Cells are complex units that dynamically respond to the chang-ing demands and stresses of daily life. They possess a maintenance function and a specialized function. The maintenance function refers to the activities that the cell must perform with respect to itself; specialized functions are those that the cell performs in re-lation to the tissues and organs of which it is a part. Individual cells may cease to function without posing a threat to the organ-ism. As the number of dead cells increases, however, the special-ized functions of the tissues are altered and the individual’s health is threatened.
Cells can adapt to environmental stress through structural and functional changes. Some of these adaptations are hypertrophy, atrophy, hyperplasia, dysplasia, and metaplasia (Table 6-2).
Hypertrophy and atrophy lead to changes in the size of cellsand hence the size of the organs they form. Compensatory hy-pertrophy is the result of an enlarged muscle mass and commonly occurs in skeletal and cardiac muscle that experiences a pro-longed, increased workload. One example is the bulging muscles of the athlete who engages in body building.
Atrophy can be the consequence of a disease or of decreaseduse, decreased blood supply, loss of nerve supply, or inadequate nutrition. Disuse of a body part is often associated with the aging process. Cell size and organ size decrease; structures principally affected are the skeletal muscles, the secondary sex organs, the heart, and the brain.
Hyperplasia is an increase in the number of new cells in anorgan or tissue. As cells multiply and are subjected to increased stimulation, the tissue mass enlarges. It is a mitotic response (a change occurring with mitosis), but it is reversible when the stimulus is removed. This distinguishes it from neoplasia or ma-lignant growth, which continues after the stimulus is removed. Hyperplasia may be hormonally induced. An example is the in-crease in the size of the thyroid gland caused by thyroid-stimulating hormone (secreted from the pituitary gland) when a deficit in thy-roid hormone is detected.
Dysplasia is the change in the appearance of cells after theyhave been subjected to chronic irritation. Dysplastic cells have a tendency to become malignant; dysplasia is seen commonly in epithelial cells in the bronchi of smokers.
Metaplasia is a cell transformation in which a highly special-ized cell changes to a less specialized cell. This serves a protective function, because the less specialized cell is more resistant to the stress that stimulated the change. For example, the ciliated columnar epithelium lining the bronchi of smokers is replaced by squamous epithelium. The squamous cells can survive; loss of the cilia and protective mucus, however, can have damaging consequences.
These adaptations allow the survival of the organism. They also reflect changes in the normal cell in response to stress. If the stress is unrelenting, the function of the adapted cell may suc-cumb, and cell injury will occur.