Many differentiated cell types are sloughed from the body or have life spans that are shorter than that of the organism. Disease and trauma can also lead to losses of differentiated cells. Since most types of differentiated cells do not divide, they must be replenished from nearby somatic stem-cell populations. In vertebrates and most invertebrates, such stem cells, in contrast to pluripotent ES cells, are multipotent in that they can give rise to some, but not all, of the cell types found in the organism. Postnatal (adult) vertebrate animals contain stem cells for many tissues, including the blood, intestine, skin, ovaries, testes, and muscle. Even some parts of the adult brain, where little cell division normally occurs, have a population of stem cells (see Chapter 22). In striated muscle, stem cells are most important in healing, as relatively little cell division occurs at other times. Some other cell types, such as liver cells (hepatocytes) and insulin-producing β islet cells, reproduce mainly by division of already differentiated cells, as exemplified by regeneration of the liver when large pieces are surgically removed. Whether these tissues also contain stem cells that can generate these types of differentiated cells is controversial.