Stem cells in mammalian development

I. Stem cells are central to three processes in an organism: development, repair of adult tissue and cancer.

A. Stem cells in mammalian development

The zygote is the ultimate stem cell. It is totipotent with the ability to produce all the cell types of the species including the trophoblast and the embryonic membranes. Development begins when the zygote undergoes several successive cell divisions, each resulting in a doubling of the cell number and a reduction in the cell size. At the 32- to 64-cell stage each cell is called a blastomere.2 The blastomeres stick together to form a tight ball of cells called a morula. Each of these cells retains totipotential. The next stage is the blastocyst which consists of a hollow ball of cells; trophoblast cells along the periphery develop into the embryonic membranes and placenta while the inner cell mass develops into the fetus. Beyond the blastocyst stage, development is characterized by cell migration in addition to cell division. The gastrula is composed of three germ layers: the ectoderm, mesoderm and endoderm. The outer layer or ectoderm gives rise to the future nervous system and the epidermis (skin and associated organs such as hair and nails). The middle layer or mesoderm gives rise to the connective tissue, muscles, bones and blood, and the endoderm (inner layer) forms the gastrointestinal tract of the future mammal.

Early in embryogenesis, some cells migrate to the primitive gonad or genital ridge. These are the precursors to the gonad of the organism and are called germinal cells. These cells are not derived from any of the three germ layers but appear to be set aside earlier.