Asymmetry in the frequency and position of mitosis in the mouse embryo epiblast at gastrulation

Abstract

At gastrulation, a subpopulation of epiblast cells constitutes a transient posteriorly located structure called the primitive streak, where cells that undergo epithelial–mesenchymal transition make up the mesoderm and endoderm lineages. Mouse embryo epiblast cells were labelled ubiquitously or in a mosaic fashion. Cell shape, packing, organization and division were recorded through live imaging during primitive streak formation. Posterior epiblast displays a higher frequency of rosettes, some of which associate with a central cell undergoing mitosis. Cells at the primitive streak, in particular delaminating cells, undergo mitosis more frequently than other epiblast cells. In pseudostratified epithelia, mitosis takes place at the apical side of the epithelium. However, mitosis is not restricted to the apical side of the epiblast, particularly on its posterior side. Non‐apical mitosis occurs specifically in the streak even when ectopically located. Posterior non‐apical mitosis results in one or two daughter cells leaving the epiblast layer. Cell rearrangement associated with mitotic cell rounding in posterior epiblast, in particular when non‐apical, might thus facilitate cell ingression and transition to a mesenchymal phenotype.

Mitosis, a springboard for epithelial-mesenchymal transition?

“…Intravital imaging of inoculated human colon cancer cells showed that cells in S/G2/M phases were more motile and invasive than G1 cells [40]. In the mouse embryo, while the EMTtriggering transcription factor Snail blocks the cell cycle in the neural epithelium or allantois at E8.5 [41], we and others found an increased mitotic index in the Snail-enriched primitive streak at E7 [42,43].…”

“…We found that the large majority of non-apical mitosis in the mouse embryo epithelial epiblast at the primitive streak gave rise to one or two mesenchymal mesoderm daughter cells, while apical mitosis resulted in epithelial epiblast daughter cells [42]. Non-apical mitosis in the primitive streak might thus help the exit of nascent mesoderm cells from the epiblast through loss of cellcell adhesion, modification of the spatial pattern of forces favoring basement membrane breakage, and/or increased epithelium fluidity (Figure 4).…”

“…Indeed, basement membrane remodeling through Nodal-mediated MMP expression is one of the earliest steps in mouse gastrulation EMT [26,90]. We found that non-apical mitosis was associated with primitive streak morphogenesis even when the streak is ectopic [42]. Since non-apical mitosis in the posterior epiblast was observed prior to the initiation of gastrulation, it is conceivable that basal mitotic cells pushing on a basement membrane might create an asymmetrical pattern of forces.…”

“…Recent work using the zebrafish retina as model pseudostratified epithelium proposed a diffusive model, in which a gradient of concentration drives the average movement of particles (nuclei) from a high concentration area (apical side) to a low concentration area (basal side) [59]. The mouse epiblast, a short pseudostratified epithelium, is particular in the way that cells undergoing mitotic rounding appear to completely detach from the basal pole, which could impact tissue reorganization [42].…”

“…Mouse embryos with subnormal cell numbers due to cell loss or disruption of cell division fail to initiate gastrulation [5]. In addition, the mitotic index is regionally regulated in the epiblast, such that mitoses are more frequent at the primitive streak [42,60]. Single-cell RNA analysis confirmed a higher proportion of cells in G2/ M at the primitive streak [43].…”