Stem cells can be classified into three broad categories, based on their ability to differentiate. Totipotent stem cells are found only in early embryos. Each cell can form a complete organism (e.g., identical twins). Pluripotent stem cells exist in the undifferentiated inner cell mass of the blastocyst and can form any of the over 200 different cell types found in the body. Multipotent stem cells are derived from fetal tissue, cord blood and adult stem cells. Although their ability to differentiate is more limited than pluripotent stem cells, they already have a track record of success in cell-based therapies. Here is a current list of the sources of stem cells:
Embryonic stem cells - are harvested from the inner cell mass of the blastocyst seven to ten days after fertilization.
Fetal stem cells - are taken from the germline tissues that will make up the gonads of aborted fetuses.
Umbilical cord stem cells - Umbilical cord blood contains stem cells similar to those found in bone marrow.
Placenta derived stem cells - up to ten times as many stem cells can be harvested from a placenta as from cord blood.
Adult stem cells - Many adult tissues contain stem cells that can be isolated.