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Hyperthrophy is an increase in the size of a cell accompanied by an increased functional capacity. Unlike hydropic swelling, the hypertrophied cell does not contain excess water or electrolytes. Hypertrophy is a response to trophic signals or increased functional demands and commonly a normal process. Physiologic hypertrophy occurs during maturation under the influence of a variety of hormones. Sex hormones at puberty lead to the hypertrophy of the juvenile sex organs and organs associated with secondary sex characteristics. The lactating women, under the influence of prolactin and estrogen, exhibits hypertrophy of breast tissue. Although hypertrophy results from certain normal hormonal signals, it is also a response to abnormal levels of hormones. Exogenous anabolic steroids has the capacity to induce muscle hypertrophy. Endogenous overproduction of TSH by the pituitary is responsible for the thyroid enlargement (goiter) seen with nutritional iodine deficiency. In the absence of sufficient iodine, thyroid hormone is not produced. Consequently there is no feedback inhibition of TSH secretion, and the unopposed TSH, acting as a trophic hormone, induces hypertrophy of the thyroid follicular cells. Increased hormone levels can also result from abnormal hormone production by tumours. For example, secretion of ACTH by primary tumours results in hypertrophy of the adrenal cortex Hypertrophy caused by an increased functional demand is exemplified by increased muscle size and strength following repeated exercise. In an analogous fashion, one places an exogenous metabolic demand on the liver cell by administering drugs that must be detoxified by the mixed-function oxidase system. Cytochrome P450 and other enzymes of this drug-metabolizing system reside in the smooth endoplasmic reticulum. The liver cell responds to the metabolic demand of detoxification by increasing the amount of smooth endoplasmic reticulum, with consequent hypertrophy of the cell. Hypolipidemic drugs cause proliferation of peroxisomes with accompanying liver cell hypertrophy. Increased demand occurs under pathologic conditions as well. - The heart may increase its contractile force because of mechanical interference with the aortic outflow, or because of systemic hypertension, a condition requiring the heart to eject blood under higher pressure. As in exercise-induced hypertrophy of skeletal muscle, the myocardial cells enlarge, and the heart may double in weight. - Increased demand also results from the loss of functional mass. If one kidney is surgically removed or rendered inoperative because of vascular occlusion, the contralateral kidney hypertrophies to accommodate the increased demand. It should be emphasized that although the stimulus for hypertrophy may assume various forms, the process must involve complex signals that eventuate in gene expression. Moreover, increased demand may itself lead to hormonal changes, which in turn may interact with other signals.
Summary: Increase in size of an organ due to increase in size of its cells. Example : - Heart in hypertension (Work hypertrophy) ; - Senile enlargement of prostate (Hormonal hypertrophy). - Uterus in pregnancy (Physiological hypertrophy). - Skeletal muscle in physical exercise.
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