Stem cells, 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 cells seen here are destined to become blood cells. 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.