Stem cells can also be classified according to their plasticity, or how versatile they may be in their development. The classifications may describe the cells’ Commitment to become particular kinds of cell: the
totipotent stem cell, the pluripotent stern cell, or the umpotent cell. Tolipotent stem cells are the most versatile type. When sperm carrying the 23 chromosomes, chock-full of genetic material, fertilizes the female
egg with its 23 chromosomes, the union occurs in the fallopian tube. The fertilized egg is called a zygote. This fertilized egg is said to be totipoteni— from the Latin torus, meaning ‘entire.” It has the potential to generate all the cells and tissues that will make up the embryo and support its development in the uterus. In mammals only the zygote and the cells resulting from the first few divisions have this all-encompassing ability to generate cells, the umbilical cord, and other structures. At this stage the cell is the ultimate mother stem cell, possessing potent sternness—the cell’s potential to generate multiple mature cell types.
Pluripotent cells are like the totipotent cells in that they can give rise to all tissue types. Unlike Lotipotent cells, pluripotent cells cannot develop into an entire organism. On about the fourth day of development, the
embryo forms into two layers that will become the tissues of the developing body. The outer layer will become the placenta, which in inner cells can form nearly any human tissue. However, because the cells cannot survive without the outer layer, they are now plunpotent. not totipotent.
However. Smith’s definition of stem cells is not all-encompassing and applies only to embryonic cells. Certain adult progenitor cells and adult stem cells have reduced capacity for self-renewal. Unipotent sem cells are those cells in adult organisms tha are capable of ditTerentiating along only one line. The root word, unus. derives from the Latin term meaningone.’” A limited number of undifferentiated stem cells are also found in the specialized tissue of the fully formed human body and are thought to function in the process of repairing. The definition of stem cells in general therefore must take into account the ability of cells to replicate.
The term adult stem cell is misleading because the cells can be found in humans of any age. Usually these adult stem cells are farther along in the process of differentiation than those found in embryos, meaning they
can give rise to only a few types of cells in the body. not to all 200 types. as embryonic cclls are capable of doing. Adult stem cells arc most frequently obtained from human bone marrow, circulating blood, and
umbilical cord blood.
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