Spontaneous decidual reactions and menstruation in the black mastiff bat, Molossus ater

Uterine function was assessed histologically in nonpregnant Molossus ater removed from a laboratory breeding colony. During the luteal phase of the cycle, bilateral decidual reactions were found to develop spontaneously in the absence of either embryos or experimental manipulation of the uterus. These included the formation of early decidual giant cells, closure of the uterine lumina, and morphological changes in the endometrial blood vessels. Some endothelial cell hypertrophy was noted in much of the decidua, but this was most pronounced in vessels associated with an unusual vascular tuft that formed in the endometrium surrounding the cranial end of the uterine lumen. These latter vessels also developed very prominent basal laminae. In pregnant bats, this tuft played a central role in the morphogenesis of the definitive discoidal chorioallantoic placenta. At the end of nonpregnant cycles, the decidua became necrotic and was sloughed off with associated bleeding. As in menstruating catarrhine primates, the endometrium of M. ater is vascularized by spiral arterioles and populated by distinctive granulocytes containing large, acidophilic granules. Increased coiling of these arterioles did not appear to be an essential element in the mechanism of mensturation in this species. M. ater is a monotocous, seasonal breeder, with a relatively long gestation period. Although it has a bicornuate uterus, ovulation and implantation appear to occur only on the right side of the tract. The ability to menstruate probably affords this bat an efficient mechanism for eliminating a highly differentiated endometrium from the usual implantation site in the event of a reproductive failure. In the wild, this may provide M. ater with another chance to establish a pregnancy at a still opportune time during the same breeding season.