life11e_ch11

Growth factors can stimulate cells to divide

Cyclin–Cdk’s provide cells with internal controls of their progress through the cell cycle. But the cell cycle is also influenced by external signals. Not all cells in an organism go through the cell cycle on a regular basis. Some cells either no longer go through the cell cycle and enter G0, or go through it slowly and divide infrequently. If such cells are to divide, they must be stimulated by external chemical signals called growth factors. Here are a few examples:

If you cut yourself and bleed, specialized cell fragments called platelets gather at the wound to initiate blood clotting. The platelets produce and release a protein called platelet-derived growth factor which diffuses to the adjacent cells in the skin and stimulates them to divide and heal the wound.
Red and white blood cells have limited lifetimes and must be replaced through the division of immature, unspecialized blood cell precursors in the bone marrow. Two types of growth factors, interleukins and erythropoietin, stimulate the division and specialization, respectively, of precursor cells of white blood cells and red blood cells.

Growth factors bind to specific receptors on target cells and activate signal transduction pathways (see Chapter 7) that end with cyclin synthesis, thereby activating Cdk’s and the cell cycle.