Observations on the vasculature of the baboon placenta (Papio SP.) with special reference to the transverse communicating artery

Angiogenesis and early vascular morphology were studied in 37 baboon embryos from 15 to 50 days gestation. Vasoformative lines were first recognized in the body stalk and chorionic mesenchyme at 15 days and open vessels are common by 18 days. Umbilical (allantoic) arteries were recognized in the body stalk by 21 days and a narrow connecting vessel may be frequently identified. This early transverse communicating artery was identified in 85% of the embryos studied. One-hundred twenty-five near-term placentae delivered by Cesarean section were investigated either by dissection or by injection-erosion for (1) vessel pattern, (2) vessel distribution, and (3) transverse communicating artery. The predominant branching pattern of the umbilical vessels was the magistral type (66.2%) while the remainder were disperse (Bacsich and Smout [1938]). Equal areas of the placenta were vascularized by branches of each umbilical artery in 73.8% of the cases. The remainder were distinctly asymmetrical in this respect. A transverse communicating artery was found in 70% of the near-term placentae and classified according to morphological characteristics. In many instances, the connection formed a direct anastomosis varying from a few millimeters to 5 cm in length rather than a distinct vessel between the umbilical arteries. The elongation of such an anastomosis from the early embryonic stages is believed to be one possible etiology of the single umbilical artery. One specimen in this group displayed a single umbilical artery.