Structural identification and characterization of arteries and veins in the placental stem villi

The vascular wall structure in the human fullterm placental villi of normal pregnancy was studied by means of light and electron microscopy with an im proved technique of perfusion fixation and tissue preparation. We observed 81 sections of stem villi that showed cross-sectional profiles of paired ve ssels in their center. Both vascular walls contained a large amount of extr acellular matrix and no elastic lamina between smooth muscle cells of the m edia, making identification of the artery and the vein quite difficult at f irst sight. We then noted that the density of the smooth muscle cell popula tion was always considerably higher in one than the other, and identified t he former as artery and the latter as vein on the basis of their connection with larger arteries and veins running on the chorionic plate. Between the paired vessels, the artery had a smaller caliber than the vein, and the ra tio of venous to arterial caliber was distributed from 1.0 to 2.5. The thic kness of media was usually thicker in the vein than in the artery. Clusters of elastic fibers were found occasionally in the media of arteries and vei ns, and basement membrane-like materials were associated frequently with th e elastic fibers and were distributed widely in the media as well as in the adventitia. In the veins, the smooth muscle cells of the most superficial part of the media contained well-developed rough endoplasmic reticulum and Golgi apparatus, indicating differentiation to secrete extracellular matric es. The present study revealed the difference of wall structure between art eries and veins in the placental stem villi for the first time at the ultra structural level, and suggested differentiation of venous smooth muscle cel ls, possibly by some influence from the luminal side.