Stem cells maturing into one of three types of mature blood cells: red blood cells, platelets, and white blood cells. Precursor cells are also shown: stem cells, myeloid blasts, lymphoid stem cells, and lymphoid blasts. 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. |