New Cells and Old Vessels: The Remodeling of the Endometrial Vasculature during Establishment of Endometrial Cups1

Endometrial cups are very poorly vascularized, in striking contrast to most endocrine glands. The nature and establishment of the vascular bed supplying the cups were examined, particularly the role of blood vessels and lymphatic vessels in relation to the maintenance and function of this unique structure. Endometrial cups were examined by light and electron microscopy between Day 36 and Day 83 of pregnancy. At the time of invasion of trophoblastic girdle cells into the endometrium, an extensive vascular bed and subepithelial capillary plexus are present as well as dilated superficial lymphatic vessels. Enlarging cup cells enter and apparently fill these lymphatic vessels and then the deeper lymphatics. Cup cells soon occupy most of the endometrial stroma. The vasculature of the mature cup is sparse, as there is no evidence of angiogenesis or hypervascularity accompanying the ingression of trophoblast cells. Moreover, the extracellular matrix surrounding the vessels thickens and sometimes contains leukocytes. The elimination of lymphatic vessels and restricted permeability of blood vessels contribute to isolation of cup cells; this may initially facilitate immunological isolation but also may eventually lead to necrosis and consequently elicit rejection of the cups.