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Involution of an organ is the shrinking or return to a former size.

 

Thymus

The thymus continues to grow between birth and puberty and then begins to atrophy, a process directed by the high levels of circulating sex hormones. Proportional to thymic size, thymic activity (T cell output) is most active before puberty. Upon atrophy, the size and activity are dramatically reduced, and the organ is primarily replaced with fat. The atrophy is due to the increased circulating level of sex hormones, and chemical or physical castration of an adult results in the thymus increasing in size and activity.

 

Uterus

It is a physiological process occurring after e.g. parturition; the hypertrophy of the uterus has to be undone since it does not need to feed the fetus anymore.

 

Mammary Gland

During pregnancy until after birth, mammary glands grow steadily to a size required for optimal milk production. At the end of breastfeeding, the number of cells in the mammary gland becomes reduced until approximately the same number is reached as before the start of pregnancy.