Although it was surmised that the IPY is an extension of the inverted yolk sac endoderm and its development commences before 14.5 dpc, little is known about the precise course of its development. We next examined the developmental course of this structure by tracing the changes in the expression pattern of PDGFRa in the placental disc. buy flovent inhaler
Sectioned and sliced specimens of 8.5-18.5-dpc placentas were subjected to immunostaining with APA5. Although PDGFRa is known to be expressed in both parietal and visceral endoderm at 5.5-6.5 dpc and our unpublished observation), it could not be detected in the visceral endoderm after 8.5 dpc. From 8.5 to 10.5 dpc, stronger signals were detected at the border of visceral and parietal endoderm (Fig. 2, A-F, Fig. 3, A-С). At 10.5 dpc, these APA5-positive cells made pits on the fetal surface of the chorioallantoic disc (Fig. 3C), and after 11.5 dpc started to invaginate and expand into the chorioallantoic disc while maintaining their epithelial structure (Fig. 2, G-L, Fig. 3D). Then finally, at 14.5 dpc, they formed structures identical to what we recognized as IPY in 14.5-15.5-dpc placentas. Their invagination seemed to occur along the border of the labyrinthine layer and chorionic mesenchyme, and differences between the squamous and columnar cells, which faced the labyrinthine layer (maternal blood sinuses) and chorion (fetal blood vessels), respectively, were distinguishable from 11.5 dpc (Fig. 3D). Although the general structure of the placenta, namely the layer structure and vascular connections, is supposed to be complete at 14.5 dpc and not to change markedly after that, the IPY continued to expand up to 18.5 dpc (Fig. 2, K-P).
FIG. 3. Development of the IPY was traced by APA5 immunostaining of sectioned specimens. DIC images of 8.5- (A), 9.5- (B), 10.5- (C), and 11.5-dpc (D) placentas are shown. Morphological differences between the columnar and squamous epithelium were recognizable after 11.5 dpc. Scale bar, 25 |j,m. vy, visceral yolk sac; py, parietal yolk sac.