Angiogenesis in the postovulatory primate endometrium: the coiled arteriolar system

The postovulatory period in the primate endometrium of the menstrual cycle is characterized by rapid growth of the coiled arterioles. A great variety of developing microvascular components occurs among a well-differentiated microvasculature of coiled arterioles, capillaries, and venules. Endometrial biopsies were obtained by hysterotomy during progesterone dominance at 5, 6, 7, 10, 12, and 14 days following the peak of the estrogen surge as determined by serum radioimmunoassay. Arteriolar ultrastructural differentiation is remarkably similar on each of these days. Ultrastructural evidence of elastogenesis in the extracellular matrix adjacent to certain endothelial tubes provides the initial sign of coiled arteriolar formation. The cellular primordia of the tunica intima and media are identified by spatial location and glycogen storage in smooth muscle cells. Endothelial projections span the incipient internal elastic membrane to make contact with the surfaces of the innermost vascular smooth muscle cells. Subsequent arteriolar differentiation centers on formation of a muscular media composed of 1 or 2 muscle layers separated by a spiraling lamellar elastic matrix that appears initially between the endothelial tube and the first muscle layer. Vascular smooth muscle cells are highly branched and linked across the elastic matrix by surface contacts. Definitive coiled arterioles consist of interlinked endothelial and smooth muscle cells within a thick, spiraling elastic matrix that provides flexibility for rapid changes in shape. Progressive differentiation of coiled arterioles continues up to the premenstrual stage. This abundant angiogenesis may reflect preparation and maintenance of a suitable uterine environment for the possibility of implantation and pregnancy during each menstrual cycle.