Tell me about stem cells (part 1)

This topic has come up quite a bit, especially a few months ago when I applied for a job at the California Institute for Regenerative Medicine. What exactly is a stem cell? Where do they come from? Why is there a big moral brouhaha about them? (That last one was from Megan) Why are we so excited about them? As you can see, there are quite a few things to talk about with regard to stem cells, so I will start with the most obvious question: What is a stem cell?

Most cells are differentiated and specialized- they have a certain purpose and they are unable to renew themselves. A muscle cell will always be a muscle cell, and it won't make more of itself (normally). Stem cells are cells that are not differentiated, in that they do not have a specific function in the body. They are also able to divide over long periods of time, replacing themselves. There are two types of stem cells that are commonly talked about, embryonic stem cells and adult stem cells.

Embryonic stem cells are pluripotent (also called multipotent). This means that they can become any kind of cell. It's sort of like your parents telling you when you're young that you can be anything when you grow up; pluripotent stem cells have the potential to become any type of cell later. The type of cells they become has to do with what environment they grow in. Because embryonic stem cells can become any type of cell (from a heart muscle cell to a neuron to a blood cell and more), scientists are really interested in figuring out how to program these cells to turn into a specific type of cell. This could make treatments for a lot of diseases possible (more on this later).

Adult stem cells that are more specialized- they are somewhat differentiated, so they can only make certain types of cells. This is sort of like how if you went to law school, you could be any kind of lawyer, but you couldn't decide to go become a practicing pediatrician right away- you're trained as a lawyer, not a doctor. Some adult stem cells are multipotent. This means that they can turn into several types of cells (but not all types of cells). Blood stem cells are multipotent; they can divide and differentiate (turn into) any kind of blood cell. Other adult stem cells can only make one type of cell; these are called unipotent. We have unipotent stem cells in our guts- their whole job is to keep making more cells to line the villi in our small intestines with. These types of stem cells are also very interesting (and all people have them, not just embryos!), but may be limited in application because they are somewhat differentiated and cannot become any type of cell.

More on this later, but for now you can find out more about this at the NIH website, or you can Google "stem cell"