Forms of stem cells

There are several types of stem cell. Embryonic stem cells are totipotent if they arise from an embryo at an early blastomere stage. It is likely that human cells remain totipotent until the 12-cell stage. Pluripotent stem cells are obtained from the embryoblast at the blastocyst stage. These cells are precursors to all human cell types that are found in later stages of development and even in adults. Embryonic sex cells are also pluripotent. Due to advances in gene manipulation in in vitro experiments, the demarcation between totipotency and pluripotency is becoming increasingly obscure.
Adult stem cells are multipotent cells. They can generate several cell types, but the spectrum of types is more limited than for pluripotent cells, e.g. to cells deriving from a particular germ layer. However, there is increasing evidence that pluripotent cells may also be found in adult stem cells. Adult stem cells include fetal stem cells (from fetal tissue after miscarriage or abortion), neonatal stem cells (from umbilical cord blood following birth) and stem cells from adult tissue (e.g. bone marrow, brain or skin).