Cellular interactions during implantation in domestic ruminants.

Implantation is a critical step in the progress of pregnancy, during which the conceptus acquires a fixed position within the uterine lumen, and leads to the establishment of the placental structures. This process implies some cellular modifications of both the uterine epithelium and the trophoblast to ensure cell adhesion between the two tissues. In ruminants, the implantation process is characterized by three main steps: a long pre-attachment period lasting 2-3 weeks during which the conceptus elongates considerably, an apposition stage when cellular contacts are established between the trophoblast and the uterine epithelium, and an adhesion stage which ends the process and gives rise to the cellular structure of an epithelio-chorial placenta. Trophoblast apposition begins in the vicinity of the embryo by day 15, 18 and 19 in sheep, goats and cows, respectively. The trophoblast cells surrounding the embryo show morphological and functional changes. These modifications are local within the conceptus since non-implanted areas of trophoblast still display the morphological and functional features that characterized this tissue during the pre-attachment period. As the implantation process spreads towards the extremity of the elongated conceptus, these cellular transformations progressively affect the whole trophoblast. Apposition is completed by a close adhesion between the interdigitating uterine microvilli and the trophoblastic plasma membrane. By this stage, trophoblastic binucleate cells migrate through the trophoblast monolayer and fuse with individual uterine cells to form a syncytial tissue. During this process placental lactogen hormones and pregnancy serum proteins (PSP) produced by the binucleate cells are transported to the endometrial tissues and then to the maternal blood circulation.