Tuesday 3 June 2003
Cells capable of division may respond to stress by undergoing both hyperplasia and hypertrophy, whereas in nondividing cells (e.g., myocardial fibers), hypertrophy occurs.
Hypertrophy can be physiologic or pathologic and is caused by increased functional demand or by specific hormonal stimulation.
- The striated muscle cells in both the heart and the skeletal muscles are capable of tremendous hypertrophy, perhaps because they cannot adequately adapt to increased metabolic demands by mitotic division and production of more cells to share the work.
- The most common stimulus for hypertrophy of muscle is increased workload. For example, the bulging muscles of bodybuilders engaged in "pumping iron" result from an increase in size of the individual muscle fibers in response to increased demand.
- The workload is thus shared by a greater mass of cellular components, and each muscle fiber is spared excess work and so escapes injury.
- The enlarged muscle cell achieves a new equilibrium, permitting it to function at a higher level of activity.
- In the heart, the stimulus for hypertrophy is usually chronic hemodynamic overload, resulting from either hypertension or faulty valves. Synthesis of more proteins and filaments occurs, achieving a balance between the demand and the cell’s functional capacity.
- The greater number of myofilaments per cell permits an increased workload with a level of metabolic activity per unit volume of cell not different from that borne by the normal cell.
endometrial mucosa in pregnancy
- The massive physiologic growth of the uterus during pregnancy is a good example of hormone-induced increase in the size of an organ that results from both hypertrophy and hyperplasia.
- The cellular hypertrophy is stimulated by estrogenic hormones acting on smooth muscle estrogen receptors, eventually resulting in increased synthesis of smooth muscle proteins and an increase in cell size.
- Similarly, prolactin and estrogen cause hypertrophy of the breasts during lactation. These are examples of physiologic hypertrophy induced by hormonal stimulation.
Although the traditional view of cardiac and skeletal muscle is that these tissues are incapable of proliferation and, therefore, their enlargement is entirely a result of hypertrophy, recent data suggest that even these cell types are capable of limited proliferation as well as repopulation from precursors. This view emphasizes the concept, mentioned earlier, that hyperplasia and hypertrophy often occur concomitantly during the responses of tissues and organs to increased stress and cell loss.
myocardial hypertrophy (cardiac hypertrophy)