The intertubular cleft and membranous whorl in the rabbit placenta

The chorio‐allantoic placenta of the rabbit is of the labyrinthine hemodichorial type where the outer trophoblastic layer next to the maternal blood space is syncytial and the inner is cellular. The syncytium has thick and thin areas. It is at the thick areas that the trophoblast from either side of the maternal blood tubule encloses a narrow intertubular cleft which has numerous interdigitating microvilli and desmosomes. Intravenously injected thorotrast (an electron dense particle) and maternal plasma readily enter the intertubular cleft. Along the intertubular cleft numerous caveolae, arrays of rough endoplasmic reticulum and mitochondria are present in the syncytium suggesting that the clefts are areas in which fluid and certain metabolites are absorbed. The agranular membranous whorls are found in the syncytial thick areas. They enclose central or eccentric masses of lipid droplets. Small whorls are found during early pregnancy; smaller and larger ones, however, are found from midpregnancy to term. The whorls progressively increase in size as pregnancy advances. Membranes of the whorl and the rough endoplasmic reticulum are interrelated and contiguous.