You have undoubtedly heard a lot about stem cells, and are aware of the political furor, but you probably don’t really understand (nor do most people who argue about stem cells) what they are all about. In the rest of this chapter, I will try to explain this in a simple way. First, since 1981, when stem cells were first cultured in mice, they have been critical for studying (in mice) human genetic diseases. Second, stem cells play an integral role in the future for curing genetic diseases. The concept of using stern cells, whether adult or embryonic, to cure disease, by trying to get undifferentiated pluripotent cells to differentiate into healthy replacement tissue, is the exact opposite of cloning. With cloning, differentiated adult tissue cells are made to become totipotential, or undifferentiated, so that their DNA can direct, all over again, development of a new embryo. In other words, cloning and stem cells go hand in hand but function in reverse directions.

The stem cell revolution actually began quietly in 1981 with a relatively unobtrusive paper by Evans and Kaufman, from Cambridge, England, in the journal Nature . The title of the paper was “Establishment in Culture of Pluripotential Cells from Mouse Embryos.” They utilized techniques that were originally used in 1975 at the Wistar Institute in Philadelphia, developed, ironically, by the same doctors who had stated categorically that mammals could never be cloned. Evans and Kaufman used early embryos from mice to culture indefinitely from them cells that were virtually immortal and could differentiate at any time into nonembryonic adult tissue.

Normally, during the first five days after fertilization, the embryo divides into two cells, four cells, eight cells, sixteen cells, etc., and eventually becomes a ball of cells called the morula on day four. By day five, this ball of cells has formed a liquid-interior cystlike structure, and is called a blastocyst. The blastocyst consists of a circumferential perimeter of cells called the trophectoderm (which will eventually become the placenta of the pregnancy), and a small glob of cells stuck to the inside of this wall, which is called the inner cell mass, or ICM. The ICM cells will go on to develop into the embryo proper, and the outer trophectoderm cells will develop into the placenta. The placenta works perfectly fine even if there are all kinds of chromosomal abnormalities in the cells, but the inner cell mass must be chromosomally normal in order to develop into a viable fetus.

Evans and Kaufman took the ICM cells from mouse embryos (blastocysts) and did what no one else had been able to do in the past. They maintained them in their undifferentiated stage of development in a culture system that allowed a perpetual replication of these cells without letting them differentiate into adult cells (see figure below). A “stem cell” is simply a type of cell that can replicate indefinitely and not develop the characteristics of any specific tissue. Its only job is to replicate over and over again, shedding off daughter cells that can differentiate into specific tissue. Previous attempts to obtain cultures of these pluripotential cells directly from the mouse embryo had been unsuccessful, even though there was always a very transient appearance in culture of healthy stem cells. Whenever attempts had been made to obtain immortal pluripotential stem cell lines from the mouse embryo inner cell mass, the embryonic cells would automatically skip the stern cell stage and differentiate into mature but disorganized adult tissue or into tumors.

There is still a very poor understanding of why Evans and Kaufman were successful, but they found out that ICM cells obtained from the early mouse embryo had to be cultured on a monolayer of mouse scar-tissue cells (i.e., fibroblasts) in order to become immortal stem cells. These mouse scar-tissue cells had to be treated with a poison called mitomycin so that they could not proliferate themselves but could produce some sort of unknown byproducts that inhibit the differentiation of the ICM cells. As soon as these stem cells were removed from this so-called feeder layer of mouse embryonic fibroblasts, they would automatically differentiate in a disorganized way into a variety of cell types. If injected into mouse tissue, they developed into tumors called teratomas. For more than twenty years no one has really understood this obscure finding. It had been hoped that a substance called leukemia-inhibiting factor (LIF) was the magic ingredient that prevented these ICM cells from developing into differentiated adult tissue and that allowed them to eternally replicate as stem cells, but this was later shown not to be valid. Stem cells derived from embryos stop being stem cells (and will usually differentiate into tumors with a variety of tissue types) if they are not inhibited from doing so by being cultured on a layer of mouse fibroblasts, and no one can explain why.