Cellular Adaptation

After you have completed this section of the chapter, you should be able to meet the following objectives:

Cite the general purpose of changes in cell structure and function that occur as the result of normal adaptive processes

Describe cell changes that occur with atrophy, hyper­trophy, hyperplasia, metaplasia, and dysplasia and state general conditions under which the changes occur Cite three sources of intracellular accumulations Compare the pathogenesis and effects of dystrophic and metastatic calcifications

Cells adapt to changes in the internal environment, just as the total organism adapts to changes in the exter­nal environment. Cells may adapt by undergoing changes in size, number, and type. These changes, occurring singly or in combination, may lead to atrophy, hypertrophy, hyperplasia, metaplasia, and dysplasia (Fig. 5-1). Adaptive cellular responses also include intracellular accumulations and storage of products in abnormal amounts.¹,²

Numerous molecular mechanisms mediate cellular adaptation, including factors produced by other cells or by the cells themselves. These mechanisms depend largely on signals transmitted by chemical messengers that exert their effects by altering gene function. In general, the genes ex­pressed in all cells fall into two categories: "housekeeping" genes that are necessary for normal function of a cell, and genes that determine the differentiating characteristics of a particular cell type. In many adaptive cellular responses, the expression of the differentiation genes is altered, whereas that of the housekeeping genes remains unaffected. Thus, a cell is able to change size or form without compromising its normal function. Once the stimulus for adaptation is re­moved, the effect on expression of the differentiating genes is removed, and the cell resumes its previous state of special­ized function. Whether adaptive cellular changes are normal or abnormal depends on whether the response was medi­ated by an appropriate stimulus. Normal adaptive responses occur in response to need and an appropriate stimulus. After the need has been removed, the adaptive response ceases.