HYPERPLASIA

Hyperplasia refers to an increase in the number of cells in an organ or tissue. It occurs in tissues with cells that are ca­pable of mitotic division, such as the epidermis, intestinal epithelium, and glandular tissue. Nerve, skeletal, and car­diac muscle cells do not divide and therefore have no ca­pacity for hyperplastic growth. There is evidence that hyperplasia involves activation of genes controlling cell proliferation and the presence of intracellular messengers that control cell replication and growth. As with other normal adaptive cellular responses, hyperplasia is a con­trolled process that occurs in response to an appropriate stimulus and ceases after the stimulus has been removed. The stimuli that induce hyperplasia may be physio­logic or nonphysiologic. There are two common types of physiologic hyperplasia: hormonal and compensatory. Breast and uterine enlargement during pregnancy are ex­amples of a physiologic hyperplasia that results from es­trogen stimulation. The regeneration of the liver that occurs after partial hepatectomy (i.e., partial removal of the liver) is an example of compensatory hyperplasia. Hyperplasia is also an important response of connective tissue in wound healing, during which proliferating fi-broblasts and blood vessels contribute to wound repair. Although hypertrophy and hyperplasia are two distinct processes, they may occur together and are often triggered by the same mechanism.¹ For example, the pregnant uterus

undergoes both hypertrophy and hyperplasia as the result of estrogen stimulation.

Most forms of nonphysiologic hyperplasia are due to excessive hormonal stimulation or the effects of growth factors on target tissues.² Excessive estrogen production can cause endometrial hyperplasia and abnormal men­strual bleeding (see Chapter 47). Benign prostatic hyper­plasia, which is a common disorder of men older than 50 years of age, is thought to be related to the action of an-drogens (see Chapter 45). Skin warts are an example of hyperplasia caused by growth factors produced by certain viruses, such as the papillomaviruses.