Tissue rheology in embryonic organization

Tissue morphogenesis in multicellular organisms is brought about by spatiotemporal coordination of mechanical and chemical signals. Extensive work on how mechanical forces together with the well‐established morphogen signalling pathways can actively shape living tissues has revealed evolutionary conserved mechanochemical features of embryonic development. More recently, attention has been drawn to the description of tissue material properties and how they can influence certain morphogenetic processes. Interestingly, besides the role of tissue material properties in determining how much tissues deform in response to force application, there is increasing theoretical and experimental evidence, suggesting that tissue material properties can abruptly and drastically change in development. These changes resemble phase transitions, pointing at the intriguing possibility that important morphogenetic processes in development, such as symmetry breaking and self‐organization, might be mediated by tissue phase transitions. In this review, we summarize recent findings on the regulation and role of tissue material properties in the context of the developing embryo. We posit that abrupt changes of tissue rheological properties may have important implications in maintaining the balance between robustness and adaptability during embryonic development

Cell Migration with Multiple Pseudopodia: Temporal and Spatial Sensing Models

Cell migration triggered by pseudopodia (or “false feet”) is the most used method of locomotion. A 3D finite element model of a cell migrating over a 2D substrate is proposed, with a particular focus on the mechanical aspects of the biological phenomenon. The decomposition of the deformation gradient is used to reproduce the cyclic phases of protrusion and contraction of the cell, which are tightly synchronized with the adhesion forces at the back and at the front of the cell, respectively. First, a steady active deformation is considered to show the ability of the cell to simultaneously initiate multiple pseudopodia. Here, randomness is considered as a key aspect, which controls both the direction and the amplitude of the false feet. Second, the migration process is described through two different strategies: the temporal and the spatial sensing models. In the temporal model, the cell “sniffs” the surroundings by extending several pseudopodia and only the one that receives a positive input will become the new leading edge, while the others retract. In the spatial model instead, the cell senses the external sources at different spots of the membrane and only protrudes one pseudopod in the direction of the most attractive one

The impact of cellular characteristics on the evolution of shape homeostasis

The importance of individual cells in a developing multicellular organism is well known but precisely how the individual cellular characteristics of those cells collectively drive the emergence of robust, homeostatic structures is less well understood. For example cell communication via a diffusible factor allows for information to travel across large distances within the population, and cell polarisation makes it possible to form structures with a particular orientation, but how do these processes interact to produce a more robust and regulated structure? In this study we investigate the ability of cells with different cellular characteristics to grow and maintain homeostatic structures. We do this in the context of an individual-based model where cell behaviour is driven by an intra-cellular network that determines the cell phenotype. More precisely, we investigated evolution with 96 different permutations of our model, where cell motility, cell death, long-range growth factor (LGF), short-range growth factor (SGF) and cell polarisation were either present or absent. The results show that LGF has the largest positive impact on the fitness of the evolved solutions. SGF and polarisation also contribute, but all other capabilities essentially increase the search space, effectively making it more difficult to achieve a solution. By perturbing the evolved solutions, we found that they are highly robust to both mutations and wounding. In addition, we observed that by evolving solutions in more unstable environments they produce structures that were more robust and adaptive. In conclusion, our results suggest that robust collective behaviour is most likely to evolve when cells are endowed with long range communication, cell polarisation, and selection pressure from an unstable environment

Deciphering cell motility and spatial sensing of intestinal cell types using an ex vivo intestinal model

The intestine is a highly organized tissue with two distinct regions: the crypt and the villus. When stem cells divide at the crypt bottom, half of their progeny migrates upwards towards the villus, where they differentiate into various cell types, including the abundant absorptive enterocytes. However, the precise mechanisms governing this migration and tissue organization remain poorly understood. In this thesis, novel methodologies, such as long-term intravital imaging and decellularization of mouse intestine, are used to study cell type-specific motility within the tissue architecture. Moreover, work in this thesis probes the mechanisms mediating intestinal regeneration and aging, and the clonal competition during tumor development. In paper 1, we employ long-term intravital imaging to identify a greater number of longterm functioning intestinal stem cells (ISCs) in the small intestine compared to the colon. We further investigate this phenomenon by combining intravital imaging and the novel ex vivo live cell imaging assay to discover that stem cells in the small intestine display downward motility directed by Wnt-ligands. In Paper 2, the ex vivo live cell imaging assay was utilized to investigate active cell migration in several cell types. Our findings reveal that both ISCs and paneth cells possess an intrinsic ability to perceive positional cues embedded in the extracellular matrix (ECM), which guides them to their native location, the crypt. In contrast, enterocytes, lack this capability. Finally, we discovered that during aging ECM loses the signals guiding crypt homing of ISCs, and that the tumor-causing mutations render cells insensitive to ECM signals resulting in loss of crypt homing. In Paper 3, we introduce an optimized intestinal decellularization protocol and demonstrate its capacity to regenerate the intestinal epithelium from single-seeded stem cells, freshly isolated crypts, or organoids. During regeneration following damage, we discovered mesenchymally produced Asporin, which promotes Tgfβ-signaling and induces fetal-like reprogramming in intestinal tissue. Additionally, we observed that chronic upregulation of Asporin in the aged intestinal tissue hampers tissue repair. In Paper 4, we elucidate how Apc-mutant ISCs gain a clonal advantage over wild-type ISCs. We reveal that Apc-mutant ISCs secrete the Wnt-inhibitor Notum, which reduces the stemness and competitiveness of wild-type ISCs. Inhibition of Notum reverted the clonal advantage of Apc-mutant cells and reduced tumor burden. In conclusion, this thesis focused on highlighting the interplay between intestinal epithelial cells and the ECM, particularly the ability of ISCs and paneth cells to sense positional cues embedded in the ECM, guiding them to their native location. Additionally, key mechanisms disrupted during aging and in intestinal cancer are elucidated

Rules of collective migration: from the wildebeest to the neural crest

Collective migration, the movement of groups in which individuals affect the behaviour of one another, occurs at practically every scale, from bacteria up to whole species' populations. Universal principles of collective movement can be applied at all levels. In this review, we will describe the rules governing collective motility, with a specific focus on the neural crest, an embryonic stem cell population that undergoes extensive collective migration during development. We will discuss how the underlying principles of individual cell behaviour, and those that emerge from a supracellular scale, can explain collective migration. This article is part of the theme issue 'Multi-scale analysis and modelling of collective migration in biological systems'

Overlooked? Underestimated? Effects of Substrate Curvature on Cell Behavior

In biological systems, form and function are inherently correlated. Despite this strong interdependence, the biological effect of curvature has been largely overlooked or underestimated, and consequently it has rarely been considered in the design of new cell–material interfaces. This review summarizes current understanding of the interplay between the curvature of a cell substrate and the related morphological and functional cellular response. In this context, we also discuss what is currently known about how, in the process of such a response, cells recognize curvature and accordingly reshape their membrane. Beyond this, we highlight state-of-the-art microtechnologies for engineering curved biomaterials at cell-scale, and describe aspects that impair or improve readouts of the pure effect of curvature on cells

Complexity in Developmental Systems: Toward an Integrated Understanding of Organ Formation

During animal development, embryonic cells assemble into intricately structured organs by working together in organized groups capable of implementing tightly coordinated collective behaviors, including patterning, morphogenesis and migration. Although many of the molecular components and basic mechanisms underlying such collective phenomena are known, the complexity emerging from their interplay still represents a major challenge for developmental biology. Here, we first clarify the nature of this challenge and outline three key strategies for addressing it: precision perturbation, synthetic developmental biology, and data-driven inference. We then present the results of our effort to develop a set of tools rooted in two of these strategies and to apply them to uncover new mechanisms and principles underlying the coordination of collective cell behaviors during organogenesis, using the zebrafish posterior lateral line primordium as a model system. To enable precision perturbation of migration and morphogenesis, we sought to adapt optogenetic tools to control chemokine and actin signaling. This endeavor proved far from trivial and we were ultimately unable to derive functional optogenetic constructs. However, our work toward this goal led to a useful new way of perturbing cortical contractility, which in turn revealed a potential role for cell surface tension in lateral line organogenesis. Independently, we hypothesized that the lateral line primordium might employ plithotaxis to coordinate organ formation with collective migration. We tested this hypothesis using a novel optical tool that allows targeted arrest of cell migration, finding that contrary to previous assumptions plithotaxis does not substantially contribute to primordium guidance. Finally, we developed a computational framework for automated single-cell segmentation, latent feature extraction and quantitative analysis of cellular architecture. We identified the key factors defining shape heterogeneity across primordium cells and went on to use this shape space as a reference for mapping the results of multiple experiments into a quantitative atlas of primordium cell architecture. We also propose a number of data-driven approaches to help bridge the gap from big data to mechanistic models. Overall, this study presents several conceptual and methodological advances toward an integrated understanding of complex multi-cellular systems

Self-organized collective cell behaviors as design principles for synthetic developmental biology

Over the past two decades, molecular cell biology has graduated from a mostly analytic science to one with substantial synthetic capability. This success is built on a deep understanding of the structure and function of biomolecules and molecular mechanisms. For synthetic biology to achieve similar success at the scale of tissues and organs, an equally deep understanding of the principles of development is required. Here, we review some of the central concepts and recent progress in tissue patterning, morphogenesis and collective cell migration and discuss their value for synthetic developmental biology, emphasizing in particular the power of (guided) self-organization and the role of theoretical advances in making developmental insights applicable in synthesis

Molecular Basis of Vertebrate Embryonic Migration

Embryology aims at understanding how a single fertilized cell develops into a complex multicellular organism. Initially the embryo is no more than a ball of cells where the three primordial layers, the ectoderm, mesoderm and the endoderm are one on top of the other. The three germ layers will go on to form all the tissues and organs of the embryo. For example, the ectoderm will give rise to epidermis and the nervous system; the mesoderm to muscles, the skeletal system, the dermis or inner layer of the skin, the circulatory, excretory, and reproductive systems; and, finally, the endoderm will give rise to the inner lining of the alimentary canal and the structures derived from it, such as lungs, the liver, pancreas, and the bladder. Correct positioning of the germ layers paves the way for the inductive interactions that are the hallmark of both axis determination and organogenesis. Therefore, the formation of the body plan requires highly integrated and regulated cell movements. The study of these movements is central to the field of embryology.Historically, the amphibian gastrula became one of the predominant models for experimental embryologists. This was partially due to the major influence of studies that lead to the eventual discovery of the organizer by Spemann and Mangold in 1924. After decades of research there is an imposing literature on the subject of inductive interactions in the amphibian and other embryos but the investigation of the movements leading from the relatively simple architecture of a blastula to the advanced and highly complex architecture of the late gastrula has been lacking. Perhaps it is not surprising, given the difficulties of studying these movements, that after almost a century of research fundamental questions still have not been answered. Haeckel first proposed the name gastrula in 1872, and although there was a long debate concerning the movements leading to the formation of the gastrula structures, the first experimental evidence for epibolic and inward morphogenetic movements was provided by Kopsch in 1895. Epiboly refers to the intercalation of cells in the animal cap (Figure IA, B and D ) while inward movements are the movements that lead to the internalization of the mesoderm, which is now referred to as involution (Figure 1 compares the location of the orange colored mesoderm at F stage 9 with that of G stage 10). The morphogenetic movements involved in gastrulation were later described by Vogt (1925, 1929) and then studied by Holtfreter (1943, 1944) and, more recently, by Keller(Gerhart and Keller 1986; Keller 1991; Weliky, Minsuk et al. 1991; Wilson and Keller 1991; Keller, Shih et al. 1992). The vast majority of this work was descriptive and only recently have we begun to gain some molecular insight regarding the pathways that are involved in specific morphogenetic movements