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The cells of the body continue to grow, divide, and differentiate throughout life. Normally, growth and differentiation are controlled in such a way as to maintain the normal structure of a particular tissue. In tissues characterized by continuous cell loss (skin, intestinal mucosa, blood), labile stem cells continuously undergo mitosis to replace lost cells. Stem cells frequently differentiate from a primitive to a mature form during this process—eg, normoblasts in bone marrow become the erythrocytes of the peripheral blood. In the skin, as superficial keratinized cells are shed from the surface, basal cells proliferate at a suitable rate to replace them. The newly produced basal cells move upward, differentiating into squamous cells that undergo nuclear and cytoplasmic changes to become the superficial cornified layer of cells. When the epidermal turnover rate is normal, the skin appears normal on histologic examination; but if the rate is greatly increased, as occurs in psoriasis, cells do not fully mature, and abnormalities are seen both on gross examination and at the histologic level.
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The growth of a tissue reflects the net balance of cell proliferation on the one hand and cell differentiation, leading to cell death, on the other. The rate of proliferation is determined by the time of passage through the cell cycle and by the interplay of a variety of growth factors with their corresponding receptors, the production of which is regulated by the interaction of growth control genes. Many of the cellular proto-oncogenes that have been identified as playing a potential role in the induction of cancer (Chapter 18: Neoplasia: II. Mechanisms & Causes of Neoplasia) encode for growth factors for receptors.
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The principal categories of growth factors are listed in Table 16–1.
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Abnormal cellular growth may result in either a decrease or an increase in the mass of the involved tissue (see Figure 16-4).
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Atrophy is a decrease in the size of a tissue or organ, resulting from a decrease either in the size of individual cells or in the number of ...