The extracellular matrices and vascularization of the developing corpus luteum in rats

The extracellular matrices and vascularization of the developing corpus luteum of adult female rats were studied with immunohistochemistry for laminin, type I, III and IV collagen and with electron microscopy. Ovaries were removed from Wistar-Imamichi rats that had a regular 4-day estrous cycle, 6 and 2 h before ovulation, immediately after ovulation, 6, 16 and 24 h after ovulation and at maturity of the corpus luteum and examined. The basement membrane of the follicles and capillaries of the theca interna started to break down around the entire circumference of the Graafian follicle 2 h before ovulation. More than 6 h after ovulation, dot-pattern positive reactions for type IV collagen and laminin were detected in the granulosa layer, and thin basement membrane structures appeared in association with the luteinizing granulosa cells. More than 16 h after ovulation, vascularization with the formation of capillary basement membrane was observed in the granulosa layer, and luteal mesenchymal cells with adjacent interstitial collagen components always appeared in close association with the newly formed capillaries. Our chronological observations revealed that both luteinizing cells and luteal mesenchymal cells are involved in the destruction and reconstruction of the follicular ECM, and that the luteal mesenchymal cells play an essential role in the neovascularization of the developing corpus luteum.