PROGRESSION OF TROPHOBLAST INTO THE ENDOMETRIUM DURING IMPLANTATION IN THE WESTERN SPOTTED SKUNK

Background: In mustelid carnivores, several blastocysts become implant ed either similar to 12 days following fertilization or after a delay of implantation. In the western spotted skunk, implantation occurs fol lowing a long period of delayed implantation and a brief activation st age. Within each implantation chamber, a large number of trophoblastic plaques form, and the syncytial trophoblast of these plaques adheres to and penetrates into the uterine luminal epithelium. The presence of multiple attachment sites was used to analyse the way in which tropho blast adheres to, penetrates, and removes uterine epithelium and its s ubsequent association with the subepithelial vascular plexus. Methods: Implantation chambers from 18 western spotted skunks were collected d uring the first week postimplantation and the tissue prepared for ligh t microscopy and scanning and transmission electron microscopy. Result s: A series of trophoblastic plaques, which form a ring peripheral to the embryonic shield, attach to and penetrate the uterine epithelium. As new trophoblastic plaques are forming, the initial plaques enlarge and spread along the basal lamina of the luminal epithelium, and troph oblastic processes project through this basal lamina. Subsequently the re is a stage of consolidation in which cytotrophoblast increases grea tly in amount, attachment sites coalesce, and the luminal epithelium i s eliminated. Syncytial trophoblast intrudes into endometrial gland lu mina and surrounds subepithelial capillaries. Conclusions: It is sugge sted that the affinity of syncytial trophoblast for apical junctional complexes of uterine epithelial cells facilitates intrusion of syncyti al trophoblast between cells, possibly guided by the marginal ridges o f the uterine cells. The trophoblast shows no tendency to adhere to or invade maternal capillaries. This lack of adhesion to endothelial cel ls suggests either a change in adhesive properties of trophoblast foll owing epithelial penetration or differences in adhesive properties bet ween surface epithelial cells and endothelial cells, Although trophobl ast differentiation appears to be chronologically regulated, it could be responding to maternally derived factors. (C) 1996 Wiley-Liss, Inc.