Pluripotent Stem Cells
A fertilized egg is totipotent, meaning that it has the potential to give rise to every type of cell needed for the development of the organism. The cell undergoes successive divisions, and at the beginning, the resulting identical cells are also totipotent. These cells form a hollow ball called a blastocyst, which contains a cluster of cells called the inner cell mass. At this point, some specialization of cells has already occurred. The outer layer of cells will form the extraembryonic membranes that will support the developing embryo, and the inner cell mass will form the embryo, itself.
The cells of the inner cell mass are known as pluripotent. Because they are incapable of giving rise to the supporting cells of the extraembryonic membranes, they are not sufficient for fetal development. Each of the inner-cell-mass cells does, however, have the potential to become any type of cell in the fetus, itself. As development proceeds, the pluripotent cells specialize further, so that each type, called a multipotent stem cell, has the ability to make only certain types of cells. Blood stem cells, for example, will make only red cells, white cells, and platelets. These multipotent cells are present throughout life.
Pluripotent stem cells have been isolated by one laboratory from the inner cell mass of human embryos in the blastocyst stage. These embryos were produced by in vitro fertilization and were in excess of those used in fertility treatment. In another laboratory, pluripotent stem cells were isolated from fetal tissue derived from terminated pregnancies. Another possible way to produce pluripotent stem cells is to fuse the nucleus of a somatic cell with an egg whose nucleus has been removed, forming a blastocyst from which inner-mast cells may be used.
There are many potential applications of pluripotent stem cells. Basic research on these cells will yield an understanding of how cell specialization occurs during normal human development. This will in turn help to explain what happens when the specialization process goes awry, for example in cancer and in birth defects. The knowledge may also be used in the development of therapies for conditions in which tissue degenerates or is destroyed, including Parkinson's and Alzheimer's diseases, spinal cord injury, burns, stroke, diabetes, and arthritis. Eventually, pluripotent cells may be manipulated to produce a particular type of tissue needed for therapy. For example, it may become possible to produce pancreatic islet cells to treat diabetes. Before that can occur, however, the changes that must occur for a pluripotent cell to become a pancreatic islet cell must be understood.
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