Physiology: Immunohistochemical characterization of human endometrial microvascular basement membrane components during the normal menstrual cycle

The expression of three basement membrane components [collagen IV (CIV), laminin and heparan sulphate proteoglycan (HSPG)] and platelet endothelial cell adhesion molecule (PECAM) were examined by immunohistochemistry in cryostat sections of normal human endometrium. Alkaline phosphatase (ALP) was detected using enzyme histochemistry. Endometrial biopsies from the menstrual (n = 4), mid-late proliferative (n = 5), early-mid secretory (n = 5) and late secretory (n = 5) stages were collected from women with a normal menstrual cycle. At all four stages of the menstrual cycle, CIV, laminin and HSPG were expressed on basement membranes of both vessels and glands whilst PECAM expression was localized specifically to endothelial cells. A similar number of vessels/mm2 stained for CIV and laminin, as well as for PECAM at each stage of the menstrual cycle, demonstrating that all vessels in endometrium stain for these two basement membrane components. By contrast, the number of vessels/mm2 that stained positively for HSPG and ALP was significantly lower, averaging approximately 55% of the total that stained positively for PECAM, CIV and laminin. During the menstrual stage, HSPG staining intensity remained strong in glandular basement membranes but decreased dramatically in vascular basement membranes. ALP activity was variable in both the vessels and glands throughout the four stages of the menstrual cycle studied. This study demonstrates heterogeneity in basement membrane components within the endometrial microvasculature. It is postulated that the disappearance of HSPG from vascular basement membranes may play a role in the process of vascular remodelling during the menstrual stage of the cycle.