Vascularization of the ovine amnion and chorion: A morphometric characterization of the surface area of the intramembranous pathway

OBJECTIVE: The purpose of this study was to characterize the vascularization of the ovine amnion, allantois, and chorion. STUDY DESIGN: A white silicone vascular casting material was infused into both umbilical arteries of nine fetal sheep ranging in age from 58 to 142 days' gestation. A morphometric analysis of photomicrographs of the membranes was then performed with computerized image analysis techniques. RESULTS: After removal of the uterus, the fetus was surrounded by a layer of white silicone-filled microvessels in the chorion. The amniotic membrane after separation from the chorion was covered by a fine mesh of microvessels, whereas the allantois was avascular. The amniotic membrane readily separated into an outer vascularized layer and an inner, avascular layer containing the amnion. Approximately 50% of the surface of the chorionic membrane was covered by microvessels; this appeared independent of gestational age. At midgestation 30% of the surface of the amnion was covered by microvessels, and this decreased to 17% at 142 days. Relative to fetal weight, the amniotic and chorionic vascular surface areas decreased from 6 to 0.3 and from 15 to 1.5 cm²/gm fetal weight, respectively, over the last half of gestation. CONCLUSIONS: There is an extensive network of microscopic fetal blood vessels within the ovine chorion and covering the outer surface of the amnion. These vessels appear to be ideally situated to facilitate a direct exchange of water and solutes between amniotic or allantoic fluids and fetal blood through the intramembranous pathway.