All of life begins as a single cell, and it is that single cell that holds the key to the formation of the organism. This stem cell has the ability to become any cell that constitutes the organism, which has attracted the interest of scientists and researchers who study degenerative disease. If these stem cells can be coaxed into differentiating into various cell types and assimilated by patients" damaged tissues, they may become the standard therapy to treat degenerative disease.
Stem cells are defined based on their behavior in vivo, or in the intact organism. For example, a zygote is a totipotent stem cell, meaning that it can differentiate into all the tissues that make up the embryo, as well as the extra-embryonic tissues, placenta, and umbilical cord. Pluripotent stem cells can give rise to cells that are derived from all three embryonic germ layers (ectoderm, mesoderm, and endoderm) throughout the course of natural embryonic development. In other words, they can differentiate into any cell that comprises the embryo. Unipotent stem cells are found in the adult organism and can only differentiate into one cell lineage in vivo. This allows for a steady state of renewal within the tissue that the unipotent cell resides.
The adult stem cell is an undifferentiated cell that is found in a differentiated tissue. It has the ability to self-replicate throughout the lifetime of the organism and can differentiate into every cell type that exists within the tissue. Sources of adult stem cells in humans have been found in the cornea, retina, dental pulp, liver, skin, vascular tissue, hematopoietic tissue, gastrointestinal lining tissue, and pancreatic islet tissue. As far as we know, all adult stem cells are unipotent. At this point in time, we have not isolated any somatic adult stem cells that are capable of differentiating into all of the cell types present within the body.
Embryonic stem cells are extracted from a blastocyst, which is one of the earliest stages of embryonic development.
