Cell divisions during embryonic development give rise to new differentiated cell types or increase the total number of cells in the embryo. In contrast, the major role of cell division in adult life is to maintain the number of differentiated cells at a constant level: to replace cells that have died or been lost through injury.
The rate at which new cells are produced in the adult is a measure of how rapidly the cell population is turning over and, on this basis, tissues can be divided into three broad categories (Leblond, 1963). In tissues with static cell populations, such as nerve and skeletal muscle, there is no cell division and most of the cells formed during development persist throughout adult life. In tissues containing conditional renewal populations, such as liver, there is generally little cell division, but in response to an appropriate stimulus most cells can divide...
