The Laboratory Shrew Placenta: Evidence for an Endothelio-Endothelial Type

Existence of an endothelio-endothelial placenta is controversial. Insectivores may have a placenta of this type or the endothelio-chorial type; thus, studies of the placentae of Suncus murinus, the laboratory shrew, were undertaken. Cellular components of the interhemal membrane included maternal endothelium, intermediate trophoblast (syncytiotrophoblast) and fetal endothelium. Up to day 20 of gestation, the intermediate trophoblast clearly intervened between the hypertrophied maternal endothelium and the fetal endothelium throughout the labyrinthine zone. From day 20 to 24, the intermediate trophoblast was sieve-like in appearance with extensive infoldings at both surfaces. Basal laminae of both the maternal endothelium and the intermediate trophoblast were broad, irregular and continuous. Therefore, at this stage the placenta was endothelio-chorial in type. After day 24 of gestation, the intermediate trophoblast could scarcely be recognized as an obvious and continuous layer. Only cytoplasmic processes of the intermediate trophoblast were present between the maternal and the fetal endothelium. Finally, projections of both the maternal and the fetal endothelium contacted each other. We conclude that the intermediate trophoblast does not play a central role within the placental barrier throughout the last week of gestation. Hence, the placenta of Suncus murinus is endothelio-endothelial in type.

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