Gastrulation in the mouse embryo: Ultrastructural and molecular aspects of germ layer morphogenesis

Abstract

Ultrastructural studies and lineage analyses of gastrulating mouse embryos have revealed that different morphogenetic tissue movements are involved in the formation of the three definitive germ layers. Definitive ectoderm is formed by epibolic expansion of the pre-existing progenitor population in the embryonic ectoderm. Formation of the mesoderm and the endoderm is initiated by cellular ingression at the primitive streak. The mesodermal layer is established by cell migration and cell sheet spreading, but the endoderm is formed by replacing the original primitive endodermal population. To this date, genes that are expressed during mouse gastrulation mostly encode cell surface adhesion or signalling molecules, growth factors and their receptors, and putative transcriptional factors. Their precise role during gastrulation remains to be investigated.

Processus and recessus adhaerentes: giant adherens cell junction systems connect and attract human mesenchymal stem cells

“…Bars 0.2 μm (d, g-i), 0.5 μm (c, e, f, j), 1.0 μm (a), 2.0 μm (b) b preparative fragility. On the other hand, we have previously noted, in an electron-microscopic study of primary mesenchymal cells and developing mesoderm of the mouse embryo in situ, "the frequency of small punctum adhaerens-like junctions", including some located "within interdigitations of processes of mesenchymal cells" (Franke et al 1983; see also Hashimoto and Nakatsuji 1989;Tam et al 1993). Therefore, we hypothesize that such "organizing" and "interlocking" cell processes are more widespread in embryonic and other tissues than is presently believed and have begun to characterize the molecular composition and developmental roles of the in situ equivalents of such junction-bearing cell processes.…”

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

“…Mesoderm cells migrate as a loosely packed cell sheet, but some cells near the area of the anterior primitive streak can be seen migrating as single cells or small groups of cells. 15,16 Molecular Pathways that Regulate EMT at the Murine Primitive Streak…”

“…14,15 Epiblast cells posses tight junctions, adherens junctions, gap junctions and contact each other by microvilli and filopodia. 15 Around E6.25 a morphologically distinct structure, the primitive streak, appears. The primitive streak breaks the bilateral symmetry and marks the posterior extremity of the embryo.…”

Ets2-dependent trophoblast signalling is required for gastrulation progression after primitive streak initiation

“…1g,j). Although DBA lectin specifically stains all VE, but not DE, mesoderm or epiblast cells 10,11 , it can be used as an indirect marker of DE formation. This is because during normal development, when DE cells intercalate into the epiblastassociated VE to form the DE layer, the initially closely packed VE cells (continuous DBA positivity) become dispersed in this part of the VE (patchy DBA positivity) to accommodate the newly formed DE cells, whereas this VE dispersal does not occur in the EXE-associated VE, where DE does not form (continuous DBA positivity) 12 (Fig.…”