Light and electron microscopic observations on the morphogenesis of the chorioallantoic placenta of the golden hamster (Cricetus auratus). Days seven through nine of gestation.

Abstract

AbstractIn the mature placental labyrinth of common myomorph rodents (rat, mouse and hamster) fetal capillaries are separated from maternal blood spaces by trabeculae composed of three distinct cytoplasmic layers. To resolve the controversy regarding the embryonic origin(s) of these layers a correlated light and electron microscopic study was made of the developing placental tissues of the golden hamster during the eighth and ninth days of gestation, the critical period of labyrinth morphogenesis in this species. It was determined that the syncytial inner layer (layer III) of the trabeculae, the layer bordering the fetal capillaries, is derived from the “true” chorionic ectoderm which fuses with the undersurface of the Träger (ectoplacental cone) early on the eighth day of gestation. Both the syncytial middle layer (layer II) and the cellular outer layer (layerI) of the trabeculae, on the other hand, were found to differentiate from the trophoblastic cells of the Träger. Allantoic mesenchyme, which approximates the fused chorionic‐Träger tissues late on day eight and is the source of the fetal capillaries and mesenchyme of the labyrinth, was noted to exert a critical inductive effect on the morphogenesis of syncytial layers II and III. On the basis of fine structure, the developing labyrinth appears not to be a major site for maternal‐fetal transfer of macromolecules nor to be involved to a significant degree in the production of proteins or steroids for secretion.