Placental permeability during early gestation in the hamster. Electron microscopic observations using horseradish...

The permeabilities of the parietal yolk sac placenta and the preplacental region of the hamster conceptus during early postimplantation (day 8) were compared by means of electron microscopy and a macromolecular protein tracer, horseradish peroxidase (HRP). HRP was administered by injection into the maternal venous system; samples of the two placental tissues were obtained for examination at intervals between 4 minutes and 1 hour later. The three layers of the parietal yolk sac wall (from outer to inner: capsular trophoblast, Reichert's membrance, parietal endoderm) appeared to provide little impediment to the passage of HRP from perivitelline maternal blood spaces to the yolk sac cavity. HRP passed through the outer trophoblast layer, both by way of intracellular fenestrae (60-200 nm diameter) and narrower intercellular channels, and completely permeated the meshwork of Reichert's membrane within minutes after injection. The inner parietal endoderm cell layer was widely discontinuous and clearly presented no barrier to HRP movement. HRP reaching the yolk sac cavity was avidly endocytosed by the viceral yolk sac epithelium. In contrast to the parietal yolk sac, the preplacental region of the conceptus was impermeable to HRP. Zonular occluding junctions located between contiguous cells of the chorionic ectoderm layer of the preplacenta were the obvious barrier to the HRP molecules. These results suggest that in this rodent species, during the early postimplantation period of gestation, the pareital yolk sac placenta potentially plays a more important role in the maternal-embryonic transfer of macromolecular substances than does the preplacenta.