Ultrastructure of the placenta of antarctic seals during the first third of pregnancy

The chorioallantoic placenta from six crabeater seals (Lobodon carcinophagus) with fetuses from 15–180 mm crown rump length (CRL), and two leopard seals (Hydrurga leptonyx) with fetuses 76 and 86 mm CRL was lobulated, zonary, labyrinthine and endotheliochorial. The fine structure of the placenta was essentially similar in both species. Maternal capillaries of the labyrinth had attenuated endothelial cells unlike the fissiped carnivores. They were surrounded by an outer syncytiotrophoblast, incomplete inner cytotrophoblast, and a core of mesenchymal tissue containing fetal vessels. In general, the cytotrophoblastic cells were less electron‐opaque than the syncytium; the former possessed abundant free ribosomes, while the latter had many nuclei, mitochondria, and numerous strands of rough endoplasmic reticulum. Mitotic figures were observed only in the cytotrophoblast. Plasma membranes between the trophoblastic layers had interdigitating processes which showed many desmosomes along the intercellular spaces, whereas membranes surrounding the maternal capillaries were usually smooth. In the limb‐bud stage embryo, the syncytiotrophoblast had thick and thin areas which became progressively attenuated in older fetuses. Instances of intratrophoblastic fetal capillaries were observed in seals with fetuses of 40 mm CRL and older, with consequent greatly thinned placental barriers. By Perl's reactions, Turnbull's method, and electron microscopy, rows of granular bodies positive for iron were demonstrated along the basement membrane of the trophoblastic cells and in the mesenchymal cells of the crabeater seals, but not the leopard seals. Furthermore, it is suggested that the deposits of granular bodies show the sites of iron storage in the placentas. In the limb‐bud stage embryo the hematome borders and isolated pouches of the central hematome were observed. The central hematome was antimesometrial. The hematomes were lined by columnar cytotrophoblastic cells which phagocytized extravasated maternal blood and absorbed histiotrophic material.