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Epiblast and trophoblast morphogenesis in the pre‐gastrulation blastocyst of the pig. A light‐ and electron‐microscopical study

The epiblast of the amniote embryo is of paramount importance during early development as it gives rise to all tissues of the embryo proper. In mammals, it emerges through segregation of the hypoblast from the inner cell mass and subsequently undergoes transformation into an epithelial sheet to create the embryonic disc. In rodents and man, the epiblast cell layer is covered by the polar trophoblast which forms the placenta. In mammalian model organisms (rabbit, pig, several non‐human primates), however, the placenta is formed by mural trophoblast whereas the polar trophoblast disintegrates prior to gastrulation and thus exposes the epiblast to the microenvironment of the uterine cavity. Both, polar trophoblast disintegration and epiblast epithelialization, thus pose special cell‐biological requirements but these are still rather ill‐understood when compared to those of gastrulation morphogenesis. This study therefore applied high‐resolution light and transmission electron microscopy and three‐dimensional (3D) reconstruction to 8‐ to 10‐days‐old pig embryos and defines the following steps of epiblast transformation: (1) rosette formation in the center of the ball‐shaped epiblast, (2) extracellular cavity formation in the rosette center, (3) epiblast segregation into two subpopulations ‐ addressed here as dorsal and ventral epiblast ‐ separated by a “pro‐amniotic” cavity. Ventral epiblast cells form between them a special type of desmosomes with a characteristic dense felt of microfilaments and are destined to generate the definitive epiblast. The dorsal epiblast remains a mass of non‐polarized cells and closely associates with the disintegrating polar trophoblast, which shows morphological features of both apoptosis and autophagocytosis. Morphogenesis of the definitive epiblast in the pig may thus exclude a large portion of bona fide epiblast cells from contributing to the embryo proper and establishes contact de novo with the mural trophoblast at the junction between the two newly defined epiblast cell populations.