Formation of a placenta requires intimate contact between the embryonic and maternal uterine epithelia in early pregnancy. Contact is accompanied by a characteristic suite of changes to the plasma membranes of uterine epithelial cells, termed the plasma membrane transformation. The plasma membrane transformation occurs in eutherian mammals and in viviparous (live-bearing) squamate reptiles, and may be fundamental to the evolution of viviparity in amniotes. Marsupials provide an excellent opportunity to test the generality of this phenomenon. Here, we present the first detailed study of the plasma membrane transformation in a marsupial. We combine electron microscopy and immunohistochemistry to describe morphological and molecular features of uterine epithelial cells during pregnancy in the fat-tailed dunnart (Sminthopsis crassicaudata; Dasyuridae). Cell morphology changes dramatically in S. crassicaudata during pregnancy. Apical microvilli are replaced by irregular blunt projections, then by spiky projections postimplantation. Cell surfaces flatten and ciliated cells are lost. Junctional complexes between adjacent cells increase in depth, then decrease just before implantation, which is consistent with junctional protein localization in this region of the cell membrane. The uterine cellular changes in S. crassicaudata are consistent with a plasma membrane transformation, and support the idea that this phenomenon is fundamental to the evolution of viviparity in amniote vertebrates.
Keywords: implantation; plasma membrane transformation; viviparity.
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