellular platform to screen for drugs, to fix a process, and so on and so forth, right? So we could do it now, we couldn't do it five years ago. Should we not do it? What is the downside of doing it? I don't see a downside at this very moment. But if we invited a lot of people, I'm sure there would be somebody who would argue against it. What would be the devil's advocate argument? Yeah. Yeah, so it's exactly perhaps what you alluded at with your question, that you are making, enabling some process of formation of the brain that could be misused at some point, or that could be showing properties that ethically we don't want to see in a tissue. So today, this is not an issue. And so you just gain dramatically from the science without because the system is so simple and so different in a way from the actual brain. But because it is the brain, we have an obligation to really consider all of this, right? And again, it's a balanced conversation where we should put disease and betterment of humanity also on that plate. What do you think, at least historically, there was some politicization of embryonic stem cells, stem cell research. Do you still see that out there? Is that still a force that we have to think about, especially in this larger discourse that we're having about the role of science in at least American society? Yeah, this is a very good question. It's very, very important. I see a very central role for scientists to inform decisions about what we should or should not do in society. And this is because the scientists have the firsthand look and understanding of really the work that they are doing. And again, this varies depending on what we're talking about here. So now we're talking about brain organoids. I think that the scientists need to be part of that conversation about what will be allowed in the future or not allowed in the future to do with the system. And I think that is very, very important because they bring reality of data to the conversation. And so they should have a voice. So data should have a voice. Data needs to have a voice because in not only data, we should also be good at communicating with non-scientists the data. So there has been, oftentimes, there is a lot of discussion and excitement and fights about certain topics just because of the way they are described. I'll give you an example. If I call the same cellular system we just talked about a brain organoid, or if I called it a human mini brain, your reaction is going to be very different to this. And so the way the systems are described, I mean, we and journalists alike need to be a bit careful that this debate is a real debate and informed by real data. That's all I'm asking. And yeah, the language matters here. So I work on autonomous vehicles and there the use of language could drastically change the interpretation and the way people feel about what is the right way to proceed forward. You are, as I've seen from a presentation, you're a parent. I saw you show a couple of pictures of your son. Is it just the one? Two. Son and a daughter. So what have you learned from the human brain by raising two of them? More than I could ever learn in a lab. What have I learned? I've learned that children really have these amazing plastic minds, right? That we have a responsibility to foster their growth in good, healthy ways. That keep them curious, that keep some adventurous, that doesn't raise them in fear of things. But also respecting who they are, which is in part coming from the genetics we talked about. My children are very different from each other, despite the fact that they're the product of the same two parents. I also learned that what you do for them comes back to you. Like, if you're a good parent, you're going to, most of the time, have perhaps a decent kids at the end. So what do you think, just a quick comment, what do you think is the source of that difference? That's often the surprising thing for parents. They can't believe that our kids, they're so different, yet they came from the same parents. Well, they are genetically different. Even they came from the same two parents because the mixing of gametes, we know this genetics, creates every time a genetically different individual, which will have a specific mix of genes that is a different mix every time from the two parents. And so they're not twins. They are genetically different. Even just that little bit of variation. Because you said, really, from a biological perspective, the brains look pretty similar. Well, so let me clarify that. So the genetics you have, the genes that you have, that play that beautiful orchestrated symphony of development, different genes will play it slightly differently. It's like playing the same piece of music, but with a different orchestra and a different director. The music will not come out. It will be still a piece by the same author, but it will come out differently if it's played by the high school orchestra instead of the... Instead of La Scala in Milan. And so you are born superficially with the same brain. It has the same cell types, similar patterns of connectivity, but the properties of the cells and how the cells will then react to the environment as you experience your world will be also shaped by who genetically you are. Speaking just as a parent, this is not something that comes from my work. I think you can tell at birth that these kids are different, that they have a different personality in a way. So both is needed, the genetics as well as the nurturing afterwards. So you are one human with a brain, sort of living through the whole mess of it, the human condition, full of love, maybe fear, ultimately mortal. How has studying the brain changed the way you see yourself? When you look in the mirror, when you think about your life, the fears, the love, when you see your own life, your own mortality. Yeah, that's a very good question. It's almost impossible to dissociate some time for me. Some of the things we do or some of the things that other people do from, oh, that's because that part of the brain is working in a certain way. Or thinking about a teenager, going through teenage years and being at times funny in the way they think. And impossible for me not to think it's because they're going through this period of time called critical periods of plasticity, where their synapses are being eliminated here and there, and they're just confused. And so from that comes perhaps a different take on that behavior or maybe I can justify it scientifically in some sort of way. I also look at humanity in general, and I am amazed by what we can do and the kind of ideas that we can come up with. And I cannot stop thinking about how the brain is continuing to evolve. I don't know if you do this, but I think about the next brain sometimes. Where are we going with this? What are the features of this brain that evolution is really playing with to get us in the future the new brain? It's not over, right? It's a work in progress. So let me just a quick comment on that. Do you think there's a lot of fascination and hope for artificial intelligence of creating artificial brains? You said the next brain. When you imagine over a period of a thousand years, the evolution of the human brain, do you sometimes envisioning that future see an artificial one? Artificial intelligence as it is hoped by many, not hoped, thought by many people would be actually the next evolutionary step in the development of humans. Yeah, I think in a way that will happen, right? It's almost like a part of the way we evolve. We evolve in the world that we created, that we interact with, that shape us as we grow up and so on and so forth. Sometimes I think about something that may sound silly, but think about the use of cell phones. Part of me thinks that somehow in their brain, there will be a region of the cortex that is attuned to that tool. And this comes from a lot of studies in model organisms where really the cortex especially adapts to the kind of things you have to do. So if we need to move our fingers in a very specific way, we have a part of our cortex that allows us to do this kind of very precise movement. An owl that has to see very, very far away with big eyes, the visual cortex very big, the brain attunes to your environment. So the brain will attune to the technologies that we will have and will be shaped by it. So the cortex very well may be shaped by it. In artificial intelligence, it may merge with it, it may get enveloped and adjusted. Even if it's not a merge of the kind of, oh, let's have a synthetic element together with a biological one. The very space around us, the fact, for example, think about we put on some goggles of virtual reality and we physically are surfing the ocean, right? Like I've done it and you have all these emotions that come to you. Your brain placed you in that reality. And it was able to do it like that just by putting the goggles on. It didn't take thousands of years of adapting to this. The brain is plastic, so adapts to new technology. So you could do it from the outside by simply hijacking some sensory capacities that we have. So clearly over recent evolution, the cerebral cortex has been a part of the brain that has known the most evolution. So we have put a lot of chips on evolving this specific part of the brain. And the evolution of cortex is plasticity, is this ability to change in response to things. So, yes, they will integrate, that we want it or not. Well, there's no better way to end it. Paola, thank you so much for talking to me. Very well, that's great. Very exciting.
 The following is a conversation with Paola Arlotta.
 She's a professor of stem cell and regenerative biology
 at Harvard University and is interested in understanding
 the molecular laws that govern the birth, differentiation,
 and assembly of the human brain's cerebral cortex.
 She explores the complexity of the brain by studying
 and engineering elements of how the brain develops.
 This was a fascinating conversation to me.
 It's part of the Artificial Intelligence Podcast.
 If you enjoy it, subscribe on YouTube,
 give it five stars on iTunes, support it on Patreon,
 or simply connect with me on Twitter
 at Lex Friedman, spelled F-R-I-D-M-A-N.
 And I'd like to give a special thank you to Amy Jefferies
 for her support of the podcast on Patreon.
 She's an artist, and you should definitely check out
 her Instagram at love, truth, good.
 Three beautiful words.
 Your support means a lot and inspires me
 to keep the series going.
 And now here's my conversation with Paola Arlotta.
 You studied the development of the human brain
 for many years, so let me ask you
 an out of the box question first.
 How likely is it that there's intelligent life out there
 in the universe, outside of Earth,
 with something like the human brain?
 So I can put it another way, how unlikely is the human brain?
 How difficult is it to build a thing
 through the evolutionary process?
 Well, it has happened here, right, on this planet?
 Once, yes. Once.
 So that simply tells you that it could, of course,
 happen again other places.
 It's only a matter of probability.
 What the probability that you would get a brain,
 like the ones that we have, like the human brain.
 So how difficult is it to make the human brain?
 It's pretty difficult, but most importantly,
 I guess we know very little about how this process
 really happens, and there is a reason for that,
 actually multiple reasons for that.
 Most of what we know about how the mammalian brain,
 so the brain of mammals, develop, comes from studying
 in labs other brains, not our own brain,
 the brain of mice, for example.
 But if I showed you a picture of a mouse brain,
 and then you put it next to a picture of a human brain,
 they don't look at all like each other.
 So they're very different.
 And therefore there is a limit to what you can learn about
 how the human brain is made by studying the mouse brain.
 There is a huge value in studying the mouse brain.
 There are many things that we have learned,
 but it's not the same thing.
 In having studied the human brain, or through the mouse,
 and through other methodologies that we'll talk about,
 do you have a sense, I mean,
 you're one of the experts in the world,
 how much do you feel you know about the brain?
 And how often do you find yourself in awe
 of this mysterious thing?
 Yeah, you pretty much find yourself in awe all the time.
 It's an amazing process.
 It's a process by which, by means that we don't
 fully understand, at the very beginning of embryogenesis,
 the structure called the neural tube literally self-assembles.
 And it happens in an embryo,
 and it can happen also from stem cells in a dish.
 Okay.
 And then from there, these stem cells that are present
 within the neural tube give rise to all of the thousands
 and thousands of different cell types
 that are present in the brain through time, right?
 With the interesting, very intriguing,
 interesting observation is that the time that it takes
 for the human brain to be made, it's human time.
 Meaning that for me and you,
 it took almost nine months of gestation to build the brain,
 and then another 20 years of learning postnatally
 to get the brain we have today
 that allows us to this conversation.
 A mouse takes 20 days or so
 for an embryo to be born.
 And so the brain is built in a much shorter period of time.
 And the beauty of it is that if you take mouse stem cells
 and you put them in a culture dish,
 the brain organoid that you get from a mouse
 is formed faster than if you took human stem cells
 and put them in the dish
 and let them make a human brain organoid.
 So the very developmental process is...
 Controlled by the speed of the species.
 Which means it's on purpose, it's not accidental,
 or there is something in that temporal dynamic
 to that development.
 Exactly, that is very important for us to get the brain.
 We have, and we can speculate for why that is.
 You know, it takes us a long time as human beings.
 After we're born to learn all the things
 that we have to learn to have the adult brain.
 It's actually 20 years.
 Think about it.
 From when a baby is born to when a teenager
 goes through puberty to adults, it's a long time.
 Do you think you can maybe talk through
 the first few months and then on to the first 20 years
 and then for the rest of their lives,
 what is the development of the human brain look like?
 What are the different stages?
 Yeah, at the beginning you have to build a brain, right?
 And the brain is made of cells.
 What's the very beginning?
 Which beginning are we talking about?
 In the embryo.
 As the embryo is developing in the womb,
 in addition to making all of the other tissues
 of the embryo, the muscle, the heart, the blood,
 the embryo is also building the brain.
 And it builds from a very simple structure
 called the neural tube,
 which is basically nothing but a tube of cells
 that spans sort of the length of the embryo
 from the head all the way to the tail,
 let's say, of the embryo.
 And then over in human beings,
 over many months of gestation, from that neural tube,
 which contains stem cell-like cells of the brain,
 you will make many, many other building blocks of the brain.
 So all of the other cell types,
 because there are many, many different types of cells
 in the brain that will form specific structures of the brain.
 So you can think about embryonic development of the brain
 as just the time in which you are making
 the building blocks, the cells.
 Are the stem cells relatively homogeneous, like uniform,
 or are they all different?
 That's a very good question.
 It's exactly how it works.
 You start with a more homogeneous,
 perhaps more multipotent type of stem cell.
 What's multipotent?
 Multipotent means that it has the potential
 to make many, many different types of other cells.
 And then with time, these progenitors
 become more heterogeneous, which means more diverse.
 There are gonna be many different types of the stem cells.
 And also they will give rise to progeny, to other cells
 that are not stem cells, that are specific cells of the brain
 that are very different from the mother stem cell.
 And now you think about this process of making cells
 from the stem cells over many, many months
 of development for humans.
 And what you're doing here, building the cells
 that physically make the brain, and then you arrange them
 in specific structures that are present in the final brain.
 So you can think about the embryonic development
 of the brain as the time where you're building the bricks.
 You're putting the bricks together to form buildings,
 structures, regions of the brain,
 and where you make the connections
 between these many different types of cells,
 especially nerve cells, neurons, right?
 That transmit action potentials and electricity.
 I've heard you also say somewhere, I think,
 correct me if I'm wrong,
 that the order of the way this builds matters.
 Oh yes.
 If you are an engineer and you think about development,
 you can think of it as, well, I could also take all the cells
 and print them all together into a brain in the end.
 But development is much more than that.
 So the cells are made in a very specific order
 that subserve the final product that you need to get.
 And so for example, all of the nerve cells,
 the neurons are made first, and all of the supportive cells
 of the neurons, like the glia, is made later.
 And there is a reason for that,
 because they have to assemble together in specific ways.
 But you also may say, well, why don't we just put them
 all together in the end?
 It's because as they develop next to each other,
 they influence their own development.
 So it's a different thing for a glia
 to be made alone in a dish,
 than a glia cell be made in a developing embryo
 with all these other cells around it
 that produce all these other signals.
 First of all, that's mind blowing,
 this development process.
 From my perspective in artificial intelligence,
 you often think of how incredible the final product is,
 the final product, the brain.
 But you're making me realize that the final product
 is just, the beautiful thing is the actual development
 process.
 Do we know the code that drives that development?
 Yeah.
 Do we have any sense?
 First of all, thank you for saying that
 it's really the formation of the brain.
 It's really its development,
 it is this incredibly choreographed dance
 that happens the same way every time each one of us
 builds the brain, right?
 And that builds an organ that allows us to do
 what we're doing today, right?
 That is mind blowing.
 And this is why developmental neurobiologists
 never get tired of studying that.
 Now you're asking about the code, what drives this?
 How is this done?
 Well, it's millions of years of evolution
 of really fine tuning gene expression programs
 that allow certain cells to be made at a certain time
 and to become a certain cell type,
 but also mechanical forces of pressure, bending.
 This embryo is not just, it will not stay a tube,
 this brain for very long, at some point,
 this tube in the front of the embryo will expand
 to make the primordium of the brain, right?
 Now the forces that control that the cells feel,
 and this is another beautiful thing,
 the very force that they feel,
 which is different from a week before, a week ago,
 will tell the cell, oh, you're being squished
 in a certain way, begin to produce these new genes,
 because now you are at the corner
 or you are in a stretch of cells or whatever it is.
 And so that mechanical physical force
 shapes the fate of the cell as well.
 So it's not only chemical, it's also mechanical.
 Mechanical.
 So from my perspective, biology is this
 incredibly complex mess, gooey mess.
 So you're saying mechanical forces.
 How different is like a computer
 or any kind of mechanical machine that we humans build
 and the biological systems?
 Have you been, because you've worked a lot with biological
 systems, are they as much of a mess as it seems
 from a perspective of an engineer, a mechanical engineer?
 Yeah.
 They are much more prone to taking alternative routes,
 right?
 So if you, we go back to printing a brain
 versus developing a brain, of course,
 if you've print a brain, given that you start
 with the same building blocks, the same cells,
 you could potentially print it the same way every time,
 but that final brain may not work the same way
 as a brain built during development does,
 because the very same building blocks that you're using
 developed in a completely different environment, right?
 It was not the environment of the brain,
 therefore they're gonna be different just by definition.
 So if you instead use development to build,
 let's say, a brain organoid, which maybe we will be talking
 about in a few minutes.
 For sure, those things are fascinating.
 Yes, so if you use processes of development,
 then when you watch it, you can see that sometimes
 things can go wrong in some organoids,
 and by wrong I mean different, one organoid from the next,
 while if you think about that embryo, it always goes right.
 So this development, for as complex as it is,
 every time a baby is born has, with very few exceptions,
 so the brain is like the next baby,
 but it's not the same if you develop it in a dish.
 And first of all, we don't even develop a brain,
 you develop something much simpler in the dish,
 but there are more options for building things differently,
 which really tells you that evolution
 has played a really tight game here
 for how in the end the brain is built in vivo.
 So just a quick, maybe dumb question,
 but it seems like the building process
 is not a dictatorship.
 It seems like there's not a centralized,
 like high-level mechanism that says,
 okay, this cell built itself the wrong way,
 I'm gonna kill it.
 It seems like there's a really strong distributed mechanism.
 Is that in your sense?
 There are a lot of possibilities, right?
 And if you think about, for example,
 different species building their brain,
 each brain is a little bit different.
 So the brain of a lizard is very different
 from that of a chicken, from that of one of us,
 and so on and so forth, and still is a brain,
 but it was built differently,
 starting from stem cells
 that pretty much had the same potential,
 but in the end, evolution builds different brains
 in different species,
 because that serves in a way the purpose of that species
 and the wellbeing of that organism.
 And so there are many possibilities,
 but then there is a way,
 and you were talking about a code.
 Nobody knows what the entire code of development is.
 Of course we don't.
 We know bits and pieces of very specific aspects
 of development of the brain,
 what genes are involved to make a certain cell type,
 how those two cells interact
 to make the next level structure.
 That we might know, but the entirety of it,
 how it's so well controlled, it's really mind blowing.
 So in the first two months in the embryo,
 or whatever, the first few weeks, months,
 so yeah, the building blocks are constructed,
 the different regions of the brain, I guess,
 and the nervous system.
 Well, this continues way longer than just the first few months.
 So over the very first few months,
 you build a lot of these cells,
 but then there is continuous building of new cell types
 all the way through birth.
 And then even postnatally,
 I don't know if you've ever heard of myelin.
 Myelin is this sort of insulation
 that is built around the cables of the neurons
 so that the electricity can go really fast from-
 The axons, I guess they're called.
 The axons are called axons, exactly.
 And so as human beings, we myelinate our cells postnatally.
 A six-year-old kid has barely started the process
 of making the mature oligodendrocytes,
 which are the cells that then eventually
 will wrap the axons into myelin.
 And this will continue, believe it or not,
 until we are about 25, 30 years old.
 So there is a continuous process of maturation
 and tweaking and additions,
 and also in response to what we do.
 I remember taking AP Biology in high school,
 and in the textbook, it said that,
 I'm going by memory here,
 that scientists disagree on the purpose of myelin
 in the brain.
 Is that totally wrong?
 So I guess it speeds up the,
 okay, I might be wrong here,
 but I guess it speeds up the electricity
 traveling down the axon or something.
 Yeah, so that's the most sort of canonical,
 and definitely that's the case.
 So you have to imagine an axon,
 and you can think about it as a cable of some type
 with electricity going through.
 And what myelin does, by insulating the outside,
 I should say there are tracts of myelin
 and pieces of axons that are naked without myelin.
 And so by having the insulation, the electricity,
 instead of going straight through the cable,
 it will jump over a piece of myelin, right,
 to the next naked little piece and jump again.
 And therefore, that's the idea that you go faster.
 And it was always thought that in order to build
 a big brain, a big nervous system,
 in order to have a nervous system that can do
 very complex type of things,
 then you need a lot of myelin because you wanna go fast
 with this information from point A to point B.
 Well, a few years ago, maybe five years ago or so,
 we discovered that some of the most evolved,
 which means the newest type of neurons
 that we have as non-human primates, as human beings
 in the top of our cerebral cortex,
 which should be the neurons that do some of the most
 complex things that we do, well, those have axons
 that have very little myelin.
 Wow.
 And they have very interesting ways in which they put
 this myelin on their axons, you know, a little piece here,
 then a long track with no myelin, another chunk there,
 and some don't have myelin at all.
 So now you have to explain
 where we are going with evolution.
 And if you think about it,
 perhaps as an electrical engineer, when I looked at it,
 I initially thought, and I'm a developmental neurobiologist,
 I thought maybe this is what we see now,
 but if we give evolution another few million years,
 we'll see a lot of myelin on these neurons too.
 But I actually think now that that's instead the future
 of the brain. Less myelin.
 Less myelin, and myelow for more flexibility
 on what you do with your axons,
 and therefore more complicated and unpredictable
 type of functions, which is also a bit mind blowing.
 So it seems like it's controlling the timing of the signal.
 So in the timing, you can encode a lot of information.
 Yeah.
 And so the brain...
 The timing, the chemistry of that little piece of axon,
 perhaps it's a dynamic process where the myelin can move.
 Now you see how many layers of variability
 you can add, and that's actually really good
 if you're trying to come up with a new function
 or a new capability or something unpredictable in a way.
 So we're going to jump around a little bit,
 but the old question of how much is nature
 and how much is nurture in terms of this incredible thing
 after the development is over,
 and how much is in the environment and the environment?
 We seem to be kind of somewhat smart, intelligent.
 Cognition, consciousness, all of these things
 are just incredible ability to reason and so on, emerge.
 In your sense, how much is in the hardware, in the nature,
 and how much is in the nurtures,
 learned with our parents,
 through interacting with the environment and so on.
 It's really both, right? If you think about it.
 Most of his cells and most of his structures,
 and that will take a few years to grow,
 to add more, to be better.
 But really, then we have these 20 years of interacting
 with the environment around us.
 And so what that brain that was so perfectly built
 or imperfectly built due to our genetic cues
 will then be used to incorporate the environment
 in its further maturation and development.
 And so your experiences do shape your brain.
 I mean, we know that.
 Like if you and I may have had a different childhood
 or a different, we have been going to different schools,
 we have been learning different things,
 and our brain is a little bit different because of that.
 We behave differently because of that.
 And so especially postnatally,
 experience is extremely important.
 We are born with a plastic brain.
 What that means is a brain that is able to change
 in response to stimuli.
 They can be sensory.
 So perhaps some of the most illuminating studies
 that were done were studies in which the sensory organs
 were not working, right?
 If you are born with eyes that don't work,
 then your very brain, piece of the brain
 that normally would process vision, the visual cortex,
 develops postnatally differently,
 and it might be used to do something different, right?
 So that's the most extreme.
 The plasticity of the brain, I guess,
 is the magic hardware that it,
 and then its flexibility in all forms
 is what enables the learning postnatally.
 Can you talk about organoids?
 What are they?
 Yes.
 And how can you use them to help us understand the brain
 and the development of the brain?
 This is very, very important.
 So the first thing I'd like to say,
 and please keep this in the video.
 The first thing I'd like to say is that an organoid,
 a brain organoid is not the same as a brain.
 Okay?
 It's a fundamental distinction.
 It's a system, a cellular system,
 that one can develop in the culture dish,
 starting from stem cells,
 that will mimic some aspects of the development of the brain,
 but not all of it.
 They are very small, maximum.
 They become about four to five millimeters in diameters.
 They are much simpler than our brain, of course,
 but yet they are the only system
 where we can literally watch a process
 of human brain development unfold.
 And by watch, I mean study it.
 Remember when I told you that we can't understand
 everything about development in our own brain
 by studying a mouse?
 Well, we can't study the actual process
 of development of the human brain
 because it all happens in utero.
 So we will never have access to that process, ever.
 And therefore, this is our next best thing.
 Like a bunch of stem cells that can be coaxed
 into starting a process of neural tube formation.
 Remember that tube that is made by the embryo reon.
 And from there, a lot of the cell types
 that are present within the brain.
 And you can simply watch it and study,
 but you can also think about diseases
 where development of the brain
 does not proceed normally, right, properly.
 Think about neurodevelopmental diseases
 that are many, many different types.
 Think about autism spectrum disorders.
 There are also many different types of autism.
 So there you could take a stem cell,
 which really means either a sample of blood
 or a sample of skin from the patient, make a stem cell,
 and then with that stem cell,
 watch a process of formation of a brain organoid
 of that person with that genetics,
 with that genetic code in it.
 And you can ask, what is this genetic code doing
 to some aspects of development of the brain?
 And for the first time, you may come to solutions
 like what cells are involved in autism, right?
 So many questions around this.
 So if you take this human stem cell
 for that particular person with that genetic code,
 how, and you try to build an organoid,
 how often will it look similar?
 What's the, yeah, so-
 Reproducibility?
 Yes, or how much variability is the flip side of that?
 Yeah, so there is much more variability
 in building organoids than there is in building brain.
 It's really true that the majority of us,
 when we are born as babies,
 our brains look a lot like each other.
 This is the magic that the embryo does,
 where it builds a brain in the context of a body,
 and there is very little variability there.
 There is disease, of course,
 but in general, little variability.
 When you build an organoid,
 we don't have the full code for how this is done.
 And so in part, the organoid somewhat builds itself
 because there are some structures of the brain
 that the cells know how to make.
 And another part comes from the investigator,
 the scientist, adding to the media factors
 that we know in the mouse, for example,
 would foster a certain step of development.
 But it's very limited.
 And so as a result,
 the kind of product you get in the end
 is much more reductionist,
 it's much more simple than what you get in vivo.
 It mimics early events of development as of today,
 and it doesn't build very complex type of anatomy
 and structure does not as of today,
 which happens instead in vivo.
 And also the variability that you see,
 one organoid to the next tends to be higher
 than when you compare an embryo to the next.
 So, okay, then the next question is,
 how hard and maybe another flip side of that expensive
 is it to go from one stem cell to an organoid?
 How many can you build in life?
 Because it sounds very complicated.
 It's work, definitely, and it's money, definitely.
 But you can really grow a very high number
 of these organoids, can go perhaps,
 I told you the maximum they become
 about five millimeters in diameter.
 Which is how many cells are in a cell?
 So this is about the size of a tiny, tiny raisin,
 or perhaps the seed of an apple.
 And so you can grow 50 to 100 of those
 inside one big bioreactors, which are these flasks
 where the media provides nutrients for the organoids.
 So the problem is not to grow more or less of them.
 It's really to figure out how to grow them in a way
 that they are more and more reproducible.
 For example, organoid to organoid,
 so they can be used to study a biological process.
 Because if you have too much variability,
 then you never know if what you see is just an exception
 or really the rule.
 So what does an organoid look like?
 Are there different neurons already emerging?
 Is there, you know, well first,
 can you tell me what kind of neurons are there?
 Are they sort of all the same?
 Are they not all the same?
 How much do we understand?
 And how much of that variance, if any,
 can exist in organoids?
 Yes.
 So you could grow,
 I told you that the brain has different parts.
 So the cerebral cortex is on the top part of the brain,
 but there is another region called the striatum
 that is below the cortex and so on and so forth.
 All of these regions have different types of cells
 in the actual brain, okay?
 And so scientists have been able to grow organoids
 that may mimic some aspects of development
 of these different regions of the brain.
 And so we are very interested in the cerebral cortex.
 That's the coolest part, right?
 Very cool.
 I agree with you.
 Yeah, sorry.
 We wouldn't be here talking if we didn't have
 a cerebral cortex.
 It's also, I like to think, the part of the brain
 that really truly makes us human,
 the most evolved in recent evolution.
 And so in the attempt to make the cerebral cortex
 and by figuring out a way to have these organoids
 continue to grow and develop for extended periods of time,
 much like it happens in the real embryo,
 months and months in culture,
 then you can see that many different types of neurons
 of the cortex appear.
 And at some point, also the astrocytes,
 so the glia cells of the cerebral cortex also appear.
 What are these?
 Astrocytes.
 Astrocytes, yeah.
 The astrocytes are not neurons,
 so they're not nerve cells,
 but they play very important roles.
 One important role is to support the neuron,
 but of course they have much more active type of roles.
 They're very important, for example,
 to make the synapses,
 which are the point of contact and communication
 between two neurons.
 They do a lot of things.
 So all that chemistry fun happens in the synapses,
 happens because of these cells?
 Are they the medium in which?
 It happens because of the interactions.
 It happens because you are making the cells
 and they have certain properties,
 including the ability to make neurotransmitters,
 which are the chemicals that are secreted to the synapses,
 including the ability of making these axons grow
 with their growth cones and so on and so forth.
 And then you have other cells around it
 that release chemicals or touch the neurons
 or interact with them in different ways
 to really foster this perfect process,
 in this case of synaptogenesis.
 And this does happen within organoids.
 Or with organoids.
 So the mechanical and the chemical stuff happens.
 Yeah, the connectivity between neurons.
 This in a way is not surprising
 because scientists have been culturing neurons forever.
 And when you take a neuron, even a very young one,
 and you culture it, eventually finds another cell
 or another neuron to talk to, it will form a synapse.
 Are we talking about mice neurons?
 Are we talking about human neurons?
 It doesn't matter, both.
 So you can culture a neuron, like a single neuron,
 and give it a little friend, and it starts interacting?
 Yes.
 So neurons are able to, it sounds,
 it's more simple than what it may sound to you.
 Neurons have molecular properties and structural properties
 that allow them to really communicate with other cells.
 And so if you put not one neuron,
 but if you put several neurons together,
 chances are that they will form synapses with each other.
 Okay, great.
 So an organoid is not a brain.
 No.
 But there's some, it's able to,
 especially what you're talking about,
 mimic some properties of the cerebral cortex, for example.
 So what can you understand about the brain
 by studying an organoid of a cerebral cortex?
 I can literally study all this incredible diversity
 of cell type, all these many, many different classes
 of cells, how are they made?
 How do they look like?
 What do they need to be made properly?
 And what goes wrong if now the genetics
 of that stem cell that I used to make the organoid
 came from a patient with a neurodevelopmental disease?
 Can I actually watch for the very first time
 what may have gone wrong years before in this kid
 when its own brain was being made?
 Think about that loop.
 In a way, it's a little tiny rudimentary window
 into the past, into the time when that brain
 in a kid that had this neurodevelopmental disease
 was being made.
 And I think that's unbelievably powerful
 because today we have no idea of what cell types,
 we barely know what brain regions are affected
 in these diseases.
 Now we have an experimental system
 that we can study in the lab and we can ask,
 what are the cells affected?
 When during development things went wrong?
 What are the molecules among the many, many different
 molecules that control brain development?
 Which ones are the ones that are really messed up here
 and we want perhaps to fix?
 And what is really the final product?
 Is it a less strong kind of circuit in brain?
 Is it a brain that lacks a cell type?
 Is it a, what is it?
 Because then we can think about treatment and care
 for these patients that is informed
 rather than just based on current diagnostics.
 So how hard is it to detect through the development
 of the process, it's a super exciting tool
 to see how different conditions develop.
 How hard is it to detect that, wait a minute,
 this is abnormal development?
 Yeah.
 How hard is it?
 How much signal is there?
 How much of it is it a mess?
 Cause things can go wrong at multiple levels, right?
 You could have a cell that is born and built
 but then doesn't work properly
 or a cell that is not even born
 or a cell that doesn't interact with other cells differently
 and so on and so forth.
 So today we have technology that we did not have
 even five years ago that allows us to look, for example,
 at the molecular picture of a cell, of a single cell
 in a sea of cells with high precision.
 And so that molecular information where you compare
 many, many single cells for the genes that they produce
 between a control individual and an individual
 with a neurodevelopmental disease,
 that may tell you what is different molecularly.
 Or you could see that some cells are not even made,
 for example, or that the process of maturation
 of the cells may be wrong.
 There are many different levels here
 and we can study the cells at the molecular level
 but also we can use the organoids to ask questions
 about the properties of the neurons,
 the functional properties,
 how they communicate with each other,
 how they respond to a stimulus and so on and so forth.
 And we may get abnormalities there, right?
 Detect those.
 So how early is this work in a,
 maybe in the history of science?
 So-
 So-
 What an easy question.
 I mean, like, so if you were to,
 if you and I time travel a thousand years into the future,
 organoids seem to be,
 maybe I'm romanticizing the notion,
 but you're building not a brain,
 but something that has properties of a brain.
 So it feels like you might be getting close to,
 in the building process to build this, to understand.
 So how,
 how far are we in this understanding process of development?
 A thousand years from now, it's a long time from now.
 So if this planet is still gonna be here
 a thousand years from now.
 So I mean, if, you know, like they write a book,
 obviously there'll be a chapter about you.
 Let's write the science fiction book today.
 Yeah, today.
 But I mean, I guess where we really understood very little
 about the brain a century ago,
 I was a big fan in high school of reading Freud and so on.
 I still am of psychiatry.
 I would say we still understand very little
 about the functional aspect of just,
 but how in the history of understanding the biology
 of the brain, the development, how far are we along?
 It's a very good question.
 And so this is just, of course, my opinion.
 I think that we did not have technology,
 even 10 years ago or certainly not 20 years ago
 to even think about experimentally investigating
 the development of the human brain.
 So we've done a lot of work in science to study the brain
 or many other organisms.
 Now we have some technologies which I'll spell out
 that allow us to actually look at the real thing
 and look at the brain, at the human brain.
 So what are these technologies?
 There has been huge progress in stem cell biology.
 The moment someone figured out
 how to turn a skin cell into an embryonic stem cell,
 basically, and that how that embryonic stem cell
 could begin a process of development again,
 to for example, make a brain, there was a huge advance.
 And in fact, there was a Nobel Prize for that.
 That started the field really of using stem cells
 to build organs.
 Now we can build on all the knowledge of development
 that we build over the many, many, many years
 to say, how do we make the stem cells
 now make more and more complex aspects
 of development of the human brain?
 So this field is young, the field of brain organoids,
 but it's moving faster.
 And it's moving fast in a very serious way
 that is rooted in labs with the right ethical framework
 and really building on solid science
 for what reality is and what is not.
 But it will go faster and it will be more and more powerful.
 We also have technology that allows us
 to basically study the properties of single cells
 across many, many millions of single cells,
 which we didn't have perhaps five years ago.
 So now with that, even an organoid that has millions
 of cells can be profiled in a way looked at
 with very, very high resolution, the single cell level
 to really understand what is going on.
 And you could do it in multiple stages of development
 and you can build your hypothesis and so on and so forth.
 So it's not gonna be a thousand years.
 It's gonna be a shorter amount of time.
 And I see this as sort of an exponential growth
 of this field enabled by these technologies
 that we didn't have before.
 And so we're gonna see something transformative
 that we didn't see at all in the prior thousand years.
 So I apologize for the crazy sci-fi questions,
 but the developmental process is fascinating
 to watch and study, but how far are we away from?
 And maybe how difficult is it to build not just an organoid
 but a human brain from a stem cell?
 Yeah, first of all, that's not the goal
 for the majority of the serial scientists
 that work on this because you don't have
 to build the whole human brain to make this model useful
 for understanding how the brain develops
 or understanding disease.
 You don't have to build the whole thing.
 So let me just comment on this, fascinating.
 It shows to me the difference between you and I
 is you're actually trying to understand the beauty
 of the human brain and to use it to really help thousands
 or millions of people with disease and so on.
 From an artificial intelligence perspective,
 we're trying to build systems that we can put in robots
 and try to create systems that have echoes
 of the intelligence about reasoning about the world,
 navigating the world.
 It's different objectives, I think.
 Yeah, that's very much science fiction.
 Science fiction, but we operate in science fiction
 a little bit, but so on that point of building a brain,
 even though that is not the focus or interest perhaps
 of the community, how difficult is it?
 Is it truly science fiction at this point?
 I think the field will progress, like I said,
 and that the system will be more and more complex
 in a way, right?
 But there are properties that emerge from the human brain
 that have to do with the mind.
 They may have to do with consciousness,
 they may have to do with intelligence or whatever.
 That we really don't understand even how they can emerge
 from an actual real brain, and therefore we can not measure
 or study in an organoid.
 So I think that this field, many, many years from now,
 may lead to the building of better neural circuits
 that really are built out of understanding
 of how this process really works.
 And it's hard to predict how complex this really will be.
 I really don't think we're so far from,
 it makes me laugh really, it's really that far
 from building the human brain.
 But you're gonna be building something
 that is always a bad version of it,
 but that may have really powerful properties.
 And might be able to respond to stimuli
 or be used in certain contexts.
 And this is why I really think that there is no other way
 to do this science, but within the right ethical framework.
 Because where you're going with this is also,
 we can talk about science fiction and write that book,
 and we could today.
 But this work happens in a specific ethical framework
 that we don't decide just as scientists,
 but also as a society.
 So the ethical framework here is a fascinating one,
 is a complicated one.
 Yes.
 Do you have a sense, a grasp of how we think about ethically
 of building organoids from human stem cells
 to understand the brain?
 It seems like a tool for helping potentially
 millions of people cure diseases,
 or at least start to cure by understanding it.
 But is there more, is there gray areas
 that we have to think about ethically?
 Absolutely, we must think about that.
 Every discussion about the ethics of this
 needs to be based on actual data
 from the models that we have today,
 and from the ones that we will have tomorrow.
 So it's a continuous conversation,
 it's not something that you decide now.
 Today, there is no issue, really.
 Very simple models that clearly can help you in many ways
 without much to think about.
 But tomorrow we need to have another conversation
 and so on and so forth.
 And so the way we do this is to actually
 really bring together constantly a group of people
 that are not only scientists, but also bioethicists,
 the lawyers, philosophers, psychiatrists,
 and so on, psychologists, and so on and so forth,
 to decide as a society, really,
 what we should and what we should not do.
 So that's the way to think about the ethics.
 Now, I also think, though, that as a scientist,
 I have a moral responsibility.
 So if you think about how transformative it could be
 for understanding and curing a neuropsychiatric disease,
 to be able to actually watch and study and treat with drugs
 the very brain of the patient that you are trying to study.
 How transformative at this moment in time this could be.
 We couldn't do it five years ago, we could do it now.
 Taking a stem cell of a particular patient.
 A patient and make an organoid for a simple and different
 from the human brain, it still is his process
 of brain development with his or her genetics.
 And we could understand perhaps what is going wrong.
 Perhaps we could use it as a platform,
 as a cellular platform to screen for drugs,
 to fix a process, and so on and so forth, right?
 So we could do it now, we couldn't do it five years ago.
 Should we not do it?
 What is the downside of doing it?
 I don't see a downside at this very moment.
 But if we invited a lot of people,
 I'm sure there would be somebody who would argue against it.
 What would be the devil's advocate argument?
 Yeah.
 Yeah, so it's exactly perhaps what you alluded at
 with your question, that you are making,
 enabling some process of formation of the brain
 that could be misused at some point,
 or that could be showing properties
 that ethically we don't want to see in a tissue.
 So today, this is not an issue.
 And so you just gain dramatically from the science
 without because the system is so simple
 and so different in a way from the actual brain.
 But because it is the brain, we have an obligation
 to really consider all of this, right?
 And again, it's a balanced conversation
 where we should put disease and betterment of humanity
 also on that plate.
 What do you think, at least historically,
 there was some politicization of embryonic stem cells,
 stem cell research.
 Do you still see that out there?
 Is that still a force that we have to think about,
 especially in this larger discourse that we're having
 about the role of science in at least American society?
 Yeah, this is a very good question.
 It's very, very important.
 I see a very central role for scientists to inform decisions
 about what we should or should not do in society.
 And this is because the scientists have the firsthand look
 and understanding of really the work that they are doing.
 And again, this varies depending on what we're talking about here.
 So now we're talking about brain organoids.
 I think that the scientists need to be part of that conversation
 about what will be allowed in the future
 or not allowed in the future to do with the system.
 And I think that is very, very important
 because they bring reality of data to the conversation.
 And so they should have a voice.
 So data should have a voice.
 Data needs to have a voice because in not only data,
 we should also be good at communicating with non-scientists the data.
 So there has been, oftentimes, there is a lot of discussion
 and excitement and fights about certain topics
 just because of the way they are described.
 I'll give you an example.
 If I call the same cellular system we just talked about a brain organoid,
 or if I called it a human mini brain,
 your reaction is going to be very different to this.
 And so the way the systems are described,
 I mean, we and journalists alike need to be a bit careful
 that this debate is a real debate and informed by real data.
 That's all I'm asking.
 And yeah, the language matters here.
 So I work on autonomous vehicles and there the use of language
 could drastically change the interpretation
 and the way people feel about what is the right way to proceed forward.
 You are, as I've seen from a presentation, you're a parent.
 I saw you show a couple of pictures of your son.
 Is it just the one?
 Two.
 Son and a daughter.
 So what have you learned from the human brain by raising two of them?
 More than I could ever learn in a lab.
 What have I learned?
 I've learned that children really have these amazing plastic minds, right?
 That we have a responsibility to foster their growth in good, healthy ways.
 That keep them curious, that keep some adventurous,
 that doesn't raise them in fear of things.
 But also respecting who they are, which is in part coming from the genetics
 we talked about.
 My children are very different from each other,
 despite the fact that they're the product of the same two parents.
 I also learned that what you do for them comes back to you.
 Like, if you're a good parent, you're going to, most of the time,
 have perhaps a decent kids at the end.
 So what do you think, just a quick comment,
 what do you think is the source of that difference?
 That's often the surprising thing for parents.
 They can't believe that our kids, they're so different,
 yet they came from the same parents.
 Well, they are genetically different.
 Even they came from the same two parents because the mixing of gametes,
 we know this genetics, creates every time a genetically different individual,
 which will have a specific mix of genes that is a different mix every time
 from the two parents.
 And so they're not twins.
 They are genetically different.
 Even just that little bit of variation.
 Because you said, really, from a biological perspective,
 the brains look pretty similar.
 Well, so let me clarify that.
 So the genetics you have, the genes that you have,
 that play that beautiful orchestrated symphony of development,
 different genes will play it slightly differently.
 It's like playing the same piece of music,
 but with a different orchestra and a different director.
 The music will not come out.
 It will be still a piece by the same author,
 but it will come out differently if it's played by the high school
 orchestra instead of the...
 Instead of La Scala in Milan.
 And so you are born superficially with the same brain.
 It has the same cell types, similar patterns of connectivity,
 but the properties of the cells and how the cells will then
 react to the environment as you experience your world
 will be also shaped by who genetically you are.
 Speaking just as a parent, this is not something that comes from my work.
 I think you can tell at birth that these kids are different,
 that they have a different personality in a way.
 So both is needed, the genetics as well as the nurturing afterwards.
 So you are one human with a brain, sort of living through the whole mess of it,
 the human condition, full of love, maybe fear, ultimately mortal.
 How has studying the brain changed the way you see yourself?
 When you look in the mirror, when you think about your life,
 the fears, the love, when you see your own life, your own mortality.
 Yeah, that's a very good question.
 It's almost impossible to dissociate some time for me.
 Some of the things we do or some of the things that other people do from,
 oh, that's because that part of the brain is working in a certain way.
 Or thinking about a teenager, going through teenage years
 and being at times funny in the way they think.
 And impossible for me not to think it's because they're going through
 this period of time called critical periods of plasticity,
 where their synapses are being eliminated here and there,
 and they're just confused.
 And so from that comes perhaps a different take on that behavior
 or maybe I can justify it scientifically in some sort of way.
 I also look at humanity in general, and I am amazed by what we can do
 and the kind of ideas that we can come up with.
 And I cannot stop thinking about how the brain is continuing to evolve.
 I don't know if you do this, but I think about the next brain sometimes.
 Where are we going with this?
 What are the features of this brain that evolution is really playing with
 to get us in the future the new brain?
 It's not over, right?
 It's a work in progress.
 So let me just a quick comment on that.
 Do you think there's a lot of fascination and hope for artificial intelligence
 of creating artificial brains?
 You said the next brain.
 When you imagine over a period of a thousand years,
 the evolution of the human brain,
 do you sometimes envisioning that future see an artificial one?
 Artificial intelligence as it is hoped by many, not hoped,
 thought by many people would be actually the next evolutionary step
 in the development of humans.
 Yeah, I think in a way that will happen, right?
 It's almost like a part of the way we evolve.
 We evolve in the world that we created, that we interact with,
 that shape us as we grow up and so on and so forth.
 Sometimes I think about something that may sound silly,
 but think about the use of cell phones.
 Part of me thinks that somehow in their brain,
 there will be a region of the cortex that is attuned to that tool.
 And this comes from a lot of studies in model organisms
 where really the cortex especially adapts to the kind of things you have to do.
 So if we need to move our fingers in a very specific way,
 we have a part of our cortex that allows us to do this kind of very precise movement.
 An owl that has to see very, very far away with big eyes,
 the visual cortex very big, the brain attunes to your environment.
 So the brain will attune to the technologies that we will have and will be shaped by it.
 So the cortex very well may be shaped by it.
 In artificial intelligence, it may merge with it, it may get enveloped and adjusted.
 Even if it's not a merge of the kind of,
 oh, let's have a synthetic element together with a biological one.
 The very space around us, the fact, for example,
 think about we put on some goggles of virtual reality
 and we physically are surfing the ocean, right?
 Like I've done it and you have all these emotions that come to you.
 Your brain placed you in that reality.
 And it was able to do it like that just by putting the goggles on.
 It didn't take thousands of years of adapting to this.
 The brain is plastic, so adapts to new technology.
 So you could do it from the outside by simply hijacking some sensory capacities that we have.
 So clearly over recent evolution,
 the cerebral cortex has been a part of the brain that has known the most evolution.
 So we have put a lot of chips on evolving this specific part of the brain.
 And the evolution of cortex is plasticity,
 is this ability to change in response to things.
 So, yes, they will integrate, that we want it or not.
 Well, there's no better way to end it.
 Paola, thank you so much for talking to me.
 Very well, that's great. Very exciting.