Polar trophoblast (Rauber's layer) of the rabbit blastocyst

Abstract

The germinal area of the rabbit blastocyst between 108 h postcoitum (pc) and 168 h pc has been examined by scanning electron and transmission electron microscopy. At 108 h and 120 h pc the polar trophoblast (Rauber's layer) is an intact epithelium overlying the epiblast of the inner cell mass. By 132 h pc the polar trophoblast cells begin to separate at multiple foci, exposing the underlying epiblast. Most of the polar trophoblast cells have become individually separated at 144 h pc. The villous, electron-dense polar trophoblast cells can be easily distinguished from the cells of the epiblast, which have smooth apical surfaces. By 162 h pc the polar trophoblast cells have disappeared from the germinal area. Before the polar trophoblast breaks up, the underlying epiblast cells are only loosely attached to one another. Concurrent with the disintegration of the trophoblast epithelium, the epiblast cells change in shape so that their lateral borders become closely apposed, and junctions develop to form a new epithelium. The epiblast becomes contiguous with the mural trophoblast, and thus the blastocyst does not lose its turgidity as the permeability seal is maintained. There are two classical theories on the fate of the polar trophoblast: the cells die, or they become incorporated into the epiblast as living cells. In newly exposed epiblast the presence of very large phagosomes, which are not found when the polar trophoblast is still intact, favors the first hypothesis and indicates that in the rabbit the epiblast is involved in the phagocytosis of the polar trophoblast.

Gastrulation events in the prestreak pig embryo: Ultrastructure and cell markers

“…In the mouse pregastrula (day ϭ 6), the ectoderm is also made of polarized epithelial cells (Jackson et al, 1981). Contrary to the situation in murine embryos, and before amniotic cavity closure, the ectoderm forms a seal with the trophectoderm to separate the uterine and blastocoelic compartments in pig (Barends et al, 1989), as well as in the rabbit (Williams and Biggers, 1990;Viebahn, 1995).…”

“…In the mouse, mitotic cells detach from the basal lamina and remain connected apically for a period of time; however, they do not form coherent clones and their separation may initiate the mixing of ectoderm cells (Gardner and Cockroft, 1998). The presence of stretched mid-bodies between pig ectoderm cells suggests a similar mode of cell proliferation and migration, also observed in the rabbit (Williams and Biggers, 1990). The apical localization of mitotic cells in the disc was used to easily identify ectoderm in wholemount preparations for light microscopy.…”

Rabbit as a reproductive model for human health

“…are w2000 in stage 1 blastocysts ) and 7000 in stage 2 (Thieme et al 2012a). Shortly prior to implantation, the polar trophoblast covering the embryonic disc (Rauber trophoblast) is shed, exposing the embryoblast cells to the uterine luminal epithelium (Williams & Biggers 1990, Tscheudschilsuren et al 1999. The rabbit embryo enzymatically dissolves the zona pellucida during day 4 (Denker & Gerdes 1979) and replaces it by the neozona during day 5 p.c.…”

Trophoblast development

“…In mice, apart from primary giant cells, all of the trophoblast tissue is derived from the polar TE covering the ICM at blastocyst stages. Yet in cattle as well as in other ungulates (pigs and sheep), carnivores, rabbits, sciurid rodents (squirrels) and insectivores, the polar TE covering the ICM -termed Rauber's layer in such species -disappears before gastrulation (Van der Stricht 1923, Flechon 1978, Williams & Biggers 1990, Stern 2004. Furthermore, the ICM flattens out to form a flat epiblast as opposed to the compact cup-shaped epiblast characteristic of the mouse (Stern 2004).…”