The amniotic duct as the key structure in the determination of the direction of growth of the human placenta and its orientation in the uterus

Abstract

G ENERAL inspection of the human placenta has led to the opinion of its radial growth from a fixed center (Strassmann). This is based primarily upon the central insertion of the umbilical cord. The exceptions to this rule are, however, so numerous that they need no comment, being represented by all. gradations from marginal to velamentous insertion. The point of reference forming the basis for most of the discussion is the body stock since this structure eventually materializes as the umbilical cord. An examination of the vascular bed of the placenta shows, however, that it is bilaterally symmetrical with the right and left umbilical arteries distributed according to a definite basic pattern consisting of an anterior, lateral, and posterior branch on either side. Furthermore, right and left may be recognized by the fixed relationship of the umbilical vein which lies anterior to the cross anastomosis between the two arteries (Fig. 1). Tha,t this condition is basic is indicated by the fact that it can be identified in the vesicle of the 3 mm. embryo, i.e.> at the time the definitive vascular bed is first established. . This basic plan is more fundamental than that of Shordania, who, recogsized a master, dispersed a special type of placenta based on the vascularity. Bacsich and Smout identify five types, all of which, however, may be seen to he bilateral. In view of the fact that the body stock becomes invaded by an allantois, the human placenta has been generally interpreted as a precocious allantoic placenta in which the chorion has taken over the allantoic vessels. It is further assumed that the human allantois spreads out in the chorionic plate in a radial direction and simulates the condition noted in Tarius. For a complete phylogenetic development of the placenta reference may he made to the monograph of Mossman. The radial spread of t,he functioning allantois from t,he outer end of the umbilical cord in the lower forms naturally leads to the concept of a. radial growth of the placenta about t,he umbilical cord insertion. In t,he human being, however, the allantois remains a blind, atrophic stock extending forward only to the chorial plate at the time of vasculat,ure is differentiated and established (Fig. 4). The steps in this process may be followed in the Mateer specimen and the Innalls embryo.