A reexamination of the chorioallantoic placental membrane of a shrew, Blarina brevicauda: Resolution of a controversy

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Abstract
The allantoic placenta of shrews has been classified as hemochorial, endotheliochorial and endothelioendothelial. Recent confusion has centered on the persistence of an intermediate cellular layer within the interhemal membrane and on its identity. Conflicting interpretations assert, (1) that the intermediate layer represents a continuous trophoblastic lamina (endotheliochorial placenta), or (2) that it consists of a discontinuous “maternal symplasma,” the trophoblast having disappeared early in development (endothelioendothelial placenta).Utilizing thin plastic sections for light microscopy and electron microscopy we find that the disputed intermediate layer comprises an exceedingly thin, but continuous layer of syncytiotrophoblast, and that the shrew placenta is indeed endotheliochorial. However, the interhemal membrane displays a most unusual organization. The trophoblast is sieve‐like in that it is honeycombed with interstitial (extracellular) spaces which open freely at both its surfaces. Into these channels project numerous processes of the maternal endothelium, and where these contact trophoblast close junctions are often apparent; in rare instances endothelial processes were seen to penetrate to the fetal surface of the trophoblast, but not through its well‐defined basal lamina. Maternal endothelium and trophoblast are separated by an irregular space filled with a dense amorphous material, but it lacks fibrillar components normally found in basal membrane. Peculiarities of the hypertrophied maternal endothelium include an abundance of oriented cytoplasmic fibrillar elements, moderate endoplasmic reticulum and ribosomes, and a marked alignment of mitochondria against the basal cell membrane. Hypertrophied fetal mesenchymal cells often intervene between the fetal endothelium and the rest of the interhemal membrane. Seldom does either abut directly against the basal lamina of the trophoblast; they are usually separated from it by a welter of cytoplasmic processes from both elements, which project into the fetal interstitial space. The fetal endothelial cells are somewhat hypertrophied, and their external processes, like those of the mesenchymal cells, often contain glycogen. Peculiarly, the fetal endothelium lacks a basal lamina. The mesenchymal cells display all the usual ultrastructural characteristics of protein synthesizing cells and appear highly active. Functional implications of the organizational peculiarities of the interhemal membrane are discussed.

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