Enzymic Activity Associated with Postpartum Involution of the Uterus and with Its Regression After Hormone Withdrawal in the Rat

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Abstract
The activity of a lysosomal enzyme, acid phosphatase, was studied by a visualization technique during postpartum involution of the rat’s uterus and in its regression following hormone withdrawal. Uteri from virgin and pregnant animals served as controls. Little enzymic activity was detectable in uteri from virgin and pregnant rats. Immediately postpartum, marked enzymic activity developed around placental detachment sites, in the epithelial cells, and in macrophages and stromal cells in all uterine layers. Smooth muscle fibers acquired moderate acid phosphatase activity. The macrophages appeared to be derived at least in part from blood monocytes. Enzymic activity remained high during the period of rapid shrinkage and reorganization of the uterine wall, and subsequently declined. Changes also were demonstrated in the amounts of free lipid and stainable collagen. Uteri from ovariectomized rats exhibited qualitatively similar enzymic responses, as did those of estrogen-plus-progesterone-treated rats following hormone withdrawal, although the degree of activity developed was lower. These results support the hypothesis that hormone withdrawal may be one factor governing the histochemical changes characteristic of postpartum uterine involution.