A morphological study of the development of the chorion of rat embryos.

The morphology of the developing chorion of the rat has been studied from its initial formation from the extra-embryonic region of the amniotic fold until its fusion with the ectoplacental cone and allantois prior to the development of the chorio-allantoic placenta (that is from 8.25 to 10.25 days of gestation). The gross structural changes, the mitotic indices, the ultrastructure and the distribution of carbohydrate in the mesodermal and ectodermal cells of the chorion have been studied throughout this period of development. The chorion developed to form a diaphragm across the egg cylinder, separating the exocoelom from the ectoplacental cavity. With further development, the ectoplacental cavity became smaller until the chorion was lying against the ectoplacental cone, to which it fused. The mesodermal cells of the chorion formed a single layer of cells covering the ectodermal cells. Shortly before the fusion of the chorion with the ectoplacental cone or the allantois, the integrity of the mesodermal cell layer in the central region of the chorion was disrupted. Intercellular contact was lost and the mesodermal cells rounded up exposing the PAS-positive extracellular coat of the ectodermal cells. The allantois was only seen to fuse with the chorion in regions in which the ectodermal cells were exposed. It was suggested that the glycocalyx of the ectoderm cells may be of importance in the fusion of the allantois and chorion.