Why do some professors have M.D. as well as/instead of a PhD? I'm interested in stem cell, regenerative medicine, developmental biology, and genetics for reverse ageing. Since these topics are dealt with in one or both of medical school and school of science and professors have one or both of MD and PhD, I'm confused about my grad school prep. Also, I'm really not interested in clinical medicine, but I want to be a researcher throughout my life. 

So, I think I need only PhD, but I'm wondering why some professors in these areas have MD? 
Is it just because they decided to become researchers although they initially intended to work as a clinical doctor?
There are two main tracks of medicine: clinical (the "standard" medical practice), and research. Typically, doctors who work in the latter tend to pursue combined MD-PhD programs, and consequently have degrees in both a traditional scientific research discipline as well as medicine.  

It probably was easier in the past to make such a switch with "just" a medical degree; however, the increased specialization in the field makes it harder for a medical doctor to switch over to a research track without the increased training (unless he or she is working on the "clinical" side of therapeutic treatments designed in research programs).