Morphology of Incipient Mesoderm Formation in the Rabbit Embryo: A Light- and Retrospective Electron-Microscopic Study

Abstract

Mesoderm formation is a hallmark of vertebrate gastrulation and, at the same time, one of the prime examples for epithelio-mesenchymal transformation. Recent advances in experimental embryology and molecular biology have clarified the role of growth factors and genes in this process; however, its microscopic anatomy in higher vertebrates is still far from clear. Therefore, the present study describes the morphology of mesoderm formation in the rabbit embryo, a species which may be representative for both the avian and the mammalian embryo in this respect. Serial semithin sections were correlated with topographical landmarks in surface views of embryonic discs at 6.4, 6.5, and 6.6 days post conceptionem, and selected semithin sections were reembedded for ultrastructural analysis. Mesoderm cells are shown to be generated by ingression of bottle-shaped epiblast cells in the area of the posterior node and the primitive streak. Here, basal endocytotic pits and absence or discontinuity of the basal lamina are taken as suggestive evidence for specific removal of extracellular matrix material. Within the bottle-shaped cells most organelles are concentrated in a narrow apical neck which will subsequently constitute the ‘trailing end’ of the ingressing mesoderm cells. These features support the assumption that most principles of epithelio-mesenchymal transformation seen during primary mesenchyme formation in the sea urchin also apply to mesoderm formation in vertebrates. However, transient tripartite zonula adherens-type junctions are formed apically between ingressing mesoderm cells and the neighboring epiblast cells. They are interpreted here as being responsible for maintaining supracellular integrity of the embryonic disc during the shedding of mesoderm cells in the amniote embryo.

Transitions between epithelial and mesenchymal states and the morphogenesis of the early mouse embryo

“…16 To maintain epithelial integrity, epiblast cells may vault over ingressing mesoderm cells as has been described in rabbits. 17 Bottle shaped cells progressively lose their contact with the apical surface as finger-like projections of surrounding epiblast cells bridge over the ingressing cell and meet apically enclosing the ingressing cell. Therefore when an ingressing cell breaks down its adherens junctions to neighboring epiblast cells, epithelial continuity is maintained as cells remaining in the epithelium seal the gap by establishing new adherens junctions.…”

“…16 Little is known about the mechanisms causing the disintegration of the basement membrane in the mouse primitive streak. However, studies in the rabbit show endocytotic pits with basal material in ingressing mesodermal cells suggesting that endocytosis may be involved, 17 while in vitro studies in carcinoma cells have demonstrated the activation of different metalloproteases by where epithelial cells undergo EMT, become motile, and migrate en masse to re-epithelialize after internalization. 7 Nonetheless, all ingression events occur by EMT, for example the ingression of primary mesenchyme in sea urchins 8 or the ingression of mesoderm through the subduction zones in the urodele.…”

Genetic ablation of FLRT3 reveals a novel morphogenetic function for the anterior visceral endoderm in suppressing mesoderm differentiation

“…Breakdown of the BM is an important event for EMT during gastrulation (Viebahn et al 1995). Recent work had shown that the BM prevents EMT and precocious differentiation of epiblast into mesoderm (Fujiwara et al 2007).…”

Mechanisms of cell positioning during C. elegans gastrulation

“…Because of their shape, these cells are often described as flask or bottle cells (Rhumbler, 1899;Rhumbler, 1902;Ruffini, 1925). Apical constriction has been suggested to play a role in morphogenetic movements in a wide range of organisms, including gastrulation in shrimp (Hertzler and Clark, 1992), fly (Young et al, 1991), jellyfish (Byrum, 2001), sea urchin (Kimberly and Hardin, 1998), white sturgeon (Bolker, 1993), rabbit (Viebahn et al, 1995) and frog (Keller, 1981;Hardin and Keller, 1988), as well as in primitive streak formation in chick and rat embryos (Solursh and Revel, 1978), and neurulation in frogs (Jacobson et al, 1986). Whether apical constriction actually drives shape changes in cell sheets has been directly tested in Xenopus, sea urchin and Drosophila gastrulation.…”