Formation of reticulated endoderm, Reichert's membrane, and amniogenesis in blastocysts of captive-bred, short-tailed fruit bats, Carollia perspicillata

Abstract

Background and methods: As part of an effort to develop the short-tailed fruit bat (Carollia perspicillata) as a new animal model for the study of interstitial implantation and trophoblast-uterine interactions, early embryogenesis was examined histologically and ultrastructurally in captive-bred females at different intervals after the first appearance of spermatozoa in daily vaginal smears (day 1 postcoitum [p.c.]).

Results: In most of the early uterine embryos examined on days 16-18 p.c., much of the endoderm appeared as a reticulated meshwork; however, a unilocular yolk sac was formed prior to the development of any mesoderm. Early blastocysts of Carollia were also unusual in that endoderm surrounded much of the inner cell mass (ICM), Reichert's membrane continued over the dorsal side of the ICM, and basal laminalike material was observed around many of the endoderm and epiblast cells. A primordial amniotic cavity was formed between days 19 and 26 p.c. by cavitation. The first mesoderm appeared between days 23 and 26 p.c., concommitant with the development of an embryonic shield.

Conclusion: The unusual reticulated appearance of early endoderm in Carollia, which is reminiscent of that seen in early human blastocysts, may be attributable to constraints imposed on growth of the blastocyst by the site and mode of implantation, temporary retardation of trophoblastic invasion by the basal laminae of endometrial epithelial elements, and endodermal proliferation in anticipation of rapid yolk sac expansion. Reichert's membrane appears to play an important role in this species in tethering the ICM and embryonic shield to the developing placenta prior to the formation of significant amounts of mesoderm.