Relationships between orientation of the blastocyst during implantation, position of the chorioallantoic placenta, and vascularization of the uterus in the noctilionoid bats Carollia perspicillata and Noctilio sp.

In most eutherian mammals, the inner cell mass (ICM) of the blastocyst assu mes an almost constantly specific orientation to the uterus at the time of implantation, and this is usually correlated with subsequent positioning of the fetal membranes and chorioallantoic placenta. Although these relations hips tend to be conserved between closely related species, this is not the case in the noctilionoid bats. In Carollia perspicillata, which has a simpl ex uterus, the ICM of the single blastocyst becomes oriented towards the ut erotubal junction on the side of ovulation, and the discoidal placenta deve lops in a fundic position. In Noctilio sp., which have partially bicornuate uteri, the ICM becomes oriented instead towards an endometrial ridge that runs along the antimesometrial to lateral side of the gravid horn. As devel opment proceeds, however, the blastocyst rotates almost 180 degrees, and th e discoidal placenta eventually assumes a mesometrial to lateral position. In these species, implantation and subsequent development of the discoidal placenta clearly seem to be targeting major maternal vessels supplying the uterus, rather than exhibiting a consistent pattern of orientation relative to its mesenteric attachments. This permits their chorioallantoic placenta e to develop a dual maternal blood supply that may be essential for the dev elopment of relatively large, precocious infants. (C) 1999 W. B. Saunders C ompany Ltd.