Stem cell, coloured scanning electron micrograph (SEM). Stem cells can differentiate into any other cell type. There are three main types of mammalian stem cell: embryonic stem cells, derived from blastocysts; adult stem cells, which are found in some adult tissues; and cord blood stem cells, which are found in the umbilical cord. The cell seen here is destined to become a blood cell. During blood cell development in adults, stem cells develop through a process known as haemopoiesis. Blood cells have short lifespans and are therefore constantly produced by the bone marrow.
Credit: SCIENCE PHOTO LIBRARY