What is a stem cell?

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Tree1You started out as one cell– wicked, right? Now look at you! Brain, skin, blood, all of these can be traced back to that first cell that was you. Since you’re bigger that one cell, this cell had to make more of itself. Since you’re more complicated than a blob of identical cells, they had to specialize to become brain, skin, and blood. These cells that 1) make more of themselves for a long time and 2) can specialize to become other cell types are called stem cells. “Stem cells” is a category more than one specific cell type. Members vary widely, and they differ in the kinds and number of cells they can become.

While people may be familiar with the most controversial kind of stem cells, embryonic stem cells (ESCs), you may not know that you have stem cells in you right now. For just one, you have blood stem cells in your bone marrow. These hematopoietic (blood-forming) stem cells can become every kind of blood cells, red or white. In fact, one true hematopoietic stem cell can remake all of the blood: if you irradiate a mouse such that all the blood cells die (akin to radiation treatment for leukemia), and then you put in one hematopoietic stem cell, this stem cell can remake all of the different kinds of blood cells, and the mouse lives.

If stem cells can go from less specialized to more specialized and remake whole systems, can we go the other way? Can we take a very specialized kind of cell and make it go back to a less specialized stem cell? Yes! Since every cell keeps the same DNA instructions but only uses different subsets, we just have to tell the cell to use a “be a stem cell” subset of DNA instructions. Yeah… just.

It’s hard to do, but you can tell a specialized blood cell to become a any-cell-making stem cell– to make them a “pluripotent” stem cell. This process is a switching-on or “induction” of a subset of DNA instructions that say “be a pluripotent stem cell.” It results in a really powerful bunch of cells that might just hold the ability to repair whole systems– brain, skin, or blood. These are induced pluripotent stem cells. These are iPS cells.

I’m writing this at a stem cell conference. It sounds like the problem has (for now) changed from “how do we make stem cells from specialized cells” to “how do we make specialized cells from stem cells.” It turns out we don’t have a great sense of how you go from that one first cell to all of the different cell types. For example, there are several stages in between fertilized egg and blood that we’re still identifying. But we’re working on it.

Our goal: take a bit of your skin or blood (big pools of cells with easy access), induce the right genes to make them iPS cells, then turn these iPS cells into the kind of cell you need replaced. Maybe we fight Parkinson’s with neurons for your brain, baldness with hair follicle cells for your skin, or HIV with helper T cells for your blood, and each of them from iPS cells.

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