Trophoblastic factors involved in lysis of the blastocyst coverings and in implantation in the rabbit: observations on inversely orientated blastocysts.

Lysis of the blastocyst coverings in the rabbit normally starts at the abembryonic hemisphere of the blastocyst facing the antimesometrial endometrium. Theoretically, this restricted effect could be caused by factors locally produced by abembryonic trophoblast, antimesometrial endometrium, or a combination of them. Blastocysts abnormally implanting with reversed orientation exhibit lysis of blastocyst coverings in the abembryonic hemisphere despite the absence of antimesometrial endometrium, and they do not show lysis in the embryonic hemisphere despite its contact with antimesometrial endometrium. Accordingly, it appears that lysis of blastocyst coverings is attributable to a factor localized in the abembryonic hemisphere of the blastocyst. Blastocyst protease activity was found to be maximal there in all blastocysts, whether oriented normally or reversed. The origin and role of this enzyme and other factors are discussed.