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Pathology, in the broadest terms, is the study of disease. Disease occurs for many reasons. Some diseases represent spontaneous alterations in the ability of a cell to proliferate and function normally, and in other cases, disease results when external stimuli produce changes in the cell's environment that make it impossible for the cell to maintain homeostasis. In such situations, cells must adapt to the new environment. These adaptations include hyperplasia, hypertrophy, atrophy, and metaplasia, and can be physiologic or pathologic, depending upon whether the stimulus is normal or abnormal. A cell can adapt to a certain point, but if the stimulus continues beyond that point, failure of the cell, and hence the organ, can result. If cells cannot adapt to the pathologic stimulus, they can die. This chapter will discuss cellular adaptation, cell injury, cellular accumulations, and cellular aging.

Overview: The four basic types of cellular adaptation to be discussed in this section are hyperplasia, hypertrophy, atrophy, and metaplasia.


Basic description: Increase in the number of cells.

Types of hyperplasia

  • Physiologic hyperplasia: Occurs due to a normal stressor. For example, increase in the size of the breasts during pregnancy, increase in thickness of endometrium during menstrual cycle, and liver growth after partial resection.
  • Pathologic hyperplasia: Occurs due to an abnormal stressor. For example, growth of adrenal glands due to production of adrenocorticotropic hormone (ACTH) by a pituitary adenoma, and proliferation of endometrium due to prolonged estrogen stimulus.

Important point regarding hyperplasia: Only cells that can divide will undergo hyperplasia; therefore, hyperplasia of the myocytes in the heart and neurons in the brain does not occur.


Basic description: Increase in the size of the cell.

Types of hypertrophy

  • Physiologic hypertrophy: Occurs due to a normal stressor. For example, enlargement of skeletal muscle with exercise.
  • Pathologic hypertrophy: Occurs due to an abnormal stressor. For example, increase in the size of the heart due to aortic stenosis. Aortic stenosis is due to a change in the aortic valve, which obstructs the orifice, resulting in the left ventricle working harder to pump blood into the aorta.

Morphology of hyperplasia and hypertrophy: Both hyperplasia and hypertrophy result in an increase in organ size; therefore, both cannot always be distinguished grossly, and microscopic examination is required to distinguish the two (Figure 1-1).

Figure 1-1.

Cross-section of the heart of a patient with systemic hypertension. The patient had high blood pressure, which increased the workload of the left ventricle and resulted in concentric hypertrophy of the left ventricular myocardium. In response to the increasing pressure load, the cardiac myocytes increased their content of contractile proteins, resulting in enlargement of individual myocytes.

mechanisms by which hyperplasia and hypertrophy ...

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