Matrix metalloproteinases have a broad spectrum of substrates ranging from extracellular matrix components and adhesion molecules to chemokines and growth factors. Despite being mostly secreted, MMPs have been detected in the cytosol, the mitochondria or the nucleus. Although most of the attention is focused on their role in matrix remodeling, the diversity of their substrates and their complex trafficking open the possibility for non-canonical functions. Yet in vivo examples and experimental demonstration of the physiological relevance of such activities are rare. Here, we have used chick neural crest (NC) cells, a highly migratory stem cell population likened to invasive cancer cells, as a model for physiological epithelialmesenchymal transition (EMT). We demonstrate that MMP14 is required for NC delamination. Interestingly, this role is independent of its cytoplasmic tail and of its catalytic activity. Our in vivo data indicate that, in addition to being a late pro-invasive factor, MMP14 is also likely to be an early player, owing to its role in EMT.

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.

Pseudostratified epithelia (PSE) are a common type of columnar epithelia found in a wealth of embryonic and adult tissues such as ectodermal placodes, the trachea, the ureter, the gut and the neuroepithelium. PSE are characterized by the choreographed displacement of cells' nuclei along the apicobasal axis according to phases of their cell cycle. Such movements, called interkinetic movements (INM), have been proposed to influence tissue expansion and shape and suggested as culprit in several congenital diseases such as CAKUT (Congenital anomalies of kidney and urinary tract) and esophageal atresia. INM rely on cytoskeleton dynamics just as adhesion, contractility and mitosis do. Therefore, long term impairment of INM without affecting proliferation and adhesion is currently technically unachievable. Here we bypassed this hurdle by generating a 2D agent-based model of a proliferating PSE and compared its output to the growth of the chick neuroepithelium to assess the interplay between INM and these other important cell processes during growth of a PSE. We found that INM directly generates apical expansion and apical nuclear crowding. In addition, our data strongly suggest that apicobasal elongation of cells is not an emerging property of a proliferative PSE but rather requires a specific elongation program. We then discuss how such program might functionally link INM, tissue growth and differentiation. Author summaryPseudostratified epithelia (PSE) are a common type of epithelia characterized by the choreographed displacement of cells' nuclei along the apicobasal axis during proliferation. These so-called interkinetic movements (INM) were proposed to influence tissue expansion and suggested as culprit in several congenital diseases. INM rely on cytoskeleton dynamics. Therefore, longer term impairment of INM without affecting proliferation and adhesion is currently technically unachievable. We bypassed this hurdle by generating a mathematical model of PSE and compared it to the growth of an epithelium of reference.Our data show that INM drive expansion of the apical domain of the epithelium and suggest that apicobasal elongation of cells is not an emerging property of a proliferative PSE but might rather requires a specific elongation program.Interkinetic movements impact the growth of pseudostratified epithelia PLOS Computational Biology | https://doi.

Mesoderm arises at gastrulation and contributes to both the mouse embryo proper and its extra‐embryonic membranes. Two‐photon live imaging of embryos bearing a keratin reporter allowed recording filament nucleation and elongation in the extra‐embryonic region. Upon separation of amniotic and exocoelomic cavities, keratin 8 formed apical cables co‐aligned across multiple cells in the amnion, allantois, and blood islands. An influence of substrate rigidity and composition on cell behavior and keratin content was observed in mesoderm explants. Embryos lacking all keratin filaments displayed a deflated extra‐embryonic cavity, a narrow thick amnion, and a short allantois. Single‐cell RNA sequencing of sorted mesoderm cells and micro‐dissected amnion, chorion, and allantois, provided an atlas of transcriptomes with germ layer and regional information. It defined the cytoskeleton and adhesion expression profile of mesoderm‐derived keratin 8‐enriched cells lining the exocoelomic cavity. Those findings indicate a novel role for keratin filaments in the expansion of extra‐embryonic structures and suggest mechanisms of mesoderm adaptation to the environment.

17Pseudostratified epithelia (PSE) are a common type of columnar epithelia found in a wealth of 18 embryonic and adult tissues such as ectodermal placodes, the trachea, the ureter, the gut and 19 the neuroepithelium. PSE are characterized by the choreographed displacement of cells' 20 nuclei along the apicobasal axis according to phases of their cell cycle. Such movements, 21 called interkinetic movements (INM) have been proposed to influence tissue expansion and 22 shape and suggested as culprit in several congenital diseases such as CAKUT and esophageal 23 atresia. INM rely on cytoskeleton dynamics just as adhesion, contractility and mitosis do. 24 Therefore, longer term impairment of INM without affecting proliferation and adhesion is 25 currently technically unachievable. Here we bypassed this hurdle by generating a 2D agent-26 based model of a proliferating PSE and compared its output to the growth of the chick 27 neuroepithelium to assess the interplay between INM and these other important cell 28 processes during growth of a PSE. We found that INM directly generates apical expansion and 29 apical nuclear crowding. In addition, our data strongly suggest that apicobasal elongation of 2 30 cells is not an emerging property of a proliferative PSE but rather requires a specific 31 elongation program. We then discuss how such program might functionally link INM, tissue 32 growth and differentiation. 33 34 Authors Summary 35 Pseudostratified epithelia (PSE) are a common type of epithelia characterized by the 36 choreographed displacement of cells' nuclei along the apicobasal axis during proliferation. 37 These so-called interkinetic movements (INM) were proposed to influence tissue expansion 38 and suggested as culprit in several congenital diseases. INM rely on cytoskeleton dynamics. 39 Therefore, longer term impairment of INM without affecting proliferation and adhesion is 40 currently technically unachievable. We bypassed this hurdle by generating a mathematical 41 model of PSE and compared it to the growth of an epithelium of reference. Our data show 42 that INM drive expansion of the apical domain of the epithelium and suggest that apicobasal 43 elongation of cells is not an emerging property of a proliferative PSE but might rather requires 44 a specific elongation program.45 3 46 48 are thin and elongated. Nuclei packing is very high and forces cells to distribute their nuclei 49 along the apicobasal axis creating multiple layers of nuclei within a monolayer of cells, hence 50 the term pseudostratification. PSE are found across the animal kingdom from invertebrates to 51 vertebrates [1]. During development, several structures adopt a pseudostratified 52 configuration such as the placodes and the central nervous system in vertebrates or the 53 imaginal discs in Drosophila. In adults, PSE can be found along the respiratory, urinary and 54 digestive tracts (e.g trachea, ureter, midgut) [2, 3] and various organs such as the gonads (e.g. 55 epididymis) or the eye (lens, retina) [1]. One characteristic fea...

Mitosis is a key process in development and remains critical to ensure homeostasis in adult tissues. Besides its primary role in generating two new cells, cell division involves deep structural and molecular changes that might have additional effects on cell and tissue fate and shape. Specific quantitative and qualitative regulation of mitosis has been observed in multiple morphogenetic events in different embryo models. For instance, during mouse embryo gastrulation, the portion of epithelium that undergoes epithelial to mesenchymal transition, where a static epithelial cell become mesenchymal and motile, has a higher mitotic index and a distinct localization of mitotic rounding, compared to the rest of the tissue. Here we explore the potential mechanisms through which mitosis may favor tissue reorganization in various models. Notably, we discuss the mechanical impact of cell rounding on the cell and its environment, and the modification of tissue physical parameters through changes in cell-cell and cell-matrix adhesion.

The mechanical properties of the different germ layers of the early mammalian embryo are likely to be critical for morphogenesis. Cytoskeleton components (actin and myosin, microtubules, intermediate filaments) are major determinants of epithelial plasticity and resilience to stress. Here, we take advantage of a mouse reporter for Keratin 8 to record the pattern of the keratin intermediate filaments network in the first epithelia of the developing mouse embryo. At the blastocyst stage, Keratin 8 is strongly expressed in the trophectoderm, and undetectable in the inner cell mass and its derivatives, the epiblast and primitive endoderm. Visceral endoderm cells that differentiate from the primitive endoderm at the egg cylinder stage display apical Keratin 8 filaments. Upon migration of the Anterior Visceral Endoderm and determination of the anterior-posterior axis, Keratin 8 becomes regionally distributed, with a stronger expression in embryonic, compared to extra-embryonic, visceral endoderm. This pattern emerges concomitantly to a modification of the distribution of Filamentous (F)-actin, from a cortical ring to a dense apical shroud, in extra-embryonic visceral endoderm only. Those regional characteristics are maintained across gastrulation. Interestingly, for each stage and region of the embryo, adjacent germ layers display contrasted levels of keratin filaments, which may play a role in their adaptation to growth and morphological changes.

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