Biology subjects

Maitre, J.-L.

Publications and source records attributed to Maitre, J.-L..

10 recordsLinked to original sources

bioRxiv2026

Rearrangement of chromosome spatial organization in mouse zygotes using optical tweezers does not affect preimplantation development

The spatial arrangement of chromosomes is thought to influence key processes such as gene expression or chromosome segregation. However, the contribution of chromosome spatial organization in physiological contexts remains disputed. Here, we use optical tweezers to displace nucleoli and physically rearrange the nuclear contents of mouse zygotes. Monitoring the preimplantation development of zygotes with physically shuffled nuclei reveals that cell dynamics, gene expression and overall blastocyst formation are not affected. This suggests that in mouse zygotes, the spatial organisation of chromosomes is not critical for blastocyst development.

developmental biology↗

bioRxiv2026

Fluid transport properties dominate blastocyst expansion over mechanics

Lumens populate most of our organs and their size, which dynamically changes throughout the day and lifetime, can influence their function. During mammalian development, the first lumen appears in the blastocyst. Surface epithelial cells pump ions to build an osmotic gradient that draws water, which inflates the embryo and stretches the epithelium as well as the zona pellucida, an elastic shell protecting the embryo. Blastocyst expansion is not steady and instead undergoes cycles of expansion interrupted by collapses during which the lumen leaks out. Early studies identified the critical role of osmotic pressure difference in regulating the expansion of the blastocyst whereas recent studies proposed that mechanical stresses would set the size of the embryo. However, the relative contributions of osmotic and mechanical stresses to lumen expansion dynamics remain unclear. Here, we investigate this using microfluidics, biophysics, machine learning and modelling on mouse embryos. Breaking down the expansion dynamics of mouse embryos, we discovered that fluid pumping rate accelerates, compensating for the increased frequency of collapses interrupting episodes of expansion. We reasoned that fluid transport could be rate limited by osmotic pumping, water permeability and/or mechanical constraints. Removing the mechanical constraint from the zona pellucida only marginally affected these expansion dynamics. Forcing fluid exchange with osmotic shocks we measured that water permeability is large enough not to restrict fluid accumulation rates. Finally, we estimate ion pumping for mouse blastocysts and interfere with it using inhibitors to measure how it quantitatively sets blastocyst expansion rates. Together, we propose that osmotic pressure differences, regulated by fluid transport properties, dominate over mechanical stresses in controlling blastocyst expansion dynamics.

developmental biology↗

bioRxiv2023

Mechanical strengthening of cell-cell adhesion during mouse embryo compaction

Compaction is the first morphogenetic movement of the eutherian mammals and involves a developmentally regulated adhesion process. Previous studies investigated cellular and mechanical aspects of compaction. During mouse and human compaction, cells spread onto each other as a result of a contractility-mediated increase in surface tension pulling at the edges of their cell-cell contacts. However, how compaction may affect the mechanical stability of cell-cell contacts remains unknown. Here, we used a dual pipette aspiration assay on cell doublets to quantitatively analyze the mechanical stability of compacting mouse embryos. We measured increased mechanical stability of contacts with rupture forces growing from 40 to 70 nN, which was highly correlated with cell-cell contact expansion. Analyzing the dynamic molecular reorganization of cell-cell contacts, we find minimal recruitment of the cell-cell adhesion molecule Cdh1 (also known as E-cadherin) to contacts but we observe its reorganization into a peripheral adhesive ring. However, this reorganization is not associated with increased effective bond density, contrary to previous reports in other adhesive systems. Using genetics, we reduce the levels of Cdh1 or replace it with a chimeric adhesion molecule composed of the extracellular domain of Cdh1 and the intracellular domain of Cdh2 (also known as N-cadherin). We find that reducing the levels of Cdh1 impairs the mechanical stability of cell-cell contacts due to reduced contact growth, which nevertheless show higher effective bond density than WT contacts of similar size. On the other hand, chimeric adhesion molecules cannot form large or strong contacts indicating that the intracellular domain of Cdh2 is unable to reorganize contacts and/or is mechanically weaker than the one of Cdh1 in mouse embryos. Together, we find that mouse embryo compaction mechanically strengthens cell-cell adhesion via the expansion of Cdh1 adhesive rings that maintain pre-compaction levels of effective bond density.

developmental biology↗

bioRxiv2023

Inverse blebs operate as hydraulic pumps during mouse blastocyst formation

During preimplantation development, mouse embryos form a fluid-filled lumen, which sets their first axis of symmetry1,2. Pressurized fluid breaks open cell-cell contacts and accumulates into pockets, which gradually coarsen into a single lumen3-5. During coarsening, the adhesive and contractile properties of cells are thought to guide intercellular fluid (IF) but what cell behavior may control fluid movements is unknown. Here, we report large fluid-filled spherical membrane intrusions called inverse blebs6,7 growing into cells at adhesive contacts. At the onset of lumen coarsening, we observed hundreds of inverse blebs throughout the embryo, each dynamically filling with IF and retracting within a minute. We find that inverse blebs grow due to pressure build-up resulting from luminal fluid accumulation and cell-cell adhesion, which locally confines fluid. Inverse blebs then retract due to actomyosin contraction, which effectively redistributes fluid within the intercellular space. Importantly, inverse blebs show topological specificity and only occur at contacts between two cells, not at contacts formed by multiple cells, which essentially serve as fluid sinks. Manipulating the topology of the embryo reveals that, in the absence of sinks, inverse blebs pump fluid into one another in a futile cycle. We propose that inverse blebs operate as hydraulic pumps to promote luminal coarsening, thereby constituting an instrument used by cells to control fluid movement.

developmental biology↗

bioRxiv2022

Ectopic activation of the polar body extrusion pathway triggers cell fragmentation in preimplantation embryos

Cell fragmentation occurs during physiological processes, such as apoptosis, migration, or germ cell development. Fragmentation is also commonly observed during preimplantation development of human embryos and is associated with poor implantation prognosis during Assisted Reproductive Technology (ART) procedures. Despite its biological and clinical relevance, the mechanisms leading to cell fragmentation are unclear. Light sheet microscopy imaging of mouse embryos reveals that compromised spindle anchoring, due to Myo1c knockout or dynein inhibition, leads to fragmentation. We further show that defective spindle anchoring brings DNA in close proximity to the cell cortex, which, in stark contrast to previous reports in mitotic cells, locally triggers actomyosin contractility and pinches off cell fragments. The activation of actomyosin contractility by DNA in preimplantation embryos is reminiscent of the signals mediated by small GTPases throughout polar body extrusion (PBE) during meiosis. By interfering with the signals driving PBE, we find that this meiotic signaling pathway remains active during cleavage stages and is both required and sufficient to trigger fragmentation. Together, we find that fragmentation happens in mitosis after ectopic activation of actomyosin contractility by signals emanating from DNA, similar to those observed during meiosis. Our study uncovers the mechanisms underlying fragmentation in preimplantation embryos and, more generally, offers insight into the regulation of mitosis during the maternal-zygotic transition.

developmental biology↗

bioRxiv2022

The filipodia-like protrusions of adjacent somatic cells shape the developmental potential of mouse oocytes

The oocyte must grow and mature before fertilization, thanks to a close dialogue with the somatic cells which surround it. Part of this communication is through filopodialike protrusions, called transzonal projections (TZPs), sent by the somatic cells to the oocyte membrane. To investigate the contribution of TZPs to oocyte quality, we impaired their structure by generating a full knockout mouse of the TZP structural component Myosin-X (MYO10). Using spinning disk and super-resolution microscopy combined with a machine learning approach to phenotype oocyte morphology, we show that the lack of Myo10 decreases TZP density during oocyte growth. Reduction in TZPs does not prevent oocyte growth but impairs oocyte-matrix integrity. Importantly, we reveal by transcriptomic analysis that gene expression is altered in TZP-deprived oocytes, and that oocyte maturation and subsequent early embryonic development are partially affected, effectively reducing mouse fertility. We propose that TZPs play a role in the structural integrity of the germline-somatic complex, which is essential for regulating gene expression in the oocyte and thus its developmental potential.

cell biology↗

bioRxiv2022

Mechanics of human embryo compaction

The shaping of the human embryo begins with compaction, during which cells come into close contact and form a tighter structure1,2. Assisted reproductive technology (ART) studies suggest that human embryos fail compaction primarily because of defective adhesion3,4. Based on our current understanding of animal morphogenesis5,6, other morphogenetic engines, such as cell contractility, could be involved in shaping the human embryo. However, the molecular, cellular and physical mechanisms driving human embryo morphogenesis remain uncharacterized. Using micropipette aspiration on human embryos donated to research, we have mapped cell surface tensions during compaction. This reveals a 4-fold increase of tension at the cellmedium interface while cell-cell contacts keep a steady tension. Comparison between human and mouse reveals qualitatively similar but quantitively different mechanical strategies, with human embryos being mechanically least efficient. Inhibition of cell contractility and cell-cell adhesion in human embryos reveal that only contractility controls the surface tension responsible for compaction. Interestingly, if both cellular processes are required for compaction, they exhibit distinct mechanical signatures when faulty. Analyzing the mechanical signature of naturally failing embryos, we find evidence that non-compacting embryos or partially compacting embryos with excluded cells have defective contractility. Together, our study reveals that an evolutionarily conserved increase in cell contractility is required to generate the forces driving the first morphogenetic movement shaping the human body.

developmental biology↗

bioRxiv2021

Zygotic contractility awakening during mouse preimplantation development

Actomyosin contractility is a major engine of preimplantation morphogenesis, which starts at the 8-cell stage during mouse embryonic development. Contractility becomes first visible with the appearance of periodic cortical waves of contraction (PeCoWaCo), which travel around blastomeres in an oscillatory fashion. How contractility of the mouse embryo becomes active remains unknown. We have taken advantage of PeCoWaCo to study the awakening of contractility during preimplantation development. We find that PeCoWaCo become detectable in most embryos only after the 2ndcleavage and gradually increase their oscillation frequency with each successive cleavage. To test the influence of cell size reduction during cleavage divisions, we use cell fusion and fragmentation to manipulate cell size across a 20-60 m range. We find that the stepwise reduction in cell size caused by cleavage divisions does not explain the presence of PeCoWaCo or their accelerating rhythm. Instead, we discover that blastomeres gradually decrease their surface tensions until the 8-cell stage and that artificially softening cells enhances PeCoWaCo prematurely. Therefore, during cleavage stages, cortical softening awakens zygotic contractility before preimplantation morphogenesis.

developmental biology↗

bioRxiv2020

Multiscale analysis of single and double maternal-zygotic Myh9 and Myh10 mutants during mouse preimplantation development

During the first days of mammalian development, the embryo forms the blastocyst, the structure responsible for implanting the mammalian embryo. Consisting of an epithelium enveloping the pluripotent inner cell mass and a fluid-filled lumen, the blastocyst results from a series of cleavages divisions, morphogenetic movements and lineage specification. Recent studies identified the essential role of actomyosin contractility in driving the morphogenesis, fate specification and cytokinesis leading to the formation of the blastocyst. However, the preimplantation development of contractility mutants has not been characterized. Here, we generated single and double maternal-zygotic mutants of non-muscle myosin-II heavy chains (NMHC) to characterize them using multiscale imaging. We find that Myh9 (NMHC II-A) is the major NMHC during preimplantation development as its maternal-zygotic loss causes failed cytokinesis, increased duration of the cell cycle, weaker embryo compaction and reduced differentiation, whereas Myh10 (NMHC II-B) maternal-zygotic loss is much less severe. Double maternal-zygotic mutants for Myh9 and Myh10 show a much stronger phenotype, failing most attempts of cytokinesis. We find that morphogenesis and fate specification are affected but nevertheless carry on in a timely fashion, regardless of the impact of the mutations on cell number. Strikingly, even when all cell divisions fail, the resulting single-celled embryo can initiate trophectoderm differentiation and lumen formation by accumulating fluid in increasingly large vacuoles. Therefore, contractility mutants reveal that fluid accumulation is a cell-autonomous process and that the preimplantation program carries on independently of successful cell division.

developmental biology↗

bioRxiv2019

Fracking and Ostwald ripening position the lumen of the mouse blastocyst

During mouse preimplantation development, the formation of the blastocoel, a fluid-filled lumen, breaks the radial symmetry of the blastocyst. What controls the formation and positioning of this basolateral lumen remains obscure. We find that accumulation of pressurized fluid fractures cell-cell contacts into hundreds of micron-size lumens. Microlumens eventually discharge their volumes into a single dominant lumen, which we model as a process akin to Ostwald ripening, underlying the coarsening of foams. Using chimeric mutant embryos, we tune the tracking of cell-cell contacts and steer the coarsening of microlumens, allowing us to successfully manipulate the final position of the lumen. We conclude that hydraulic fracture of cell-cell contacts followed by directed coarsening of microlumens sets the first axis of symmetry of the mouse embryo.

developmental biology↗