ABSTRACT
Complex organisms such as mammals are composed of two fundamentally different types of somatic cells: postmitotic cells and mitotic or mitotically competent cells. Postmitotic cells are those that have irreversibly lost the ability to proliferate (‘proliferation’ is used here interchangeably with ‘growth’). Mitotic cells, in contrast, are those that can divide when there is a physiological need. Postmitotic cells include mature neurons, skeletal and cardiac muscle cells, and differentiated adipocytes. Mitotic cells include the epithelial and stromal cells that comprise organs such as the skin, liver, breast, prostate, etc., the lymphocytes and precursor cells of the hematopoietic system, and support cells such as the glial cells of the
nervous system. One of the most striking differences between tissues composed mostly of postmitotic cells and those composed mostly of mitotic cells is the type of pathology to which each becomes increasingly susceptible with advancing age. Because postmitotic cells cannot be replaced by the proliferation of neighboring postmitotic cells, postmitotic tissues are particularly vulnerable to degeneration. Mitotic tissues, on the other hand, are particularly vulnerable to hyperproliferation, which can lead to cancer (Figure 10.1).
