Morphometric and histochemical study of involution in the rat uterus after parturition

Morphological changes to and collagen loss from the rat uterus during postpartum involution were investigated. The expression patterns of collagen type III and myeloperoxidase (MPO) were also determined. Morphological changes were studied on days 1, 3, 5, 10, 15, 20, 22 and 25 postpartum. As a control, diestrus rats’ uterine were used. Specimens from the uterine horn were embedded in paraffin, cut into 8 µm coronal sections, and stained with hematoxylin‐eosin. The thickness of the longitudinal and circular smooth muscle layers and of the endometrium were measured. The collagen content was determined using hydroxyproline analysis. Immunostaining was used to examine the expression of collagen type III on days 1, 3, 5, 10, 15 and 20 postpartum; and MPO on days 1, 3, 5, 10 and 22 postpartum. The thickness of the smooth muscle layers was found to decrease rapidly postpartum: the circular smooth muscle layer returned to that of a non‐pregnant, control uterus by day 5 postpartum and the longitudinal smooth muscle layer by day 15 postpartum. Eosinophilic cells were observed in the endometrial stroma adjacent to the myometrium on days 10, 15 and 20 postpartum, and were confirmed as collagenous cells. Immunostaining identified collagen type III positive cells in the vessel‐rich layer adjacent to the placental site on days 1, 3, 5 and 10 postpartum, and these cells were confirmed to be phagocytic. Postpartum reduction in the weight of the uterus was accompanied by decreases in both the collagen content and the thickness of the longitudinal and circular smooth muscle layers. Furthermore, the phagocytic cells were shown to express MPO during postpartum involution of the uterus.