Vascular histodifferentiation in the chick chorioallantoic membrane: a morphometric study.

This study presents a quantitative description of ultrastructural changes associated with focal histodifferentiation of the chick chorioallantoic membrane (CAM) microvasculature. Early vessels (4-8 days of a 21-day incubation period) were lined by a thin, but continuous endothelium which was characterized by punctate appositional contacts, an incomplete or absent basal lamina, and few plasmalemmal vesicles. During this stage, there was little ultrastructural distinction between CAM arterioles, capillaries, and venules. During the intermediate stage (9-13 days), the arteriolar endothelium displayed more extensive junctional apposition. In addition, the interstitial (periendothelial) space contained more collagen fibrils and increased cell volume densities, some of which resembled presumptive smooth muscle cells. In contrast to the arterioles, endothelial appositions of the venules and capillaries remained punctate during the intermediate stage. The interstitial space of the venules displayed an increase in cell and fibril densities but to a lesser extent than that observed in the arterioles. The capillaries did not exhibit any significant changes in their periendothelial spaces. By the late stage (14-18 days), a circular tunic containing several layers of presumptive smooth muscle cells surrounded only the arteriolar endothelium. The endothelium of both the arterioles and venules increased in mean thickness. The arterioles also displayed complex interdigitating endothelial appositions. All CAM vessels exhibited a progressive increase in the number of endothelial plasmalemmal vesicles and a more complete basal lamina during the developmental stages. These observations served to distinguish differential patterns of vessel maturation along the length of the microvascular system.