Changes in extracellular matrix materials in the uterine myometrium of rats during pregnancy and postparturition

Morphological changes in extracellular matrix materials in the uterine myometrium of rats during pregnancy and postparturition were studied by light and electron microscopy together with immunofluorescence microscopy for type III and IV collagens, fibronectin and laminin. The main components present in late pregnancy were 1) various-sized collagen fibrils, 2) thick elastic fibers adjacent to smooth muscle cells, and 3) continuous and thick basement membranes of hypertrophic smooth muscle cells. These findings are considered to indicate degradation of collagen fibrils and development of elastic fibers and basement membranes of smooth muscle cells. This change in extracellular matrix materials in the late stage of pregnancy may be important in the process of uterine enlargement associated with elasticity and preparation for labor. In the postpartum stage, myofibroblastic interstitial cells were seen to phagocytize collagen fibrils, and elastic fibers accumulated mainly around the bundles of smooth muscle cells. These changes in the postpartum stage are thought to be important for the process in which the uterus returns to the nonpregnant condition. It is suggested that smooth muscle cells participate in regulating the development of their basement membranes and elastic fibers, and that myofibroblastic interstitial cells function by clearing degraded collagen fibrils from the uterine myometrium.