Structure of trophoblast papillae on the sheep conceptus at implantation.

A study of 13-18 days old sheep conceptuses has consistently demonstrated the presence of multicellular protrusions (papillae) from the trophectoderm surface. These papillae were shown to be restricted to the embryonic region of conceptuses flushed out of the uterus. After perfusion fixation of the uterus on day 16 of pregnancy, the papillae can be observed penetrating well down into the lumina of the uterine glands. The papillae have not been observed at or after day 20. It is suggested that the papillae may play an important but transient role in anchoring the embryonic region of the conceptus against the uterine epithelium to allow the initiation of the cellular changes characteristic of implantation.