Molecular and mechanical signals determine morphogenesis of the cerebral hemispheres in the chicken embryo

Measuring intraventricular pressure in developing mouse embryos: Uncovering a repetitive mechanical cue for brain development

In living organisms, including humans, the developmental processes that construct their morphology from a single fertilized egg are influenced not only by genetic regulation but also by various external factors. One such factor is mechanical stimulation. Although mechanical forces are suggested to contribute to brain formation during development, quantitative information on intraventricular pressure during neurogenesis remains limited. We developed a high time-resolution system efficiently using a piezoresistive sensor to measure brain intraventricular pressure in mouse embryos from E12.5 to E16.5 (embryonic stages in days). Ex utero measurements revealed intraventricular pressure increasing from 53.76 ± 4.16 Pa at E12.5 to 158.10 ± 19.94 Pa by E16.5. In utero analyses uncovered striking periodicity in sync with uterine contractions, reaching up to 1430 ± 195.2 Pa at E12.5, indicating dynamic mechanical stimuli beyond ex utero observations. Additionally, perforation experiments at E9.0-E15.5 showed rapid neuroepithelial thickening and apical surface contraction upon pressure release, indicative of a tensile effect by the positive intraventricular pressure. This effect diminished after E15.5, implying that tension wanes or the neuroepithelium becomes more robust. These results highlight the dynamic nature of embryonic intraventricular pressure, governed by internal fluid production and uterine forces, and emphasize the importance of mechanical cues in neuroepithelial architecture. Our findings provide a steppingstone to clarify how mechanical forces integrate with genetic and molecular processes to shape normal brain development and may render new perspectives on brain evolution.

Differential tissue deformability underlies fluid pressure-driven shape divergence of the avian embryonic brain and spinal cord

An enlarged brain underlies the complex central nervous system of vertebrates. The dramatic expansion of the brain that diverges its shape from the spinal cord follows neural tube closure during embryonic development. Here, we show that this differential deformation is encoded by a pre-pattern of tissue material properties in chicken embryos. Using magnetic droplets and atomic force microscopy, we demonstrate that the dorsal hindbrain is more fluid than the dorsal spinal cord, resulting in a thinning versus a resisting response to increasing lumen pressure, respectively. The dorsal hindbrain exhibits reduced apical actin and a disorganized laminin matrix consistent with tissue fluidization. Blocking the activity of neural-crest-associated matrix metalloproteinases inhibits hindbrain expansion. Transplanting dorsal hindbrain cells to the spinal cord can locally create an expanded brain-like morphology in some cases. Our findings raise questions in vertebrate head evolution and suggest a general role of mechanical pre-patterning in sculpting epithelial tubes.

Mechanical stress connects cortical folding to fiber organization in the developing brain

During development of the gyrencephalic brain, both the formation of cortical folds and the establishment of axonal tracts require large, coordinated mechanical deformations. Cortical folding enables a high ratio of cortical surface area to brain volume, which is thought to enhance overall processing power. Meanwhile, a complex network of axonal connections facilitates communication between distant brain regions. The mechanisms underlying the formation of brain folds and axon tract organization remain widely debated. However, evidence emerging from measurements of mechanical stress, combined with physical and mathematical models, suggests that constrained cortical expansion generates folds via mechanical instability. In this opinion article, we highlight recent models and experimental data suggesting that mechanical stress induced by cortical folding also mediates axonal growth. We propose a key role for mechanics in establishing brain morphology and the organization of white matter fascicles of the mature brain.

Anatomical and Embryological Development of the Chick Cerebrum in Different Embryonic Periods

The objective of this study is to assess the embryological and morphometric development of the chick cerebrum during specific incubation periods. The cerebrums of 24 Babcock White Leghorn chicks, six each from the 10th, 13th, 16th and 21st days of the incubation period, were used in the study. After removing the heads of fixed embryos from the upper edge of the atlas, the brains were taken out of the cranial cavity. Morphometric measurements were performed on the removed brains, and paraffin blocks were prepared following the routine histological procedure. Sections 5 µm thick were taken from the blocks, with an additional 10 µm thick section taken every 50th section. The slides were then stained using Crossmon's triple stain and Klüver-Barrera staining methods and photographed. The sectional images were transferred to the ImageJ programme, brain volume was calculated using stereological methods, and histological measurements were performed. The development of brain parts in selected embryonic periods was examined in detail, focusing on anatomical and histological aspects. According to the results, it was determined that all measured parameters, except the third ventricle width, increased and were statistically significant (p < 0.05). It is believed that the findings of this study will enhance the understanding of the region's anatomy. The new morphometric data can serve as reference data in neurotoxicity and embryotoxicity studies.

Zonal patterning of extracellular matrix and stromal cell populations along a perfusable cellular microchannel

The spatial organization of biophysical and biochemical cues in the extracellular matrix (ECM) in concert with reciprocal cell-cell signaling is vital to tissue patterning during development. However, elucidating the role an individual microenvironmental factor plays using existing in vivo models is difficult due to their inherent complexity. In this work, we have developed a microphysiological system to spatially pattern the biochemical, biophysical, and stromal cell composition of the ECM along an epithelialized 3D microchannel. This technique is adaptable to multiple hydrogel compositions and scalable to the number of zones patterned. We confirmed that the methodology to create distinct zones resulted in a continuous, annealed hydrogel with regional interfaces that did not hinder the transport of soluble molecules. Further, the interface between hydrogel regions did not disrupt microchannel structure, epithelial lumen formation, or media perfusion through an acellular or cellularized microchannel. Finally, we demonstrated spatially patterned tubulogenic sprouting of a continuous epithelial tube into the surrounding hydrogel confined to local regions with stromal cell populations, illustrating spatial control of cell-cell interactions and signaling gradients. This easy-to-use system has wide utility for modeling three-dimensional epithelial and endothelial tissue interactions with heterogeneous hydrogel compositions and/or stromal cell populations to investigate their mechanistic roles during development, homeostasis, or disease.

Expanding understanding of chick embryo's nervous system development at HH22-HH41 embryonic stages using X-ray microcomputed tomography

Assessing the embryotoxicity and teratogenicity of various substances and processes is crucial due to their complexity and resource intensity. The chicken embryo (CE) serves an ideal model for simulating the first months of mammalian embryonic development. This makes the CE a reliable model for testing teratogenic effects, particularly in relation to the nervous system (NS), which experiences developmental abnormalities second in frequency only to cardiovascular teratogenic disorders. Microcomputed tomography (μCT) is a promising method for studying these processes. The advantages of μCT include relatively high research speed, diagnostic accuracy, high resolution and the ability to visualize the entire internal 3D structure of an object while preserving for other types of research. At the same time, there are practically no available databases of normative μCT data, both qualitative and quantitative, which would act as a starting point for screening detection of abnormalities in the development of the NS. In this study, we present a simple method for obtaining very detailed quantitative sets of 2D and 3D μCT data of NS structures of the CE (Gallus Gallus domesticus) at HH22-HH41 embryonic stages with contrasting by 1% phosphotungstic acid. The results of μCT demonstrate the exact boundaries, high general and differentiated contrast of the main and specific structures of the NS of CE, which are quantitatively and qualitatively similar to results of histological analysis. Calculations of the X-ray density and volume of the main structures of the NS at constant exponential growth are presented. In addition to the increase in linear dimensions, significant changes in the structures of various parts of the brain were identified and visualized during the CE development at HH22 to HH41 embryonic stages. The data presented establish the first methodology for obtaining normative data, including subtle localized differences in the NS in CE embryogenesis. The data obtained open up new opportunities for modern embryology, teratology, pharmacology and toxicology.

Direct quantitative perturbations of physical parameters in vivo to elucidate vertebrate embryo morphogenesis

Physical parameters such as tissue interplay forces, luminal pressure, fluid flow, temperature, and electric fields are crucial regulators of embryonic morphogenesis. While significant attention has been given to cellular and molecular responses to these physical parameters, their roles in morphogenesis are not yet fully elucidated. This is largely due to a shortage of methods for spatiotemporal modulation and direct quantitative perturbation of physical parameters in embryos. Recent advancements addressing these challenges include microscopes equipped with devices to apply and adjust forces, direct perturbation of luminal pressure, and the application of micro-forces to targeted cells and cilia in vivo. These methods are critical for unveiling morphogenesis mechanisms, highlighting the importance of integrating molecular and physical approaches for a comprehensive understanding of morphogenesis.

Photo-induced changes in tissue stiffness alter epithelial budding morphogenesis in the embryonic lung

Extracellular matrix (ECM) stiffness has been shown to influence the differentiation of progenitor cells in culture, but a lack of tools to perturb the mechanical properties within intact embryonic organs has made it difficult to determine how changes in tissue stiffness influence organ patterning and morphogenesis. Photocrosslinking of the ECM has been successfully used to stiffen soft tissues, such as the cornea and skin, which are optically accessible, but this technique has not yet been applied to developing embryos. Here, we use photocrosslinking with Rose Bengal (RB) to locally and ectopically stiffen the pulmonary mesenchyme of explanted embryonic lungs cultured ex vivo . This change in mechanical properties was sufficient to suppress FGF-10-mediated budding morphogenesis along the embryonic airway, without negatively impacting patterns of cell proliferation or apoptosis. A computational model of airway branching was used to determine that FGF-10-induced buds form via a growth-induced buckling mechanism and that increased mesenchymal stiffness is sufficient to inhibit epithelial buckling. Taken together, our data demonstrate that photocrosslinking can be used to create regional differences in mechanical properties within intact embryonic organs and that these differences influence epithelial morphogenesis and patterning. Further, this photocrosslinking assay can be readily adapted to other developing tissues and model systems.

Zonal Patterning of Extracellular Matrix and Stromal Cell Populations Along a Perfusable Cellular Microchannel

The spatial organization of biophysical and biochemical cues in the extracellular matrix (ECM) in concert with reciprocal cell-cell signaling is vital to tissue patterning during development. However, elucidating the role an individual microenvironmental factor plays using existing in vivo models is difficult due to their inherent complexity. In this work, we have developed a microphysiological system to spatially pattern the biochemical, biophysical, and stromal cell composition of the ECM along an epithelialized 3D microchannel. This technique is adaptable to multiple hydrogel compositions and scalable to the number of zones patterned. We confirmed that the methodology to create distinct zones resulted in a continuous, annealed hydrogel with regional interfaces that did not hinder the transport of soluble molecules. Further, the interface between hydrogel regions did not disrupt microchannel structure, epithelial lumen formation, or media perfusion through an acellular or cellularized microchannel. Finally, we demonstrated spatially patterned tubulogenic sprouting of a continuous epithelial tube into the surrounding hydrogel confined to local regions with stromal cell populations, illustrating spatial control of cell-cell interactions and signaling gradients. This easy-to-use system has wide utility for modeling three-dimensional epithelial and endothelial tissue interactions with heterogeneous hydrogel compositions and/or stromal cell populations to investigate their mechanistic roles during development, homeostasis, or disease.

A chemo-mechanical model of endoderm movements driving elongation of the amniote hindgut

Although mechanical and biochemical descriptions of development are each essential, integration of upstream morphogenic cues with downstream tissue mechanics remains understudied during vertebrate morphogenesis. Here, we developed a two-dimensional chemo-mechanical model to investigate how mechanical properties of the endoderm and transport properties of fibroblast growth factor (FGF) regulate avian hindgut morphogenesis in a coordinated manner. Posterior endoderm cells convert a gradient of FGF ligands into a contractile force gradient, leading to a force imbalance that drives collective cell movements that elongate the forming hindgut tube. We formulated a 2D reaction-diffusion-advection model describing the formation of an FGF protein gradient as a result of posterior displacement of cells transcribing unstable Fgf8 mRNA during axis elongation, coupled with translation, diffusion and degradation of FGF protein. The endoderm was modeled as an active viscous fluid that generates contractile stresses in proportion to FGF concentration. With parameter values constrained by experimental data, the model replicates key aspects of hindgut morphogenesis, suggests that graded isotropic contraction is sufficient to generate large anisotropic cell movements, and provides new insight into how chemo-mechanical coupling across the mesoderm and endoderm coordinates hindgut elongation with axis elongation.

Cellular mechanotransduction in health and diseases: from molecular mechanism to therapeutic targets

Cellular mechanotransduction, a critical regulator of numerous biological processes, is the conversion from mechanical signals to biochemical signals regarding cell activities and metabolism. Typical mechanical cues in organisms include hydrostatic pressure, fluid shear stress, tensile force, extracellular matrix stiffness or tissue elasticity, and extracellular fluid viscosity. Mechanotransduction has been expected to trigger multiple biological processes, such as embryonic development, tissue repair and regeneration. However, prolonged excessive mechanical stimulation can result in pathological processes, such as multi-organ fibrosis, tumorigenesis, and cancer immunotherapy resistance. Although the associations between mechanical cues and normal tissue homeostasis or diseases have been identified, the regulatory mechanisms among different mechanical cues are not yet comprehensively illustrated, and no effective therapies are currently available targeting mechanical cue-related signaling. This review systematically summarizes the characteristics and regulatory mechanisms of typical mechanical cues in normal conditions and diseases with the updated evidence. The key effectors responding to mechanical stimulations are listed, such as Piezo channels, integrins, Yes-associated protein (YAP) /transcriptional coactivator with PDZ-binding motif (TAZ), and transient receptor potential vanilloid 4 (TRPV4). We also reviewed the key signaling pathways, therapeutic targets and cutting-edge clinical applications of diseases related to mechanical cues.

A chemo-mechanical model of endoderm movements driving elongation of the amniote hindgut

While mechanical and biochemical descriptions of development are each essential, integration of upstream morphogenic cues with downstream tissue mechanics remains understudied in many contexts during vertebrate morphogenesis. A posterior gradient of Fibroblast Growth Factor (FGF) ligands generates a contractile force gradient in the definitive endoderm, driving collective cell movements to form the hindgut. Here, we developed a two-dimensional chemo-mechanical model to investigate how mechanical properties of the endoderm and transport properties of FGF coordinately regulate this process. We began by formulating a 2-D reaction-diffusion-advection model that describes the formation of an FGF protein gradient due to posterior displacement of cells transcribing unstable Fgf8 mRNA during axis elongation, coupled with translation, diffusion, and degradation of FGF protein. This was used together with experimental measurements of FGF activity in the chick endoderm to inform a continuum model of definitive endoderm as an active viscous fluid that generates contractile stresses in proportion to FGF concentration. The model replicated key aspects of hindgut morphogenesis, confirms that heterogeneous - but isotropic - contraction is sufficient to generate large anisotropic cell movements, and provides new insight into how chemo-mechanical coupling across the mesoderm and endoderm coordinates hindgut elongation with outgrowth of the tailbud.

Biomechanical models and mechanisms of cellular morphogenesis and cerebral cortical expansion and folding

Morphogenesis of the nervous system involves a highly complex spatio-temporal pattern of physical forces (mainly tension and pressure) acting on cells and tissues that are pliable but have an intricately organized cytoskeletal infrastructure. This review begins by covering basic principles of biomechanics and the core cytoskeletal toolkit used to regulate the shapes of cells and tissues during embryogenesis and neural development. It illustrates how the principle of 'tensegrity' provides a useful conceptual framework for understanding how cells dynamically respond to forces that are generated internally or applied externally. The latter part of the review builds on this foundation in considering the development of mammalian cerebral cortex. The main focus is on cortical expansion and folding - processes that take place over an extended period of prenatal and postnatal development. Cortical expansion and folding are likely to involve many complementary mechanisms, some related to regulating cell proliferation and migration and others related to specific types and patterns of mechanical tension and pressure. Three distinct multi-mechanism models are evaluated in relation to a set of 18 key experimental observations and findings. The Composite Tension Plus (CT+) model is introduced as an updated version of a previous multi-component Differential Expansion Sandwich Plus (DES+) model (Van Essen, 2020); the new CT+ model includes 10 distinct mechanisms and has the greatest explanatory power among published models to date. Much needs to be done in order to validate specific mechanistic components and to assess their relative importance in different species, and important directions for future research are suggested.

Development and characterization of ferret ex vivo tracheal injury and cell engraftment model

The field of airway biology research relies primarily on in vitro and in vivo models of disease and injury. The use of ex vivo models to study airway injury and cell-based therapies remains largely unexplored although such models have the potential to overcome certain limitations of working with live animals and may more closely replicate in vivo processes than in vitro models can. Here, we characterized a ferret ex vivo tracheal injury and cell engraftment model. We describe a protocol for whole-mount staining of cleared tracheal explants, and showed that it provides a more comprehensive structural overview of the surface airway epithelium (SAE) and submucosal glands (SMGs) than 2D sections, revealing previously underappreciated structural anatomy of tracheal innervation and vascularization. Using an ex vivo model of tracheal injury, we evaluated the injury responses in the SAE and SMGs that turned out to be consistent with published in vivo work. We used this model to assess factors that influence engraftment of transgenic cells, providing a system for optimizing cell-based therapies. Finally, we developed a novel 3D-printed reusable culture chamber that enables live imaging of tracheal explants and differentiation of engrafted cells at an air-liquid interface. These approaches promise to be useful for modeling pulmonary diseases and testing therapies. Graphical abstract1,2. We describe here a method for differential mechanical injury of ferret tracheal explants that can be used to evaluate airway injury responses ex vivo. 3. Injured explants can be cultured at ALI (using the novel tissue-transwell device on the right) and submerged long-term to evaluate tissue-autonomous regeneration responses. 4. Tracheal explants can also be used for low throughput screens of compounds to improve cell engraftment efficiency or can be seeded with particular cells to model a disease phenotype. 5. Lastly, we demonstrate that ex vivo-cultured tracheal explants can be evaluated by various molecular assays and by immunofluorescent imaging that can be performed live using our custom-designed tissue-transwell.

Developmentally interdependent stretcher-compressor relationship between the embryonic brain and the surrounding scalp in the preosteogenic head

BACKGROUND

How developing brains mechanically interact with the surrounding embryonic scalp layers (ie, epidermal and mesenchymal) in the preosteogenic head remains unknown. Between embryonic day (E) 11 and E13 in mice, before ossification starts in the skull vault, the angle between the pons and the medulla decreases, raising the possibility that when the elastic scalp is directly pushed outward by the growing brain and thus stretched, it recoils inward in response, thereby confining and folding the brain.

RESULTS

Stress-release tests showed that the E11-13 scalp recoiled and that the in vivo prestretch prerequisite for this recoil was physically dependent on the brain (pressurization at 77-93 Pa) and on actomyosin and elastin within the scalp. In scalp-removed heads, brainstem folding was reduced, and the spreading of ink from the lateral ventricle to the spinal cord that occurred in scalp-intact embryos (with >5 μL injection) was lost, suggesting roles of the embryonic scalp in brain morphogenesis and cerebrospinal fluid homeostasis. Under nonstretched conditions, scalp cell proliferation declined, while the restretching of the shrunken scalp rescued scalp cell proliferation.

CONCLUSIONS

In the embryonic mouse head before ossification, a stretcher-compressor relationship elastically develops between the brain and the scalp, underlying their mechanically interdependent development.

Neuromechanobiology: An Expanding Field Driven by the Force of Greater Focus

The brain processes information by transmitting signals through highly connected and dynamic networks of neurons. Neurons use specific cellular structures, including axons, dendrites and synapses, and specific molecules, including cell adhesion molecules, ion channels and chemical receptors to form, maintain and communicate among cells in the networks. These cellular and molecular processes take place in environments rich of mechanical cues, thus offering ample opportunities for mechanical regulation of neural development and function. Recent studies have suggested the importance of mechanical cues and their potential regulatory roles in the development and maintenance of these neuronal structures. Also suggested are the importance of mechanical cues and their potential regulatory roles in the interaction and function of molecules mediating the interneuronal communications. In this review, the current understanding is integrated and promising future directions of neuromechanobiology are suggested at the cellular and molecular levels. Several neuronal processes where mechanics likely plays a role are examined and how forces affect ligand binding, conformational change, and signal induction of molecules key to these neuronal processes are indicated, especially at the synapse. The disease relevance of neuromechanobiology as well as therapies and engineering solutions to neurological disorders stemmed from this emergent field of study are also discussed.

Mechanical forces in avian embryo development

Research using avian embryos has led to major conceptual advances in developmental biology, virology, immunology, genetics and cell biology. The avian embryo has several significant advantages, including ready availability and ease of accessibility, rapid development with marked similarities to mammals and a high amenability to manipulation. As mechanical forces are increasingly recognised as key drivers of morphogenesis, this powerful model system is shedding new light on the mechanobiology of embryonic development. Here, we highlight progress in understanding how mechanical forces direct key morphogenetic processes in the early avian embryo. Recent advances in quantitative live imaging and modelling are elaborating upon traditional work using physical models and embryo manipulations to reveal cell dynamics and tissue forces in ever greater detail. The recent application of transgenic technologies further increases the strength of the avian model and is providing important insights about previously intractable developmental processes.

Mechanics of Development

Mechanical forces are integral to development-from the earliest stages of embryogenesis to the construction and differentiation of complex organs. Advances in imaging and biophysical tools have allowed us to delve into the developmental mechanobiology of increasingly complex organs and organisms. Here, we focus on recent work that highlights the diversity and importance of mechanical influences during morphogenesis. Developing tissues experience intrinsic mechanical signals from active forces and changes to tissue mechanical properties as well as extrinsic mechanical signals, including constraint and compression, pressure, and shear forces. Finally, we suggest promising avenues for future work in this rapidly expanding field.

Integration of luminal pressure and signalling in tissue self-organization

Many developmental processes involve the emergence of intercellular fluid-filled lumina. This process of luminogenesis results in a build up of hydrostatic pressure and signalling molecules in the lumen. However, the potential roles of lumina in cellular functions, tissue morphogenesis and patterning have yet to be fully explored. In this Review, we discuss recent findings that describe how pressurized fluid expansion can provide both mechanical and biochemical cues to influence cell proliferation, migration and differentiation. We also review emerging techniques that allow for precise quantification of fluid pressure in vivo and in situ. Finally, we discuss the intricate interplay between luminogenesis, tissue mechanics and signalling, which provide a new dimension for understanding the principles governing tissue self-organization in embryonic development.