title: Embryonic-maternal cell interactions at implantation in the fat-tailed dunnart, a dasyurid marsupial.
abstract: BACKGROUND
  In marsupials implantation occurs about two-thirds the way through the short gestation before which time the embryo is surrounded by the permeable shell membrane which prevents physical contact between the trophoblast and uterine epithelium. Although the trophoblast has been shown to be invasive to varying degrees in several species of marsupials, the ultrastructure of the embryonic-uterine cell interactions at the time of implantation has not been described in this group.
  
  
  METHODS
  Thick plastic sections and transmission electron microscopy were employed to investigate the cellular interactions at implantation in the fat-tailed dunnart (Sminthopsis crassicaudata), a dasyurid Australian marsupial.
  
  
  RESULTS
  Our results show that epithelial penetration begins when the embryo is at the late presomite/early somite stage. In the trilaminar region of the yolk sac (TYS), trophoblast cells adjacent to the embryo form desmosomes with uterine epithelial cells and also appear to fuse with them to form hybrid cells, the cytoplasm of which resembles that of trophoblast. Later in the TYS, as the placenta develops, trophoblast microvilli and larger cell processes invaginate, and interdigitate with, the highly folded maternal epithelium but do not invade it. At this time in the bilaminar, or avascular, yolk sac (BYS), multinucleate trophoblast giant cells (TGCs) from an annular region adjacent to the sinus terminalis intrude between, and possibly fuse with, the maternal epithelium. The invading TGCs spread laterally above the residual basal lamina before migrating into the stroma.
  
  
  CONCLUSIONS
  In this species of marsupial at least, the cell interactions at the time of implantation are similar to those seen in some eutherian species despite the fact that the fetal chorion is of yolk sac rather than allantoic origin.
authors: Department of Anatomy and Histology, University of Adelaide, Australia.
authors: C T
authors: CT
authors: Roberts
authors: W G
authors: WG
authors: Breed
mesh_terms: D000818:Animals; D002196:Capillaries / Q000648:ultrastructure; D010064:Embryo Implantation; D005314:Embryonic and Fetal Development; D004717:Endometrium / Q000098:blood supply / Q000196:embryology / Q000648:ultrastructure*; D005260:Female; D008394:Marsupialia / Q000196:embryology*; D008431:Maternal-Fetal Exchange; D011247:Pregnancy; D014327:Trophoblasts / Q000648:ultrastructure*; D015017:Yolk Sac / Q000196:embryology / Q000648:ultrastructure*