What Are Stem Cells?
Stem cells are relatively undifferentiated and unspecialized, but give rise to the differentiated and specialized cells of the body such as liver, kidney, and brain cells. All specialized cells arise originally from stem cells, and ultimately from a small number of embryonic cells that appear in the first few days of development of the human embryo.
Embryonic stem cells are taken from the inner cell mass of embryos at the blastocyte stage, roughly five to nine days after fertilization. This is after about 200 cells have accumulated, but before they have begun to differentiate into specialized cells.
Germ stem cells are derived from the gonadal ridges (structures appearing in human embryos at about five weeks) and will eventually develop into reproductive cells (sperm or eggs).
Adult [non-embryonic] stem cells can be found in various tissues in fetuses and children, as well as adults. They are more differentiated than embryonic (or egg) cells, but they are not yet fully differentiated. Although they can give rise to various cell types, adult stem cells are generally all within the same broad type of tissue, such as muscle stem cells or nerve stem cells.
All stem cells have both the capacity for (1) unlimited or prolonged self-renewal (the capability to maintain a pool of undifferentiated stem cells) and (2) the potential to produce differentiated descendant cell types. As stem cells within a developing human embryo grow and differentiate, their capacity to diversify generally becomes more limited. Stem cells and their derivatives may prove a valuable source of transplantable cells and tissues for repair and regeneration.
