Cell sorting at embryonic boundaries

Mechanical convergence in mixed populations of mammalian epithelial cells

Tissues consist of cells with different molecular and/or mechanical properties. Measuring the forces and stresses in mixed-cell populations is essential for understanding the mechanisms by which tissue development, homeostasis, and disease emerge from the cooperation of distinct cell types. However, many previous studies have primarily focused their mechanical measurements on dissociated cells or aggregates of a single-cell type, leaving the mechanics of mixed-cell populations largely unexplored. In the present study, we aimed to elucidate the influence of interactions between different cell types on cell mechanics by conducting in situ mechanical measurements on a monolayer of mammalian epithelial cells. Our findings revealed that while individual cell types displayed varying magnitudes of traction and intercellular stress before mixing, these mechanical values shifted in the mixed monolayer, becoming nearly indistinguishable between the cell types. Moreover, by analyzing a mixed-phase model of active tissues, we identified physical conditions under which such mechanical convergence is induced. Overall, the present study underscores the importance of in situ mechanical measurements in mixed-cell populations to deepen our understanding of the mechanics of multicellular systems.

From soap bubbles to multicellular organisms: Unraveling the role of cell adhesion and physical constraints in tile pattern formation and tissue morphogenesis

Tile patterns, in which numerous cells are arranged in a regular pattern, are found in a variety of multicellular organisms and play important functional roles. Such regular arrangements of cells are regulated by various cell adhesion molecules. On the other hand, cell shape is also known to be regulated by physical constraints similar to those of soap bubbles. In particular, circumference minimization plays an important role, and cell adhesion negatively affects this process, thereby regulating tissue morphogenesis based on physical properties. Here, we focus on the Drosophila compound eye and the mouse auditory epithelium, and summarize the mechanisms of tile pattern formation by cell adhesion molecules such as cadherins, Irre Cell Recognition Modules (IRMs), and nectins. Phenomena that cannot be explained by physical stability based on cortical tension alone have been reported in the tile pattern formation in the compound eye, suggesting that previously unexplored forces such as cellular concentric expansion force may play an important role. We would like to summarize perspectives for future research on the mechanisms of tissue morphogenesis.

Different temporal requirements for tartan and wingless in the formation of contractile interfaces at compartmental boundaries

Compartmental boundaries physically separate developing tissues into distinct regions, which is fundamental for the organisation of the body plan in both insects and vertebrates. In many examples, this physical segregation is caused by a regulated increase in contractility of the actomyosin cortex at boundary cell-cell interfaces, a property important in developmental morphogenesis beyond compartmental boundary formation. We performed an unbiased screening approach to identify cell surface receptors required for actomyosin enrichment and polarisation at parasegmental boundaries (PSBs) in early Drosophila embryos, from the start of germband extension at gastrulation and throughout the germband extended stages (stages 6 to 11). First, we find that Tartan is required during germband extension for actomyosin enrichment at PSBs, confirming an earlier report. Next, by following in real time the dynamics of loss of boundary straightness in tartan mutant embryos compared with wild-type and ftz mutant embryos, we show that Tartan is required during germband extension but not beyond. We identify candidate genes that could take over from Tartan at PSBs and confirm that at germband extended stages, actomyosin enrichment at PSBs requires Wingless signalling.

Expanding the boundaries of synthetic development

Embryonic tissue boundaries are critical to not only cement newly patterned structures during development, but also to serve as organizing centers for subsequent rounds of morphogenesis. ​Although this latter role is especially difficult to study in vivo, synthetic embryology offers a new vantage point and fresh opportunities. In this review, we cover recent progress towards understanding and controlling in vitro boundaries and how they impact synthetic model systems. A key point this survey highlights is that the outcome of self-organization is strongly dependent on the boundary imposed, and new insight into the complex functions of embryonic boundaries will be necessary to create better self-organizing tissues for basic science, drug development, and regenerative medicine.