How are Adult and Embryonic Stem Cells Different?
Perhaps the most distinguishing feature of embryonic stem cells and adult stem cells is their source. Most scientists now agree that adult stem cells exist in many tissues of the human body (in vivo), although the cells are quite rare. In contrast, it is less certain that embryonic stem cells exist as such in the embryo. Instead, embryonic stem cells and embryonic germ cells develop in tissue culture after they are derived from the inner cell mass of the early embryo or from the gonadal ridge tissue of the fetus, respectively. Depending on the culture conditions, embryonic stem cells may form clumps of cells that can differentiate spontaneously to generate many cell types. This property has not been observed in cultures of adult stem cells. Also, if undifferentiated embryonic stem cells are removed from the culture dish and injected into a mouse with a compromised immune system, a benign tumor called a teratoma can develop. A teratoma typically contains a mixture of partially different
