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5.10Stem Cells

Zygotes and embryonic cells during early development of animals have the potential to develop into various kinds of cells that form the body. For example, it is possible to make a complete individual from just one cell of a two-cell-stage embryo. A good example of this potential is the method by which the cells of the two-cell-stage embryo naturally separate and form identical twins. However, except in germ cells, the capability to develop into various kinds of cells is gradually lost as development progresses (Fig. 5-7). Furthermore, once the individual has become an adult with a completely developed body, most of its cells can only develop into certain predetermined cells. For example, the epidermal cells of the skin cannot develop into other types of cells (such as muscle or nerve cells) unless they are artificially manipulated.

Fig. 5-7. The Regeneration Process and Differential Ability

During early development, embryonic cells have the ability to develop into various kinds of cells. However, this ability gradually becomes limited as development progresses. Ultimately, they can each develop into only one kind of cell.

However, there are cells in many tissues of adults that have the potential to become many kinds of cells. These cells are present in the bone marrow, which continuously produces various kinds of blood cells (e.g., red and white blood cells) in skin, which is exposed to the external environment and easily damaged by it, and in the mucosal tissue of the gastrointestinal tract. Such cells are required for regeneration and repair of tissues as they can turn into the various types of cells that form those tissues.
Cells that have the potential to develop into various types of cells in this manner are generally called "stem cells." Furthermore, stem cells derived from embryos are called "embryonic stem cells" (ES cells), and ES cells present in adult tissue other than reproductive tissue are called "somatic stem cells." Embryonic cells can develop into almost any type of tissue or organ cells, but somatic stem cells can only develop into a limited number of cell types.

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