Session W08: Morphogenesis I

Mesenchymal cell motility is required for sacculation during lung development

During the final stages of embryonic lung development, the distal tips of the epithelial airways expand into the surrounding mesenchyme to generate multiple small sacs called saccules, which form the gas-exchange surfaces of the neonatal mouse lung. The cellular mechanisms that drive the dramatic changes in tissue morphology that accompany sacculation, including epithelial expansion and mesenchymal thinning, are poorly understood. Using tissue-specific knockout mice, we found experimentally that proteins associated with cell motility are required in the mesenchyme for normal lung sacculation. In this study, we build a computational model of sacculation to test our hypothesis that mesenchymal cell motility is required for epithelial expansion and mesenchymal thinning. By independently tuning both luminal pressure and mesenchymal motility, we show that the sacculation process requires mesenchymal cells to undergo neighbor exchanges. Despite the simplicity of the model, our numerical results recapitulate the morphologies of saccules observed in our wild-type embryos (high luminal pressure and high mesenchymal motility) and mutant embryos (high luminal pressure and low mesenchymal motility). The latter indicates that normal lung sacculation requires a motile, and thus more fluid population of mesenchymal cells surrounding the epithelium.   

How do cell intrinsic cues and external signals interplay with cell motility to affect cellular organization?

Multicellular bodies are organized into many levels, such as tissues and organs. The manner in which cellular organization is achieved is different in plants and fungi, which have immotile cells, as compared to animals where cells can migrate. In general, cellular organization requires cells to decide whether to divide, differentiate or migrate in response to chemical and mechanical cues. But cells vary in the extent to which they respond to cell intrinsic versus extrinsic cues. For example, early embryonic cells in invertebrates depend completely on cell intrinsic cues, whereas cells in sea urchin blastomeres interact extensively with neighboring cells. Here, we describe a rule based model to investigate how the relative dependence of cellular decisions on intrinsic versus extrinsic cues affects cellular organization, and how is the effect different in organisms with motile versus non-motile cells.

    

Learning About Teeth: Data-driven Continuum Models of Amelogenesis

Amelogenesis, the process by which the enamel layer of a tooth is formed, features the robust coordination of the collective motion of epithelial cells over a multi-year time-span and a length-scale vastly exceeding individual ameloblast size. Based on a minimal set of assumptions about the behavior of individual cells, we develop a coarse-grained continuum model of epithelial front motion and enamel deposition, which we solve using recently developed deep learning techniques for inverse problems. We validate our simulations by replicating several empirical features using histological sections of six clinically extracted upper premolars from the United Kingdom. Our modeling approach allows us to quantitatively evaluate competing theories on the regulatory mechanisms required for reliable tooth formation. Finally, we discuss how investigating internal and incremental connections between ameloblast movement, perikymata distributions, enamel thickness, and overall enamel crown shape allows us to significantly improve our ability to interpret developmental variables in fossil hominin teeth.   

Anisotropic Growth in the presence of Noise and Mechanical Feedback in Elastic Tissues

Growth processes in biology are generally anisotropic and noisy. In elastic tissues, growth inhomogeneities induced by noise generate mechanical stresses. The mechanical stresses feed into the growth process and regulate it such that consequent growth relieves these stresses. How do closed curves, or clone boundaries, in the tissue evolve under such feedback controlled noisy growth? How does anisotropy in the background growth reflect in these shapes? We model a growing elastic tissue in two dimensions using continuum mechanics, and study the effects of noise and mechanical feedback on a background of exponential anisotropic growth. We examine some subtleties associated with setting up a perturbative treatment for noise that arise from anisotropy of the background growth, and discuss the implications of growth anisotropy on the evolution of clone boundaries.   

A morphogen gradient orchestrates pattern-preserving tissue morphogenesis via motility-driven (un) jamming

Embryo development requires biochemical signalling to generate patterns of cell fates and active mechanical forces to drive tissue shape changes. However, how these processes are coordinated, and how tissue patterning is preserved despite the cellular flows occurring during morphogenesis, remains poorly understood. Gastrulation is a crucial embryonic stage that involves both patterning and internalization of the mesendoderm germ layer tissue. Here we show that, in zebrafish embryos, a gradient in Nodal signalling orchestrates pattern-preserving internalization movements by triggering a motility-driven unjamming transition. In addition to its role as a morphogen determining embryo patterning, graded Nodal signalling mechanically subdivides the mesendoderm into a small fraction of highly protrusive leader cells, able to autonomously internalize via local unjamming, and less protrusive followers, which need to be pulled inwards by the leaders. The Nodal gradient further enforces a code of preferential adhesion coupling leaders to their immediate followers, resulting in a collective and ordered mode of internalization that preserves mesendoderm patterning. Integrating this dual mechanical role of Nodal signalling into minimal active particle simulations quantitatively predicts both physiological and experimentally perturbed internalization movements. This provides a quantitative framework for how a morphogen-encoded unjamming transition can bidirectionally couple tissue mechanics with patterning during complex three-dimensional morphogenesis.   

Cell Division and Active Motility Drive Hexatic Order in Biological Tissues

During many fundamental physiological events, biological tissues undergo a transition from a solid-like state to a liquid-like state. Recent experimental observations further suggest that in 2D epithelial tissues, these solid-fluid transitions can happen via intermediate states akin to the intermediate hexatic phases observed during two-dimensional melting. The hexatic phase is characterized by a quasi-long-range orientational order without any translational order. While it has been shown that hexatic order in tissue models can be induced by active motility and thermal fluctuations, the role of cell division and apoptosis has not been explored, despite its importance in driving physiological events. In this work, we study the effect of cell division and apoptosis on global hexatic order within the framework of the self-propelled Voronoi model. Although cell division naively destroys order, and active propulsion facilitates deformations, their competition drives the transition from liquid to hexatic to liquid, as the active speed increases. The hexatic phase is promoted by the balance of defect generation and healing.   

The cellular mechanics of cephalic furrow formation in the Drosophila embryo investigated using an advanced vertex model

Cephalic furrow formation (CFF) is one of the major morphogenetic movements in the Drosophila (fruit fly) embryo.  The furrow initially develops at the lateral sides of the embryo and then extends over the entire embryo perimeter.  An analysis of the cell shape changes that occur in the active region of the epithelial cell layer reveals that the CFF occurs in two distinct phases: the initiation and progression phases.  In the initiation phase, the epithelial fold starts to invaginate into the yolk sac and during the progression phase, additional pairs of cells are gradually added to the furrow. While the geometry of the cell shape changes during these processes was imaged in detail, the underlying mechanical forces and stresses

are poorly understood.  To investigate the invagination mechanics, we have developed an advanced vertex model that takes into account experimentally observed membrane curvature and relates it to a pressure jump across the membrane.  Our model shows that the cell shape sequences observed in the progression phase can be fully explained by a corresponding succession of cell membrane length changes and the corresponding time variation of the membrane tension and cell pressure.  However, the initiation of the CFF requires an additional inward force associated with the tension that develops along the curved line of the furrow cleft.   

A mechanical finger-trap mechanism controls cell width in Bacillus subtilis

The Gram-positive bacteria B.subtilis retains its rod shape during growth, meaning that the cell elongates as maintaining the same width. In this organism, the cytoplasmatic turgor pressure promotes cell elongation by enlarging the cell membrane, thus, the cell wall. Despite the force generated by the turgor pressure, cell width remains constant. In bacteria, the cell wall determines cell shape. This structure is formed of circumferentially oriented stiff glycans crosslinked by soft peptides. It is known that the constitutive properties of the cell wall are fundamental to control the width. However, we observed that during the hydrolysis of the cell wall, which is essential for growth, the cell length and width increase in response to hydrolases cleavage, indicating that an alternative mechanism to counteract the turgor pressure must exist. By studying the mechanical properties of the cell wall using osmotic shocks, we found that the cell responded to an increase in turgor pressure with longitudinal stretching and circumferential compaction. Vice versa, when the pressure decreased, the cell shrank and swelled in diameter, similar to a finger trap. These data suggested that the compaction forces to counteract the turgor pressure are generated in the cell wall. Accordingly, when we genetically reduced the synthesis of new peptidoglycan, the cell could not compact in response to increasing pressure. We conclude that the synthesis of new peptidoglycan prevents swelling of the diameter during growth.    

Receptor promiscuity and feedback control mediate accurate and robust encoding and decoding of positional information during morphogenesis

Spatial profiles of morphogens provide positional information in a developing tissue during morphogenesis. The cells read the local morphogen concentrations via receptors and process this information via signalling mechanisms before inferring their position in terms of a transcriptional readout. However, the processes of binding to receptors and their internalisation by signalling mechanisms also mediate the dynamics of the morphogen profile through the effect on transport and degradation/removal of morphogen molecules. In this study, we show how receptor promiscuity and feedback control on receptors can act as local cell autonomous control mechanisms that allow for sculpting of morphogen profiles in tandem with accurate and robust positional inference. In effect, the same cellular processes provide robustness to encoding and decoding of positional information in morphogens simultaneously.   

Control of tissue fluidity by optogenetic manipulation of actomyosin

Epithelial tissue sheets can be shaped into complex forms through series of stretching, folding and flowing events, driven by myosin-generated forces. However, it is not well understood how different patterns of tension can influence tissue mechanical properties and cell behaviors that contribute to rapid tissue deformations. Here we use optogenetic tools to manipulate actomyosin contractility in the germband epithelium, which exhibits rapid flow during Drosophila body axis elongation. The ability to flexibly induce changes in myosin contractility allows us to analyze the effects of distinct myosin patterns on cell rearrangements, tissue tensions, and tissue mechanical properties. We find that either optogenetic activation or deactivation of actomyosin at the apical surface of the tissue disrupts tissue tension anisotropy and cell packings, leading to fewer cell rearrangements and reduced tissue-level flow. These results directly link the distribution of myosin II to tissue tension and cell packings and suggest that actomyosin influences not only the anisotropic forces that drive tissue flow but also the mechanical properties of the tissue, leading to a complex relationship between cell-generated forces and tissue fluidity.

    

An action principle for the morphogenesis of thin sheets

How does growth encode form in developing organisms? Many different spatiotemporal growth profiles may sculpt 2D epithelial sheets into the same target 3D shapes, but only specific growth patterns are observed in animal and plant development. The criteria that select for these stereotypic growth patterns and the ubiquity of anisotropic growth remain poorly understood. We propose that nature settles on the 'simplest' growth patterns. Using the geometric formalism of quasiconformal transformations, we demonstrate that growth pattern selection can be formulated as an optimization problem and solved for the trajectories that minimize spatiotemporal variation in areal growth rates and deformation anisotropy.  The result is a complete prediction for the growth of the surface, including not only a set of intermediate shapes, but also a prediction for how cells flow along those surfaces. Optimization of growth trajectories for both idealized surfaces and experimentally acquired data show that relative growth rates can be uniformized at the cost of introducing anisotropy. Minimizing complexity can therefore be viewed as a generic mechanism for growth pattern selection and may help to understand the prevalence of anisotropy in developmental programs. Application to appendage outgrowth in the crustacean Parhyale hawaiensis generates dynamic developmental trajectories that are consistent with experimentally observed growth patterns.   

Hatching on a budget: The economized energy expenditure of beetle embryos

Oviparous animals are a biological example of a thermodynamically quasi-isolated system. Additional to the genetic code, most energy and matter necessary for embryo development is stored inside the egg from the time point of fertilization up to the time point of hatching. It remains controversial how energy is stored and transferred, especially in quiescent periods between large-scale deformations. Here, we study extra-embryonic tissue dynamics in two key processes during embryo development, gastrulation and dorsal closure, in the serosa membrane of the red flour beetle Tribolium castaneum. We argue that energy in the serosa tissue is close-to-fully conserved between serosa window closure during gastrulation and dorsal closure. We identify active and passive processes with sub-cellular resolution using 3D non-invasive velocity and stress estimation techniques. We localize the point of origin of forces driving serosa tissue rupture and retraction. We believe that this study of extra-embryonic tissue dynamics signifies the importance of studying both mechanics and thermodynamics in embryogenesis.