There is More than One Source for Stem Cells
Stem cells are often classified with regard to their origin. Stem cells are present both in the embryo and in the adult. Depending on where the stem cells originate, they have different properties. According to this classification scheme, the three kinds of stem cells are: Embryonic stem cells, embryonic germ cells, and adult stem cells.
Embryonic stem cells are from the inner cell mass, which is part of the early (4-5 day old) embryo called the blastocyst. Once removed, the cells of the inner cell mass can be cultured into embryonic stem cells.
Embryonic germ cells have characteristics similar to embryonic stem cells. Embryonic germ cells, however, are collected from the fetus later in the developmental process from a region known as the gonadal ridge (which would eventually develop into the sex organs). Though these cells can give rise to the three germ layers that make all the specific organs of the body, the cell types that can develop from embryonic germ cells are slightly more limited than those that develop from embryonic stem cells. This is because embryonic germ cells are further along in the developmental process [16].
Adult stem cells originate from mature adults. These can also be referred to as multipotent stem cells, as the number of cell types which they can differentiate into are limited. Adult stem cells serve as a fresh source of cells in living organisms. They replace cells that need to be replaced on a regular basis in a living organism, such as blood (which has a 120 day lifespan) and other connective tissues. It is generally believed that adult stem cell therapies will complement but not replace embryonic stem cell therapies. One advantage of adult stem cells is that they offer the opportunity to utilize small samples of adult tissues of a patient's own cells for expansion and subsequent implantation. This avoids the ethical issues of embryonic stem cells, as well as the issues that accompany allogeneic donations.
No one type of stem cell is necessarily better than the other, rather the different stem cells have different advantages [16].
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