Carnegie stages

In embryology, Carnegie stages are a standardized system of 23 stages used to provide a unified developmental chronology of the vertebrate embryo.
The stages are delineated through the development of structures, not by size or the number of days of development, and so the chronology can vary between species, and to a certain extent between embryos. In the human being only the first 60 days of development are covered; at that point the term embryo is usually replaced with the term fetus.
It was based on work by Streeter (1942) and O'Rahilly and Müller (1987). The name "Carnegie stages" comes from the Carnegie Institution of Washington.
While the Carnegie stages provide a universal system for staging and comparing the embryonic development of most vertebrates, other systems are occasionally used for the common model organisms in developmental biology, such as the Hamburger–Hamilton stages in the chick.
Stages.
Days are approximate, and reflect the days since the last ovulation before pregnancy ("Postovulatory age").
Stage 1: 1 days.
Carnegie stage 1 is the unicellular embryo. This stage is divided into three substages.
Stage 1 a.
Primordial embryo. All the genetic material necessary for a new individual, along with some redundant chromosomes, are present within a single plasmalemma. Penetration of the fertilising sperm allows the oocyte to resume meiosis and the polar body is extruded.
Stage 1 b.
Pronuclear embryo. Two separate haploid components are present - the maternal and paternal pronuclei. The pronunclei move towards each other and eventually compress their envelopes where they lie adjacent near the centre of the wall.
Stage 1 c.
Syngamic embryo. The last phase of fertilisation. The pronuclear envelopes disappear and the parental chromosomes come together in a process called syngamy.
Stage 2: 2-3 days.
Carnegie stage 2 begins when the zygote undergoes its first cell division, and ends when the blastocyst forms a cavity and comprises more than 16 cells. At this point, it is called a morula.
The cleavage divisions of CS2 embryos do not occur synchronously. And the fate of the blastomeres is not yet determined.
The two-cell embryo is spherical and surrounded by the transparent zona pellucida. Each of the blastomeres that form are also spherical.
On approximately day 3, at the eight-cell stage, compaction usually begins.
Stage 3: 4-5 days.
Carnegie stage 3 begins when a cavity first appears in the morula and ends after the hatching from the zona pellucida when the embryo makes contact with the endometrial lining of the uterus.
There are only two stage 3 embryos in the Carnegie collection.
There are four characteristic processes that CS3 embryos go through: cavitation, collapse and expansion, hatching, and discarding of cells.
Cavitation.
The initiation of cavitation indicates the start of CS3. This process leads to the differentiation of blastocysts into outer trophoblast cells and inner embryoblasts.
Collapse and expansion.
This process is seen in vitro and it is not known whether this occurs in vivo. In vitro, the blastocyst rapidly collapses and slowly re-expands before hatching from the zona pellucida.
Hatching.
During this process, the blastocyst breaks through and escapes from the zona pellucida. This process must occur prior to implantation into the endometrium.
Discarding of cells.
TEM inspection of in vitro blastocysts has allowed us to identify two types of cells that the developing embryo apparently discards. These are sequestered cells and isolated cells. Sequestered cells are groups of cells that are located in between the zona pellucida and the trophoblast. Isolated cells are mainly found in the blastocystic cavity.