Biology subjects

Bal, P. K.

Publications and source records attributed to Bal, P. K..

3 recordsLinked to original sources

bioRxiv2025

Multiscale wrinkling dynamics in epithelial shells

Thin shells buckle and wrinkle when compressed. While this behavior is generally detrimental in engineering, it has been widely implicated in epithelial morphogenesis and patterning during development. Yet the rules governing buckling of active viscoelastic shells like epithelia remain unclear. Here we delineate those rules by combining an experimental system that allows us to sculpt epithelial shells and subject them to controlled deflation with a 3D computational model linking cytoskeletal dynamics to tissue mechanics. Experiments and simulations across several orders of magnitude in time and space reveal that buckling emerges for fast deflation relative to the cortexs relaxation time, and is suppressed by high contractility. We show, further, that the tissue develops wrinkle patterns with different degrees of symmetry breaking that depend on its size and viscous confinement. Strikingly, we find that epithelial buckling is a multiscale phenomenon involving long-lived supracellular folds but also short-lived subcellular wrinkles in the actin cortex. Finally, by forming epithelial shells with anisotropic curvature we rationally direct buckling into predictable wrinkle patterns. Our study shows that epithelial tissues can be understood as hierarchical materials with mechanical instabilities that can be harnessed to engineer epithelial morphogenesis.

biophysics↗

bioRxiv2025

Guidance of cellular nematics into shape-programmable living surfaces

Engineering living materials capable of autonomously morphing into predetermined shapes holds great potential for applications ranging from synthetic tissue morphogenesis to soft robotics. In this regard, harnessing the inherent ability of cellular tissues to self-organize and generate robust force fields offers a promising strategy for creating self-shaping living surfaces. However, achieving precise control over tissue mechanics to direct specific morphogenetic outcomes remains a challenge. Here, we introduce a strategy to program tissue shape transformations through the nematic organization of cellular forces. By precisely controlling nematic order and topological defects, we generate millimeter-scale tissues programmed with specific multicellular tension fields. Using a theoretical framework that integrates contractile nematics with thin sheet mechanics, we explore the role of nematically guided tensions in shape morphogenesis. Experimentally, upon tissue detachment, nematically guided tension fields drive out-of-plane deformations, generating reproducible 3D shapes. By integrating tissue contractility and nematic patterning, our approach offers a robust framework for the rational design of shape-programmable living surfaces.

biophysics↗

bioRxiv2025

Continuum theory for the mechanics of curved epithelial shells by coarse-graining an ensemble of active gel cellular surfaces

Epithelial tissues undergo complex morphogenetic transformations driven by cellular and cytoskeletal dynamics. To understand the emergent tissue mechanics resulting from sub-cellular mechanisms, we formulate a fully nonlinear continuum theory for epithelial shells that coarse-grains an underlying 3D vertex model, whose surfaces are in turn patches of active viscoelastic gel undergoing turnover. Our theory relies on two ingredients. First, we relate the deformation of apical, basal and lateral surfaces of cells to the continuum deformation of the tissue mid-surface and a thickness director field. We explore two variants of the theory, a Cosserat theory accommodating through-thickness tilt of cells, and a Kirchhoff theory assuming that lateral cell surfaces remain perpendicular to the mid-surface. Second, by adopting a variational formalism of irreversible thermodynamics, we construct an effective Rayleighian functional of the tissue constrained by the cellular-continuum kinematic relations, which therefore depends on continuum fields only. This functional allows us to obtain the governing equations of the continuum theory and is the basis for efficient finite element simulations. Verification against explicit 3D cellular model simulations demonstrates the accuracy of the proposed theory in capturing epithelial buckling dynamics. Furthermore, we show that the Cosserat theory is required to model tissues exhibiting apicobasal asymmetry of active tension. Our work provides a general frame-work for further studies integrating refined subcellular models into continuum descriptions of epithelial mechanobiology.

biophysics↗