Mechanical Patterning in Animal Morphogenesis

Interplay between Notch signaling and mechanical forces during developmental patterning processes

The coordination between biochemical signals and cell mechanics has emerged in recent years as a crucial mechanism driving developmental patterning processes across a variety of developing and homeostatic systems. An important class of such developmental processes relies on local communication between neighboring cells through Notch signaling. Here, we review how the coordination between Notch-mediated differentiation and cell mechanics can give rise to unique cellular patterns. We discuss how global and local mechanical cues can affect, and be affected by, cellular differentiation and reorganization controlled by Notch signaling. We compare recent studies of such developmental processes, including the mammalian inner ear, Drosophila ommatidia, intestinal organoids, and zebrafish myocardium, to draw shared general concepts and their broader implications in biology.

The mechanical journey of primordial germ cells

Primordial germ cells (PGCs) are the earliest progenitors of germline cells of the gonads in animals. The tissues that arise from primordial germ cells give rise to male and female gametes and are thus responsible for transmitting genetic information to subsequent generations. The development of gonads, from single cells to fully formed organs, is of great interest to the reproductive biology community. In most higher animals, PGCs are initially specified at a site away from the gonads. They then migrate across multiple tissue microenvironments to reach a mesodermal mass of cells called the genital ridge, where they associate with somatic cells to form sex-specific reproductive organs. Their migratory behavior has been studied extensively to identify which tissues they interact with and how this might affect gonad development. A crucial point overlooked by classical studies has been the physical environment experienced by PGCs as they migrate and the mechanical challenges they might encounter along the way. It has long been understood that migrating cells can sense and adapt to physical forces around them via a variety of mechanisms, and studies have shown that these mechanical signals can guide stem cell fate. In this review, we summarize the mechanical microenvironment of migrating PGCs in different organisms. We describe how cells can adapt to this environment and how this adaptation can influence cell fate. Finally, we propose that mechanical signals play a crucial role in the normal development of the germline and shed light on this unexplored area of developmental biology.

Mechanochemical topological defects in an active nematic

We propose a reaction-diffusion system that converts topological information of an active nematic into chemical signals. We show that a curvature-activated reaction dipole is sufficient for creating a system that dynamically senses topology by producing a concentration field possessing local extrema coinciding with ±1/2 defects. The enabling term is analogous to polarization charge density seen in dielectric materials. We demonstrate the ability of this system to identify defects in both passive and active nematics. Our results illustrate that a relatively simple feedback scheme, expressed as a system of partial differential equations, is capable of producing chemical signals in response to inherently nonlocal structures in anisotropic media. We posit that such coarse-grained systems can help generate testable hypotheses for regulated processes in biological systems, such as morphogenesis, and motivate the creation of bio-inspired materials that utilize dynamic coupling between nematic structure and biochemistry.

Control of Modular Tissue Flows Shaping the Embryo in Avian Gastrulation

Avian gastrulation requires coordinated flows of thousands of cells to form the body plan. We quantified these flows using their fundamental kinematic units: one attractor and two repellers constituting its Dynamic Morphoskeleton (DM). We have also elucidated the mechanistic origin of the attractor, marking the primitive streak (PS), and controlled its shape, inducing gastrulation flows in the chick embryo that are typical of other vertebrates. However, the origins of repellers and dynamic embryo shape remain unclear. Here, we address these questions using active matter physics and experiments. Repeller 1, separating the embryo proper (EP) from extraembryonic (EE) tissues, arises from the tug-of-war between EE epiboly and EP isotropic myosin-induced active stress. Repeller 2, bisecting the anterior and posterior PS and associated with embryo shape change, arises from anisotropic myosin-induced active intercalation in the mesendoderm. Combining mechanical confinement with inhibition of mesendoderm induction, we eliminated either one or both repellers, as predicted by our model. Our results reveal a remarkable modularity of avian gastrulation flows delineated by the DM, uncovering the mechanistic roles of EE epiboly, EP active constriction, mesendoderm intercalation and ingression. These findings offer a new perspective for deconstructing morphogenetic flows, uncovering their modular origin, and aiding synthetic morphogenesis.

Creative processes during vertebrate organ morphogenesis: Biophysical self-organization at the supracellular scale

Here, we review recent developments in the literature that provide insight into self-organization at supracellular scales in vertebrate organ morphogenesis. We briefly present a historical and conceptual analysis of the term "self-organization." Based on this analysis, we suggest that self-organizing processes, at their root, possess a form of causal relationship, reciprocal causality, that is markedly distinct from linear causal chains. We survey the extent to which reciprocal causality can be used to interpret or clarify supracellular studies in development and disease. Finally, we explore how reciprocal causality can exist across length-scales, identifying situations where multiple scales require simultaneous analysis.

Multiple feedbacks on self-organized morphogenesis during plant regeneration

Decades of research have primarily emphasized genetic blueprint as the driving force behind plant regeneration. The flow of information from genetics, which manifests as biochemical properties, including hormones, has been extensively implicated in plant regeneration. However, recent advancements have unveiled additional intrinsic modules within this information flow. Here, we explore the three core modules of plant regeneration: biochemical properties, mechanical forces acting on cells, and cell geometry. We debate their roles and interactions during morphogenesis, emphasizing the potential for multiple feedbacks between these core modules to drive pattern formation during regeneration. We propose that de novo organ regeneration is a self-organized event driven by multidirectional information flow between these core modules.

A mechanochemical model recapitulates distinct vertebrate gastrulation modes

During vertebrate gastrulation, an embryo transforms from a layer of epithelial cells into a multilayered gastrula. This process requires the coordinated movements of hundreds to tens of thousands of cells, depending on the organism. In the chick embryo, patterns of actomyosin cables spanning several cells drive coordinated tissue flows. Here, we derive a minimal theoretical framework that couples actomyosin activity to global tissue flows. Our model predicts the onset and development of gastrulation flows in normal and experimentally perturbed chick embryos, mimicking different gastrulation modes as an active stress instability. Varying initial conditions and a parameter associated with active cell ingression, our model recapitulates distinct vertebrate gastrulation morphologies, consistent with recently published experiments in the chick embryo. Altogether, our results show how changes in the patterning of critical cell behaviors associated with different force-generating mechanisms contribute to distinct vertebrate gastrulation modes via a self-organizing mechanochemical process.

Forceful patterning: theoretical principles of mechanochemical pattern formation

Biological pattern formation is essential for generating and maintaining spatial structures from the scale of a single cell to tissues and even collections of organisms. Besides biochemical interactions, there is an important role for mechanical and geometrical features in the generation of patterns. We review the theoretical principles underlying different types of mechanochemical pattern formation across spatial scales and levels of biological organization.

Tuneable defect-curvature coupling and topological transitions in active shells

Recent experimental observations have suggested that topological defects can facilitate the creation of sharp features in developing embryos. Whereas these observations echo established knowledge about the interplay between geometry and topology in two-dimensional passive liquid crystals, the role of activity has mostly remained unexplored. In this article we focus on deformable shells consisting of either polar or nematic active liquid crystals and demonstrate that activity renders the mechanical coupling between defects and curvature much more involved and versatile than previously thought. Using a combination of linear stability analysis and three-dimensional computational fluid dynamics, we demonstrate that such a coupling can in fact be tuned, depending on the type of liquid crystal order, the specific structure of the defect (i.e. asters or vortices) and the nature of the active forces. In polar systems, this can drive a spectacular transition from spherical to toroidal topology, in the presence of large extensile activity. Our analysis strengthens the idea that defects could serve as topological morphogens and provides a number of predictions that could be tested in in vitro studies, for instance in the context of organoids.

Tissue interplay during morphogenesis

The process by which biological systems such as cells, tissues and organisms acquire shape has been named as morphogenesis and it is central to a plethora of biological contexts including embryo development, wound healing, or even cancer. Morphogenesis relies in both self-organising properties of the system and in environmental inputs (biochemical and biophysical). The classical view of morphogenesis is based on the study of external biochemical molecules, such as morphogens. However, recent studies are establishing that the mechanical environment is also used by cells to communicate within tissues, suggesting that this mechanical crosstalk is essential to synchronise morphogenetic transitions and self-organisation. In this article we discuss how tissue interaction drive robust morphogenesis, starting from a classical biochemical view, to finalise with more recent advances on how the biophysical properties of a tissue feedback with their surroundings to allow form acquisition. We also comment on how in silico models aid to integrate and predict changes in cell and tissue behaviour. Finally, considering recent advances from the developmental biomechanics field showing that mechanical inputs work as cues that promote morphogenesis, we invite to revisit the concept of morphogen.

Precise alternating cellular pattern in the inner ear by coordinated hopping intercalations and delaminations

The mammalian hearing organ, the organ of Corti, is one of the most organized tissues in mammals. It contains a precisely positioned array of alternating sensory hair cells (HCs) and nonsensory supporting cells. How such precise alternating patterns emerge during embryonic development is not well understood. Here, we combine live imaging of mouse inner ear explants with hybrid mechano-regulatory models to identify the processes that underlie the formation of a single row of inner hair cells (IHCs). First, we identify a previously unobserved morphological transition, termed "hopping intercalation," that allows cells differentiating toward IHC fate to "hop" under the apical plane into their final position. Second, we show that out-of-row cells with low levels of the HC marker Atoh1 delaminate. Last, we show that differential adhesion between cell types contributes to straightening of the IHC row. Our results support a mechanism for precise patterning based on coordination between signaling and mechanical forces that is likely relevant for many developmental processes.

Reconstruction of distinct vertebrate gastrulation modes via modulation of key cell behaviors in the chick embryo

The morphology of gastrulation driving the internalization of the mesoderm and endoderm differs markedly among vertebrate species. It ranges from involution of epithelial sheets of cells through a circular blastopore in amphibians to ingression of mesenchymal cells through a primitive streak in amniotes. By targeting signaling pathways controlling critical cell behaviors in the chick embryo, we generated crescent- and ring-shaped mesendoderm territories in which cells can or cannot ingress. These alterations subvert the formation of the chick primitive streak into the gastrulation modes seen in amphibians, reptiles, and teleost fish. Our experimental manipulations are supported by a theoretical framework linking cellular behaviors to self-organized multicellular flows outlined in detail in the accompanying paper. Together, this suggests that the evolution of gastrulation movements is largely determined by changes in a few critical cell behaviors in the mesendoderm territory across different species and controlled by a relatively small number of signaling pathways.

Genome size drives morphological evolution in organ-specific ways

Morphogenesis is an emergent property of biochemical and cellular interactions during development. Genome size and the correlated trait of cell size can influence these interactions through effects on developmental rate and tissue geometry, ultimately driving the evolution of morphology. We tested whether variation in genome and body size is related to morphological variation in the heart and liver using nine species of the salamander genus Plethodon (genome sizes 29-67 gigabases). Our results show that overall organ size is a function of body size, whereas tissue structure changes dramatically with evolutionary increases in genome size. In the heart, increased genome size is correlated with a reduction of myocardia in the ventricle, yielding proportionally less force-producing mass and greater intertrabecular space. In the liver, increased genome size is correlated with fewer and larger vascular structures, positioning hepatocytes farther from the circulatory vessels that transport key metabolites. Although these structural changes should have obvious impacts on organ function, their effects on organismal performance and fitness may be negligible because low metabolic rates in salamanders relax selective pressure on function of key metabolic organs. Overall, this study suggests large genome and cell size influence the developmental systems involved in heart and liver morphogenesis.