Origins and types of stem cells

This chapter is about stem cells: what we can do with them and what they can mean for future health. To begin though, and to distinguish fact from fiction, we have to understand what they are. They are best defined as cells that can divide over long periods but at the same time have the ability to form one or more different cell types through a process called differentiation. Many organs and tissue types in embryos and adult individuals, both animals and human, contain stem cells. In Chapter 3 we explained that embryonic stem cells, derived from embryos 3–5 days after fertilization of an egg in a laboratory test tube, are considered to be pluripotent, that is they are able to make all cells of the fetus. By contrast, stem cells derived from tissues after birth are generally considered to be multi- or unipotent. Even though literally, multipotency and pluripotency actually mean the same (the ability to become many cell types by differentiation), conventionally we use the term multipotency for adult (or “postnatal”) stem cells, to refer to the more limited range of cell types that these stem cells can form. Unipotency usually refers to spermatogonia, cells which can divide but are only able to form sperm cells. Totipotency, by contrast, is the exact opposite: not only can totipotent cells form all cells of the body (just like pluripotent cells) but they can also form all of the cell types needed to support fetal development in the mother: the yolk sac, placenta, and umbilical cord. We will give examples of these different types of “developmental potency” and exactly what it means. We will also describe how through altering DNA in cells by genetic engineering we can recreate the state of pluripotency in normal cells of the body or drive these cells to become different cell types through a process called transdifferentiation.