The existence of a control mechanism for cell reproduction is evident in the fact that some cells reproduce continually, some occasionally, and some not at all. Skin cells, blood-forming cells, and the cells that line the intestine, for example, reproduce continually throughout life. Those that compose some organs, such as the liver, seem to reproduce until a particular number of cells are present, and then they cease reproducing. Interestingly, if the number of liver cells is reduced by injury or surgery, the remaining cells are somehow stimulated to reproduce again. Still other cells, such as nerve cells, apparently lose their ability to reproduce as they become differentiated; therefore, damage to nerve cells is likely to result in permanent loss of nerve function. How the reproductive capacities of cells are controlled is not well understood, but the mechanism may involve the release of growth-inhibiting substances. Such a substance might slow or stop the growth and reproduction of particular cells when their numbers reach a certain level.
Surface - Volume Relationships
Another factor involved with the control of cell reproduction is the relationship between a cell's membrane surface area and its volume. The quantity of nutrients needed to maintain a cell is directly related to the volume of its living material. The quantity of nutrients that can enter the cell is related directly to the surface area of its membrane. As a cell grows, however, its membrane surface area increases proportionately less than its volume.
Consequently, the surface area will eventually become inadequate for the needs of the living
material inside. A cell can solve this growth problem by dividing. The resulting daughter cells arc smaller than the parent cell and thus have a more favorable surface volume relationship.
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| Control Of Cell Reproduction |
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