Cell Renewal

Somatic cells in the adult organism may be classified according to their mitotic activity

The level of mitotic activity in a cell can be assessed by the number of mitotic metaphases visible in a single high-magnification light microscopic field or by autoradiographic studies of the incorporation of tritiated thymidine into the newly synthesized DNA prior to mitosis. Using these methods, cell populations may be classified either static, stable, or renewing:

• Static cell populations consist of cells that no longer divide (postmitotic cells), such as cells of the central nervous system, or cells that divide only rarely, such as skeletal and cardiac muscle cells.

• Stable cell populations consist of cells that divide episodically and slowly to maintain normal tissue or organ structure. These cells may be stimulated by injury to become more mitotically active. Periosteal and perichondral cells, smooth muscle cells and endothelial cells of blood vessels, and fibroblasts of the connective tissue may be included in this category.

• Renewing cell populations may be slowly or rapidly renewing but display regular mitotic activity. Division of such cells usually results in two daughter cells that differentiate both morphologically and functionally or two cells that remain as stem cells. Daughter cells may divide one or more times before their mature state is reached. The differentiated cell may ultimately be lost from the body.

Slowly renewing populations include smooth muscle cells of most hollow organs, fibroblasts of the uterine wall, and epithelial cells of the lens of the eye. Slowly renewing populations may actually slowly increase in size during life, as do the smooth muscle cells of the gastrointestinal tract and the epithelial cells of the lens.

Rapidly renewing popidations include blood cells, epithelial cells and dermal fibroblasts of the skin, and the epithelial cells and subepithelial fibroblasts of the mucosal lining of the alimentary tract.

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