Stem cells are
undifferentiated biological cells that can
differentiate into specialized cells and can
divide (through
mitosis) to produce more stem cells. They are found in multicellular
organisms. In
mammals, there are two broad types of stem cells:
embryonic stem cells, which are isolated from the
inner cell mass of
blastocysts, and
adult stem cells, which are found in various tissues. In
adult organisms, stem cells and
progenitor cells act as a repair system for the body, replenishing adult tissues. In a developing embryo, stem cells can differentiate into all the specialized cells—ectoderm, endoderm and mesoderm (see
induced pluripotent stem cells)—but also maintain the normal turnover of regenerative organs, such as blood, skin, or intestinal tissues.