CPLANE protein INTU regulates growth and patterning of the mouse lungs through cilia-dependent Hh signaling

Congenital lung malformations are fatal at birth in their severe forms. Prevention and early intervention of these birth defects require a comprehensive understanding of the molecular mechanisms of lung development. We find that the loss of inturned (Intu), a cilia and planar polarity effector gene, severely disrupts growth and branching morphogenesis of the mouse embryonic lungs. Consistent with our previous results indicating an important role for Intu in ciliogenesis and hedgehog (Hh) signaling, we find greatly reduced number of primary cilia in both the epithelial and mesenchymal tissues of the lungs. We also find significantly reduced expression of Gli1 and Ptch1, direct targets of Hh signaling, suggesting disruption of cilia-dependent Hh signaling in Intu mutant lungs. An agonist of the Hh pathway activator, smoothened, increases Hh target gene expression and tubulogenesis in explanted wild type, but not Intu mutant, lungs, suggesting impaired Hh signaling response underlying lung morphogenetic defects in Intu mutants. Furthermore, removing both Gli2 and Intu completely abolishes branching morphogenesis of the lung, strongly supporting a mechanism by which Intu regulates lung growth and patterning through cilia-dependent Hh signaling. Moreover, a transcriptomics analysis identifies around 200 differentially expressed genes (DEGs) in Intu mutant lungs, including known Hh target genes Gli1, Ptch1/2 and Hhip. Genes involved in muscle differentiation and function are highly enriched among the DEGs, consistent with an important role of Hh signaling in airway smooth muscle differentiation. In addition, we find that the difference in gene expression between the left and right lungs diminishes in Intu mutants, suggesting an important role of Intu in asymmetrical growth and patterning of the mouse lungs.

Leaf Vein Patterning

Leaves form veins whose patterns vary from a single vein running the length of the leaf to networks of staggering complexity where huge numbers of veins connect to other veins at both ends. For the longest time, vein formation was thought to be controlled only by the polar, cell-to-cell transport of the plant hormone auxin; recent evidence suggests that is not so. Instead, it turns out that vein patterning features are best accounted for by a combination of polar auxin transport, facilitated auxin diffusion through plasmodesma intercellular channels, and auxin signal transduction-though the latter's precise contribution remains unclear. Equally unclear remain the sites of auxin production during leaf development, on which that vein patterning mechanism ought to depend. Finally, whether that vein patterning mechanism can account for the variety of vein arrangements found in nature remains unknown. Addressing those questions will be the exciting challenge of future research.

Mesenchyme instructs growth while epithelium directs branching in the mouse mammary gland

The mammary gland is a unique organ that undergoes dynamic alterations throughout a female's reproductive life, making it an ideal model for developmental, stem cell and cancer biology research. Mammary gland development begins in utero and proceeds via a quiescent bud stage before the initial outgrowth and subsequent branching morphogenesis. How mammary epithelial cells transit from quiescence to an actively proliferating and branching tissue during embryogenesis and, importantly, how the branch pattern is determined remain largely unknown. Here, we provide evidence indicating that epithelial cell proliferation and onset of branching are independent processes, yet partially coordinated by the Eda signaling pathway. Through heterotypic and heterochronic epithelial-mesenchymal recombination experiments between mouse mammary and salivary gland tissues and ex vivo live imaging, we demonstrate that unlike previously concluded, the mode of branching is an intrinsic property of the mammary epithelium whereas the pace of growth and the density of ductal tree are determined by the mesenchyme. Transcriptomic profiling and ex vivo and in vivo functional studies in mice disclose that mesenchymal Wnt/ß-catenin signaling, and in particular IGF-1 downstream of it critically regulate mammary gland growth. These results underscore the general need to carefully deconstruct the different developmental processes producing branched organs.

Isthmin-1: A critical regulator of branching morphogenesis and metanephric mesenchyme condensation during early kidney development

Isthmin-1 (Ism1) was first described to be syn-expressed with Fgf8 in Xenopus. However, its biological role has not been elucidated until recent years. Despite of accumulated evidence that Ism1 participates in angiogenesis, tumor invasion, macrophage apoptosis, and glucose metabolism, the cognate receptors for Ism1 remain largely unknown. Ism1 deficiency in mice results in renal agenesis (RA) with a transient loss of Gdnf transcription and impaired mesenchyme condensation at E11.5. Ism1 binds to and activates Integrin α8β1 to positively regulate Gdnf/Ret signaling, thus promoting mesenchyme condensation and ureteric epithelium branching morphogenesis. Here, we propose the hypothesis underlying the mechanism by which Ism1 regulates branching morphogenesis during early kidney development.

Reconfigurable Growth of Engineered Living Materials

The growth of multicellular organisms is a process akin to additive manufacturing where cellular proliferation and mechanical boundary conditions, among other factors, drive morphogenesis. Engineers have limited ability to engineer morphogenesis to manufacture goods or to reconfigure materials comprised of biomass. Herein, a method that uses biological processes to grow and regrow magnetic engineered living materials (mELMs) into desired geometries is reported. These composites contain Saccharomyces cerevisiae and magnetic particles within a hydrogel matrix. The reconfigurable manufacturing process relies on the growth of living cells, magnetic forces, and elastic recovery of the hydrogel. The mELM then adopts a form in an external magnetic field. Yeast within the material proliferates, resulting in 259 ± 14% volume expansion. Yeast proliferation fixes the magnetic deformation, even when the magnetic field is removed. The shape fixity can be up to 99.3 ± 0.3%. The grown mELM can recover up to 73.9 ± 1.9% of the original form by removing yeast cell walls. The directed growth and recovery process can be repeated at least five times. This work enables ELMs to be processed and reprocessed into user-defined geometries without external material deposition.

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.

Fibroblast-induced mammary epithelial branching depends on fibroblast contractility

Epithelial branching morphogenesis is an essential process in living organisms, through which organ-specific epithelial shapes are created. Interactions between epithelial cells and their stromal microenvironment instruct branching morphogenesis but remain incompletely understood. Here, we employed fibroblast-organoid or fibroblast-spheroid co-culture systems and time-lapse imaging to reveal that physical contact between fibroblasts and epithelial cells and fibroblast contractility are required to induce mammary epithelial branching. Pharmacological inhibition of ROCK or non-muscle myosin II, or fibroblast-specific knock-out of Myh9 abrogate fibroblast-induced epithelial branching. The process of fibroblast-induced branching requires epithelial proliferation and is associated with distinctive epithelial patterning of yes associated protein (YAP) activity along organoid branches, which is dependent on fibroblast contractility. Moreover, we provide evidence for the in vivo existence of contractile fibroblasts specifically surrounding terminal end buds (TEBs) of pubertal murine mammary glands, advocating for an important role of fibroblast contractility in branching in vivo. Together, we identify fibroblast contractility as a novel stromal factor driving mammary epithelial morphogenesis. Our study contributes to comprehensive understanding of overlapping but divergent employment of mechanically active fibroblasts in developmental versus tumorigenic programs.

Integrating mechanical cues with engineered platforms to explore cardiopulmonary development and disease

Mechanical forces provide critical biological signals to cells during healthy and aberrant organ development as well as during disease processes in adults. Within the cardiopulmonary system, mechanical forces, such as shear, compressive, and tensile forces, act across various length scales, and dysregulated forces are often a leading cause of disease initiation and progression such as in bronchopulmonary dysplasia and cardiomyopathies. Engineered in vitro models have supported studies of mechanical forces in a number of tissue and disease-specific contexts, thus enabling new mechanistic insights into cardiopulmonary development and disease. This review first provides fundamental examples where mechanical forces operate at multiple length scales to ensure precise lung and heart function. Next, we survey recent engineering platforms and tools that have provided new means to probe and modulate mechanical forces across in vitro and in vivo settings. Finally, the potential for interdisciplinary collaborations to inform novel therapeutic approaches for a number of cardiopulmonary diseases are discussed.

Spatial patterning of energy metabolism during tissue morphogenesis

Biophysical signaling organizes forces to drive tissue morphogenesis, a process co-opted during disease progression. The systematic buildup of forces at the tissue scale is energetically demanding. Just as mechanical forces, gene expression, and concentrations of morphogens vary spatially across a developing tissue, there might similarly be spatial variations in energy consumption. Recent studies have started to uncover the connections between spatial patterns of mechanical forces and spatial patterns of energy metabolism. Here, we define and review the concept of energy metabolism during tissue morphogenesis. We highlight experiments showing spatial variations in energy metabolism across several model systems, categorized by morphogenetic motif, including convergent extension, branching, and migration. Finally, we discuss approaches to further enable quantitative measurements of energy production and consumption during morphogenesis.

TubULAR: tracking in toto deformations of dynamic tissues via constrained maps

A common motif in biology is the arrangement of cells into tubes, which further transform into complex shapes. Traditionally, analysis of dynamic tissues has relied on inspecting static snapshots, live imaging of cross-sections or tracking isolated cells in three dimensions. However, capturing the interplay between in-plane and out-of-plane behaviors requires following the full surface as it deforms and integrating cell-scale motions into collective, tissue-scale deformations. Here, we present an analysis framework that builds in toto maps of tissue deformations by following tissue parcels in a static material frame of reference. Our approach then relates in-plane and out-of-plane behaviors and decomposes complex deformation maps into elementary contributions. The tube-like surface Lagrangian analysis resource (TubULAR) provides an open-source implementation accessible either as a standalone toolkit or as an extension of the ImSAnE package used in the developmental biology community. We demonstrate our approach by analyzing shape change in the embryonic Drosophila midgut and beating zebrafish heart. The method naturally generalizes to in vitro and synthetic systems and provides ready access to the mechanical mechanisms relating genetic patterning to organ shape change.

Topology recapitulates morphogenesis of neuronal dendrites

Branching allows neurons to make synaptic contacts with large numbers of other neurons, facilitating the high connectivity of nervous systems. Neuronal arbors have geometric properties such as branch lengths and diameters that are optimal in that they maximize signaling speeds while minimizing construction costs. In this work, we asked whether neuronal arbors have topological properties that may also optimize their growth or function. We discovered that for a wide range of invertebrate and vertebrate neurons the distributions of their subtree sizes follow power laws, implying that they are scale invariant. The power-law exponent distinguishes different neuronal cell types. Postsynaptic spines and branchlets perturb scale invariance. Through simulations, we show that the subtree-size distribution depends on the symmetry of the branching rules governing arbor growth and that optimal morphologies are scale invariant. Thus, the subtree-size distribution is a topological property that recapitulates the functional morphology of dendrites.

Shape-driven confluent rigidity transition in curved biological tissues

Collective cell motions underlie structure formation during embryonic development. Tissues exhibit emergent multicellular characteristics such as jamming, rigidity transitions, and glassy dynamics, but there remain questions about how those tissue-scale dynamics derive from local cell-level properties. Specifically, there has been little consideration of the interplay between local tissue geometry and cellular properties influencing larger-scale tissue behaviors. Here, we consider a simple two-dimensional computational vertex model for confluent tissue monolayers, which exhibits a rigidity phase transition controlled by the shape index (ratio of perimeter to square root area) of cells, on surfaces of constant curvature. We show that the critical point for the rigidity transition is a function of curvature such that positively curved systems are likely to be in a less rigid, more fluid, phase. Likewise, negatively curved systems (saddles) are likely to be in a more rigid, less fluid, phase. A phase diagram we generate for the curvature and shape index constitutes a testable prediction from the model. The curvature dependence is interesting because it suggests a natural explanation for more dynamic tissue remodeling and facile growth in regions of higher surface curvature. Conversely, we would predict stability at the base of saddle-shaped budding structures without invoking the need for biochemical or other physical differences. This concept has potential ramifications for our understanding of morphogenesis of budding and branching structures.

'Chip'-ing away at morphogenesis - application of organ-on-chip technologies to study tissue morphogenesis

Emergent cell behaviors that drive tissue morphogenesis are the integrated product of instructions from gene regulatory networks, mechanics and signals from the local tissue microenvironment. How these discrete inputs intersect to coordinate diverse morphogenic events is a critical area of interest. Organ-on-chip technology has revolutionized the ability to construct and manipulate miniaturized human tissues with organotypic three-dimensional architectures in vitro. Applications of organ-on-chip platforms have increasingly transitioned from proof-of-concept tissue engineering to discovery biology, furthering our understanding of molecular and mechanical mechanisms that operate across biological scales to orchestrate tissue morphogenesis. Here, we provide the biological framework to harness organ-on-chip systems to study tissue morphogenesis, and we highlight recent examples where organ-on-chips and associated microphysiological systems have enabled new mechanistic insight in diverse morphogenic settings. We further highlight the use of organ-on-chip platforms as emerging test beds for cell and developmental biology.

Self-organized and directed branching results in optimal coverage in developing dermal lymphatic networks

Branching morphogenesis is a ubiquitous process that gives rise to high exchange surfaces in the vasculature and epithelial organs. Lymphatic capillaries form branched networks, which play a key role in the circulation of tissue fluid and immune cells. Although mouse models and correlative patient data indicate that the lymphatic capillary density directly correlates with functional output, i.e., tissue fluid drainage and trafficking efficiency of dendritic cells, the mechanisms ensuring efficient tissue coverage remain poorly understood. Here, we use the mouse ear pinna lymphatic vessel network as a model system and combine lineage-tracing, genetic perturbations, whole-organ reconstructions and theoretical modeling to show that the dermal lymphatic capillaries tile space in an optimal, space-filling manner. This coverage is achieved by two complementary mechanisms: initial tissue invasion provides a non-optimal global scaffold via self-organized branching morphogenesis, while VEGF-C dependent side-branching from existing capillaries rapidly optimizes local coverage by directionally targeting low-density regions. With these two ingredients, we show that a minimal biophysical model can reproduce quantitatively whole-network reconstructions, across development and perturbations. Our results show that lymphatic capillary networks can exploit local self-organizing mechanisms to achieve tissue-scale optimization.

Spatially coordinated cell cycle activity and motility govern bifurcation of mammary branches

Branching morphogenesis is an evolutionary solution to maximize epithelial function in a compact organ. It involves successive rounds of branch elongation and branch point formation to generate a tubular network. In all organs, branch points can form by tip splitting, but it is unclear how tip cells coordinate elongation and branching. Here, we addressed these questions in the embryonic mammary gland. Live imaging revealed that tips advance by directional cell migration and elongation relies upon differential cell motility that feeds a retrograde flow of lagging cells into the trailing duct, supported by tip proliferation. Tip bifurcation involved localized repression of cell cycle and cell motility at the branch point. Cells in the nascent daughter tips remained proliferative but changed their direction to elongate new branches. We also report the fundamental importance of epithelial cell contractility for mammary branching morphogenesis. The co-localization of cell motility, non-muscle myosin II, and ERK activities at the tip front suggests coordination/cooperation between these functions.

Taking shape: Intercalation within the mammary terminal end bud

In this issue of Developmental Cell, Pfannenstein and Macara leverage the experimental versatility of the mammary gland to show that intercalation within the terminal end bud is regulated by the tight junction organizing protein ZO-1 and dynamic actin cytoskeletal rearrangements to propel ductal elongation.

Organ function is preserved despite reorganization of niche architecture in the hair follicle

The ability of stem cells to build and replenish tissues depends on support from their niche. Although niche architecture varies across organs, its functional importance is unclear. During hair follicle growth, multipotent epithelial progenitors build hair via crosstalk with their remodeling fibroblast niche, the dermal papilla, providing a powerful model to functionally interrogate niche architecture. Through mouse intravital imaging, we show that dermal papilla fibroblasts remodel individually and collectively to form a morphologically polarized, structurally robust niche. Asymmetric TGF-β signaling precedes morphological niche polarity, and loss of TGF-β signaling in dermal papilla fibroblasts leads them to progressively lose their stereotypic architecture, instead surrounding the epithelium. The reorganized niche induces the redistribution of multipotent progenitors but nevertheless supports their proliferation and differentiation. However, the differentiated lineages and hairs produced by progenitors are shorter. Overall, our results reveal that niche architecture optimizes organ efficiency but is not absolutely essential for organ function.

Configurational entropy is an intrinsic driver of tissue structural heterogeneity

Tissues comprise ordered arrangements of cells that can be surprisingly disordered in their details. How the properties of single cells and their microenvironment contribute to the balance between order and disorder at the tissue-scale remains poorly understood. Here, we address this question using the self-organization of human mammary organoids as a model. We find that organoids behave like a dynamic structural ensemble at the steady state. We apply a maximum entropy formalism to derive the ensemble distribution from three measurable parameters - the degeneracy of structural states, interfacial energy, and tissue activity (the energy associated with positional fluctuations). We link these parameters with the molecular and microenvironmental factors that control them to precisely engineer the ensemble across multiple conditions. Our analysis reveals that the entropy associated with structural degeneracy sets a theoretical limit to tissue order and provides new insight for tissue engineering, development, and our understanding of disease progression.

Inflationary theory of branching morphogenesis in the mouse salivary gland

The mechanisms that regulate the patterning of branched epithelia remain a subject of long-standing debate. Recently, it has been proposed that the statistical organization of multiple ductal tissues can be explained through a local self-organizing principle based on the branching-annihilating random walk (BARW) in which proliferating tips drive a process of ductal elongation and stochastic bifurcation that terminates when tips encounter maturing ducts. Here, applied to mouse salivary gland, we show the BARW model struggles to explain the large-scale organization of tissue. Instead, we propose that the gland develops as a tip-driven branching-delayed random walk (BDRW). In this framework, a generalization of the BARW, tips inhibited through steric interaction with proximate ducts may continue their branching program as constraints become alleviated through the persistent expansion of the surrounding tissue. This inflationary BDRW model presents a general paradigm for branching morphogenesis when the ductal epithelium grows cooperatively with the domain into which it expands.

Macrophages maintain mammary stem cell activity and mammary homeostasis via TNF-α-PI3K-Cdk1/Cyclin B1 axis

Adult stem cell niche is a special environment composed of a variety stromal cells and signals, which cooperatively regulate tissue development and homeostasis. It is of great interest to study the role of immune cells in niche. Here, we show that mammary resident macrophages regulate mammary epithelium cell division and mammary development through TNF-α-Cdk1/Cyclin B1 axis. In vivo, depletion of macrophages reduces the number of mammary basal cells and mammary stem cells (MaSCs), while increases mammary luminal cells. In vitro, we establish a three-dimensional culture system in which mammary basal cells are co-cultured with macrophages, and interestingly, macrophage co-culture promotes the formation of branched functional mammary organoids. Moreover, TNF-α produced by macrophages activates the intracellular PI3K/Cdk1/Cyclin B1 signaling in mammary cells, thereby maintaining the activity of MaSCs and the formation of mammary organoids. Together, these findings reveal the functional significance of macrophageal niche and intracellular PI3K/Cdk1/Cyclin B1 axis for maintaining MaSC activity and mammary homeostasis.

Isthmin-1 (Ism1) modulates renal branching morphogenesis and mesenchyme condensation during early kidney development

The outgrowth of epithelial bud followed by reiterated bifurcations during renal development is driven by the ligand-receptor interactions between the epithelium and the surrounding mesenchyme. Here, by exploring ligand-receptor interactions in E10.5 and E11.5 kidneys by single cell RNA-seq, we find that Isthmin1 (Ism1), a secreted protein, resembles Gdnf expression and modulates kidney branching morphogenesis. Mice deficient for Ism1 exhibit defective ureteric bud bifurcation and impaired metanephric mesenchyme condensation in E11.5 embryos, attributable to the compromised Gdnf/Ret signaling, ultimately leading to renal agenesis and hypoplasia/dysplasia. By HRP-induced proximity labelling, we further identify integrin α8β1 as a receptor of Ism1 in E11.5 kidney and demonstrate that Ism1 promoted cell-cell adhesion through interacting with Integrin α8β1, the receptor whose activation is responsible for Gdnf expression and mesenchyme condensation. Taken together, our work reveals Ism1 as a critical regulator of cell-cell interaction that modulates Gdnf/Ret signaling during early kidney development.

Mechanobiological approaches to synthetic morphogenesis: learning by building

Tissue morphogenesis occurs in a complex physicochemical microenvironment with limited experimental accessibility. This often prevents a clear identification of the processes that govern the formation of a given functional shape. By applying state-of-the-art methods to minimal tissue systems, synthetic morphogenesis aims to engineer the discrete events that are necessary and sufficient to build specific tissue shapes. Here, we review recent advances in synthetic morphogenesis, highlighting how a combination of microfabrication and mechanobiology is fostering our understanding of how tissues are built.

Dynamics, structure and assembly of the basement membrane in developing salivary glands revealed by an exogenous EGFP-tagged nidogen probe

Most epithelial tissues rapidly become complex during embryonic development while being surrounded by the basement membrane (BM). Thus, the BM shape is thought to change dramatically as the epithelium grows, but the underlying mechanism is not yet clear. Nidogen-1 is ubiquitous in the BM and binds to various other BM components, including laminin and type IV collagen. To elucidate the behavior of the BM during epithelial morphogenesis, we attempted to live-label the developing BM with recombinant human nidogen-1 fused to an enhanced green fluorescent protein (hNid1-EGFP). Submandibular glands of mouse embryos were cultured in glass-bottomed dishes and incubated in media containing hNid1-EGFP. Subsequent confocal microscopy clearly visualized the BMs surrounding the epithelial end buds. On three-dimensional reconstruction from Z-series confocal sections, the epithelial BM was observed as a thin sheet that expanded continuously around the entire epithelial basal surface. Because the explants continued to grow well in the presence of hNid1-EGFP, time-lapse confocal microscopy was performed to follow the dynamics of the BM. We found that the epithelial BM is an adaptive structure that deforms in accordance with the rapid shape changes of the developing epithelium. Furthermore, hNid1-EGFP was found to be incorporated differently into the epithelial BM compared with that reported for fibronectin or type IV collagen, suggesting that individual BM components assemble in different ways to form the BM.

mTORC1 signaling facilitates differential stem cell differentiation to shape the developing murine lung and is associated with mitochondrial capacity

Formation of branched organs requires sequential differentiation of stem cells. In this work, we find that the conducting airways derived from SOX2⁺ progenitors in the murine lungs fail to form without mTOR complex 1 (mTORC1) signaling and are replaced by lung cysts. Proximal-distal patterning through transitioning of distal SOX9⁺ progenitors to proximal SOX2⁺ cells is disrupted. Mitochondria number and ATP production are reduced. Compromised mitochondrial capacity results in a similar defect as that in mTORC1-deficient lungs. This suggests that mTORC1 promotes differentiation of SOX9⁺ progenitors to form the conducting airways by modulating mitochondrial capacity. Surprisingly, in all mutants, saccules are produced from lung cysts at the proper developmental time despite defective branching. SOX9⁺ progenitors also differentiate into alveolar epithelial type I and type II cells within saccules. These findings highlight selective utilization of energy and regulatory programs during stem cell differentiation to produce distinct structures of the mammalian lungs.

The reduced number of nephrons with shortening renal tubules in mouse postnatal adverse environment

BACKGROUND

The intrauterine adverse environment during nephrogenesis reduces the nephron number, probably associates with impaired ureteric bud (UB) branching.

METHODS

The kidneys in C57/BL6 mice were irradiated with a single dose of 10 gray (10 Gy) as adverse environment on postnatal day 3 (irradiated PND3 kidneys) after UB branching ceased. The renal functions and pathological findings of irradiated PND3 kidneys were compared with those of non-irradiated control and 10 Gy irradiation on PND14 (irradiated PND14 kidney) from 1 to 18 months.

RESULTS

The number and density of glomeruli in irradiated PND3 kidneys were reduced by 1 month with renal dysfunction at 6 months. The morphologically incomplete glomeruli with insufficient capillaries were involuted by 1 month in the superficial cortex. Reduced tubular numbers and developmental disability with shortening renal tubules occurred in irradiated PND3 kidneys with impaired urine concentration at 6 months. Hypertrophy of glomeruli developed, and occasional sclerotic glomeruli appeared in the juxtamedullary cortex with hypertension and albuminuria at 12 to 18 months.

CONCLUSIONS

The reduced number of nephrons with shortening renal tubules occurred with impaired renal functions in a postnatal adverse environment after cessation of UB branching, and glomerular hypertrophy with occasional glomerulosclerosis developed accompanied with hypertension and albuminuria in the adulthood.

IMPACT

The reduced number of nephrons with shortening renal tubules occurred with impaired renal functions in a postnatal adverse environment after cessation of ureteric bud branching. The reduced number of glomeruli were associated with not only the impaired formation of glomeruli but also involution of morphologically small incomplete glomeruli after an adverse environment. The insufficiently developed nephrons were characterized by the shortening renal tubules with impaired urine concentration. In addition, glomerular hypertrophy and occasional glomerulosclerosis developed with hypertension and albuminuria in adulthood. The present study can help to understand the risk of alternations of premature nephrons in preterm neonates.

Spatiotemporal dynamics of self-organized branching in pancreas-derived organoids

The development dynamics and self-organization of glandular branched epithelia is of utmost importance for our understanding of diverse processes ranging from normal tissue growth to the growth of cancerous tissues. Using single primary murine pancreatic ductal adenocarcinoma (PDAC) cells embedded in a collagen matrix and adapted media supplementation, we generate organoids that self-organize into highly branched structures displaying a seamless lumen connecting terminal end buds, replicating in vivo PDAC architecture. We identify distinct morphogenesis phases, each characterized by a unique pattern of cell invasion, matrix deformation, protein expression, and respective molecular dependencies. We propose a minimal theoretical model of a branching and proliferating tissue, capturing the dynamics of the first phases. Observing the interaction of morphogenesis, mechanical environment and gene expression in vitro sets a benchmark for the understanding of self-organization processes governing complex organoid structure formation processes and branching morphogenesis.

Pancreatic ductal adenocarcinomas (PDAC) exhibit complex morphologies challenging to capture in organoid models. Here, the authors develop PDAC organoids that can recreate branched structures and, with the use of a minimal mathematical model, shed light to pathways and processes directing the dynamics of self-organization and branching morphogenesis.

Toward Measuring the Mechanical Stresses Exerted by Branching Embryonic Airway Epithelial Explants in 3D Matrices of Matrigel

Numerous organs in the bodies of animals, including the lung, kidney, and mammary gland, contain ramified networks of epithelial tubes. These structures arise during development via a process known as branching morphogenesis. Previous studies have shown that mechanical forces directly impact this process, but the patterns of mechanical stress exerted by branching embryonic epithelia are not well understood. This is, in part, owing to a lack of experimental tools. Traditional traction force microscopy assays rely on the use of compliant hydrogels with well-defined mechanical properties. Isolated embryonic epithelial explants, however, have only been shown to branch in three-dimensional matrices of reconstituted basement membrane protein, or Matrigel, a biomaterial with poorly characterized mechanical behavior, especially in the regime of large deformations. Here, to compute the traction stresses generated by branching epithelial explants, we quantified the finite-deformation constitutive behavior of gels of reconstituted basement membrane protein subjected to multi-axial mechanical loads. We then modified the mesenchyme-free assay for the ex vivo culture of isolated embryonic airway epithelial explants by suspending fluorescent microspheres within the surrounding gel and tracking their motion during culture. Surprisingly, the tracked bead motion was non-zero in regions of the gel far away from the explants, suggestive of passive swelling deformations within the matrix. To compute accurate traction stresses, these swelling deformations must be decomposed from those generated by the branching explants. We thus tracked the motion of beads suspended within cell-free matrices and quantified spatiotemporal patterns of gel swelling. Taken together, these passive swelling data can be combined with the measured mechanical properties of the gel to compute the traction forces exerted by intact embryonic epithelial explants.

Hedgehog Signaling Pathway Orchestrates Human Lung Branching Morphogenesis

The Hedgehog (HH) signaling pathway plays an essential role in mouse lung development. We hypothesize that the HH pathway is necessary for branching during human lung development and is impaired in pulmonary hypoplasia. Single-cell, bulk RNA-sequencing data, and human fetal lung tissues were analyzed to determine the spatiotemporal localization of HH pathway actors. Distal human lung segments were cultured in an air-liquid interface and treated with an SHH inhibitor (5E1) to determine the effect of HH inhibition on human lung branching, epithelial-mesenchymal markers, and associated signaling pathways in vitro. Our results showed an early and regulated expression of HH pathway components during human lung development. Inhibiting HH signaling caused a reduction in branching during development and dysregulated epithelial (SOX2, SOX9) and mesenchymal (ACTA2) progenitor markers. FGF and Wnt pathways were also disrupted upon HH inhibition. Finally, we demonstrated that HH signaling elements were downregulated in lung tissues of patients with a congenital diaphragmatic hernia (CDH). In this study, we show for the first time that HH signaling inhibition alters important genes and proteins required for proper branching of the human developing lung. Understanding the role of the HH pathway on human lung development could lead to the identification of novel therapeutic targets for childhood pulmonary diseases.

DICER1-associated hepatic cystic neoplasm with pleuropulmonary blastoma-like features: a novel clinicopathologic diagnosis

This report documents a unique multicystic neoplasm of the liver in an 8-month-old boy with a heterozygous germline pathogenic DICER1 variant. This neoplasm, initially considered most likely a mesenchymal hamartoma based on imaging, demonstrated the characteristic histologic pattern of embryonal rhabdomyosarcoma residing in the subepithelial or cambium layer-like zone of the epithelial-lined cysts. Thus, although the differential diagnosis includes mesenchymal hamartoma, a young child with a multicystic mass lesion in the liver, lung, or kidney should both raise the possibility of a germline pathogenic DICER1 variant and also not be mistaken for one of the other hepatic neoplasms of childhood.

ADAMTS18 regulates early branching morphogenesis of lacrimal gland and has a significant association with the risk of dry eye in mice

ADAMTS18 is an orphan member of the ADAMTS family of metalloproteinase. ADAMTS18 mutation has been linked to developmental eye disorders, such as retinal dystrophies and ectopia lentis. Here, we report a new function of ADAMTS18 in modulating the lacrimal gland (LG) branching morphogenesis, and an association with dry eye in mice. Adamts18 mRNA was found to be enriched in the epithelium of branching tips of embryonic (E) LG, but its expression was barely detectable after 2 weeks of birth. Histological analyses of E16.5-E17.5 LG showed that ADAMTS18 deficiency resulted in a significant reduction of epithelial branching in embryonic LG. In vitro culture of E15.5 LG explants showed that the numbers of epithelial buds and branches in Adamts18 knockout (Adamts18^-/-) LGs were significantly decreased when compared to those of wild type (Adamts18^+/+) LGs after 0 h, 24 h, and 48 h of culture. Increased fibronectin deposition was detected in LG mesenchyme of E16.5 Adamts18^-/- mice. At 14 months of age, Adamts18^-/- mice manifested multiple LG pathological changes, including acinar atrophy and irregular duct ectasis with periductal fibrosis. The tear volume was significantly decreased in Adamts18^-/- mice at 4 months of age, which corresponds to early adulthood in humans.

LGL1 binds to Integrin β1 and inhibits downstream signaling to promote epithelial branching in the mammary gland

Branching morphogenesis is a fundamental process by which organs in invertebrates and vertebrates form branches to expand their surface areas. The current dogma holds that directional cell migration determines where a new branch forms and thus patterns branching. Here, we asked whether mouse Lgl1, a homolog of the Drosophila tumor suppressor Lgl, regulates epithelial polarity in the mammary gland. Surprisingly, mammary glands lacking Lgl1 have normal epithelial polarity, but they form fewer branches. Moreover, we find that Lgl1 null epithelium is unable to directionally migrate, suggesting that migration is not essential for mammary epithelial branching as expected. We show that LGL1 binds to Integrin β1 and inhibits its downstream signaling, and Integrin β1 overexpression blocks epithelial migration, thus recapitulating the Lgl1 null phenotype. Altogether, we demonstrate that Lgl1 modulation of Integrin β1 signaling is essential for directional migration and that epithelial branching in invertebrates and the mammary gland is fundamentally distinct.

Ma et al. show that Lgl1 is essential for mammary gland branching morphogenesis but not epithelial polarity. Lgl1 is required for directional migration by regulating Integrin β1 signaling levels and focal adhesion strengths. Finally, branching mechanisms are distinct between mammary gland and Drosophila systems where directional migration is indispensable.

DICER1 tumor predisposition syndrome: an evolving story initiated with the pleuropulmonary blastoma

DICER1 syndrome (OMIM 606241, 601200) is a rare autosomal dominant familial tumor predisposition disorder with a heterozygous DICER1 germline mutation. The most common tumor seen clinically is the pleuropulmonary blastoma (PPB), a lung neoplasm of early childhood which is classified on its morphologic features into four types (IR, I, II and III) with tumor progression over time within the first 4-5 years of life from the prognostically favorable cystic type I to the unfavorable solid type III. Following the initial report of PPB, its association with other cystic neoplasms was demonstrated in family studies. The detection of the germline mutation in DICER1 provided the opportunity to identify and continue to recognize a number seemingly unrelated extrapulmonary neoplasms: Sertoli-Leydig cell tumor, gynandroblastoma, embryonal rhabdomyosarcomas of the cervix and other sites, multinodular goiter, differentiated and poorly differentiated thyroid carcinoma, cervical-thyroid teratoma, cystic nephroma-anaplastic sarcoma of kidney, nasal chondromesenchymal hamartoma, intestinal juvenile-like hamartomatous polyp, ciliary body medulloepithelioma, pituitary blastoma, pineoblastoma, primary central nervous system sarcoma, embryonal tumor with multilayered rosettes-like cerebellar tumor, PPB-like peritoneal sarcoma, DICER1-associated presacral malignant teratoid neoplasm and other non-neoplastic associations. Each of these neoplasms is characterized by a second somatic mutation in DICER1. In this review, we have summarized the salient clinicopathologic aspects of these tumors whose histopathologic features have several overlapping morphologic attributes particularly the primitive mesenchyme often with rhabdomyoblastic and chondroid differentiation and an uncommitted spindle cell pattern. Several of these tumors have an initial cystic stage from which there is progression to a high grade, complex patterned neoplasm. These pathologic findings in the appropriate clinical setting should serve to alert the pathologist to the possibility of a DICER1-associated neoplasm and initiate appropriate testing on the neoplasm and to alert the clinician about the concern for a DICER1 mutation.

Systems for intricate patterning of the vertebrate anatomy

Periodic patterns form intricate arrays in the vertebrate anatomy, notably the hair and feather follicles of the skin, but also internally the villi of the gut and the many branches of the lung, kidney, mammary and salivary glands. These tissues are composite structures, being composed of adjoined epithelium and mesenchyme, and the patterns that arise within them require interaction between these two tissue layers. In embryonic development, cells change both their distribution and state in a periodic manner, defining the size and relative positions of these specialized structures. Their placement is determined by simple spacing mechanisms, with substantial evidence pointing to a variety of local enhancement/lateral inhibition systems underlying the breaking of symmetry. The nature of the cellular processes involved, however, has been less clear. While much attention has focused on intercellular soluble signals, such as protein growth factors, experimental evidence has grown for contributions of cell movement or mechanical forces to symmetry breaking. In the mesenchyme, unlike the epithelium, cells may move freely and can self-organize into aggregates by chemotaxis, or through generation and response to mechanical strain on their surrounding matrix. Different modes of self-organization may coexist, either coordinated into a single system or with hierarchical relationships. Consideration of a broad range of distinct biological processes is required to advance understanding of biological pattern formation. This article is part of the theme issue 'Recent progress and open frontiers in Turing's theory of morphogenesis'.

Negative Transpulmonary Pressure Disrupts Airway Morphogenesis by Suppressing Fgf10

Mechanical forces are increasingly recognized as important determinants of cell and tissue phenotype and also appear to play a critical role in organ development. During the fetal stages of lung morphogenesis, the pressure of the fluid within the lumen of the airways is higher than that within the chest cavity, resulting in a positive transpulmonary pressure. Several congenital defects decrease or reverse transpulmonary pressure across the developing airways and are associated with a reduced number of branches and a correspondingly underdeveloped lung that is insufficient for gas exchange after birth. The small size of the early pseudoglandular stage lung and its relative inaccessibility in utero have precluded experimental investigation of the effects of transpulmonary pressure on early branching morphogenesis. Here, we present a simple culture model to explore the effects of negative transpulmonary pressure on development of the embryonic airways. We found that negative transpulmonary pressure decreases branching, and that it does so in part by altering the expression of fibroblast growth factor 10 (Fgf10). The morphogenesis of lungs maintained under negative transpulmonary pressure can be rescued by supplementing the culture medium with exogenous FGF10. These data suggest that Fgf10 expression is regulated by mechanical stress in the developing airways. Understanding the mechanical signaling pathways that connect transpulmonary pressure to FGF10 can lead to the establishment of novel non-surgical approaches for ameliorating congenital lung defects.

Repetitive accumulation of interstitial cells generates the branched structure of Cladonema medusa tentacles

The shaping of tissues and organs in many animals relies on interactions between the epithelial cell layer and its underlying mesoderm-derived tissues. Inductive signals, such as receptor tyrosine kinase (RTK) signaling emanating from mesoderm, act on cells of the epithelium to initiate three-dimensional changes. However, how tissues are shaped in a diploblastic animal with no mesoderm remains largely unknown. In this study, the jellyfish Cladonema pacificum was used to investigate branch formation. The tentacles on its medusa stage undergo branching, which increases the epithelial surface area available for carrying nematocytes, thereby maximizing prey capture. Pharmacological and cellular analyses of the branching process suggest a two-step model for tentacle branch formation, in which mitogen-activated protein kinase kinase signaling accumulates interstitial cells in the future branch-forming region, and fibroblast growth factor signaling regulates branch elongation. This study highlights an essential role for these pluripotent stem cells in the tissue-shaping morphogenesis of a diploblastic animal. In addition, it identifies a mechanism involving RTK signaling and cell proliferative activity at the branch tip for branching morphogenesis that is apparently conserved across the animal kingdom.

Rac1 promotes kidney collecting duct integrity by limiting actomyosin activity

A polarized collecting duct (CD), formed from the branching ureteric bud (UB), is a prerequisite for an intact kidney. The small Rho GTPase Rac1 is critical for actin cytoskeletal regulation. We investigated the role of Rac1 in the kidney collecting system by selectively deleting it in mice at the initiation of UB development. The mice exhibited only a mild developmental phenotype; however, with aging, the CD developed a disruption of epithelial integrity and function. Despite intact integrin signaling, Rac1-null CD cells had profound adhesion and polarity abnormalities that were independent of the major downstream Rac1 effector, Pak1. These cells did however have a defect in the WAVE2–Arp2/3 actin nucleation and polymerization apparatus, resulting in actomyosin hyperactivity. The epithelial defects were reversible with direct myosin II inhibition. Furthermore, Rac1 controlled lateral membrane height and overall epithelial morphology by maintaining lateral F-actin and restricting actomyosin. Thus, Rac1 promotes CD epithelial integrity and morphology by restricting actomyosin via Arp2/3-dependent cytoskeletal branching.

Collagen polarization promotes epithelial elongation by stimulating locoregional cell proliferation

Epithelial networks are commonly generated by processes where multicellular aggregates elongate and branch. Here, we focus on understanding cellular mechanisms for elongation using an organotypic culture system as a model of mammary epithelial anlage. Isotropic cell aggregates broke symmetry and slowly elongated when transplanted into collagen 1 gels. The elongating regions of aggregates displayed enhanced cell proliferation that was necessary for elongation to occur. Strikingly, this locoregional increase in cell proliferation occurred where collagen 1 fibrils reorganized into bundles that were polarized with the elongating aggregates. Applying external stretch as a cell-independent way to reorganize the extracellular matrix, we found that collagen polarization stimulated regional cell proliferation to precipitate symmetry breaking and elongation. This required β1-integrin and ERK signaling. We propose that collagen polarization supports epithelial anlagen elongation by stimulating locoregional cell proliferation. This could provide a long-lasting structural memory of the initial axis that is generated when anlage break symmetry.

Salivary gland: A budding genius

During salivary gland development, branches arise through formation of new end buds (budding) and division of existing buds (clefting). In a recent Cell study, Wang et al., (2021) report that mouse salivary gland budding is reliant on a delicate interplay between epithelial cells and the extracellular matrix surrounding them.

Tissue mechanics in stem cell fate, development, and cancer

Cells in tissues experience a plethora of forces that regulate their fate and modulate development and homeostasis. Cells sense mechanical cues through localized mechanoreceptors or by influencing cytoskeletal or plasma membrane organization. Cells translate force and modulate their behavior through a process termed mechanotransduction. Cells tune their tension upon exposure to chronic force by engaging cellular machinery that modulates actin tension, which in turn stimulates matrix remodeling and stiffening and alters cell-cell adhesions until cells achieve a state of tensional homeostasis. Loss of tensional homeostasis can be induced through oncogene activity and/or tissue fibrosis, accompanies tumor progression, and is associated with increased cancer risk. The mechanical stresses that develop in tumors can also foster the mesenchymal-like transdifferentiation of cells to induce a stem-like phenotype that contributes to their aggression, metastatic dissemination, and treatment resistance. Thus, strategies that ameliorate tumor mechanics may comprise an effective strategy to prevent aggressive tumor behavior.

Organ-Specific Branching Morphogenesis

A common developmental process, called branching morphogenesis, generates the epithelial trees in a variety of organs, including the lungs, kidneys, and glands. How branching morphogenesis can create epithelial architectures of very different shapes and functions remains elusive. In this review, we compare branching morphogenesis and its regulation in lungs and kidneys and discuss the role of signaling pathways, the mesenchyme, the extracellular matrix, and the cytoskeleton as potential organ-specific determinants of branch position, orientation, and shape. Identifying the determinants of branch and organ shape and their adaptation in different organs may reveal how a highly conserved developmental process can be adapted to different structural and functional frameworks and should provide important insights into epithelial morphogenesis and developmental disorders.

Building the complex architectures of vascular networks: Where to branch, where to connect and where to remodel?

The cardiovascular system is the first organ to become functional during vertebrate embryogenesis and is responsible for the distribution of oxygen and nutrients to all cells of the body. The cardiovascular system constitutes a circulatory loop in which blood flows from the heart through arteries into the microvasculature and back through veins to the heart. The vasculature is characterized by the heterogeneity of blood vessels with respect to size, cellular architecture and function, including both larger vessels that are found at defined positions within the body and smaller vessels or vascular beds that are organized in a less stereotyped manner. Recent studies have shed light on how the vascular tree is formed and how the interconnection of all branches is elaborated and maintained. In contrast to many other branched organs such as the lung or the kidney, vessel connection (also called anastomosis) is a key process underlying the formation of vascular networks; each outgrowing angiogenic sprout must anastomose in order to allow blood flow in the newly formed vessel segment. It turns out that during this "sprouting and anastomosis" process, too many vessels are generated, and that blood flow is subsequently optimized through the removal (pruning) of low flow segments. Here, we reflect on the cellular and molecular mechanisms involved in forming the complex architecture of the vasculature through sprouting, anastomosis and pruning, and raise some questions that remain to be addressed in future studies.

Evaluating snail-trail frameworks for leader-follower behavior with agent-based modeling

Branched networks constitute a ubiquitous structure in biology, arising in plants, lungs, and the circulatory system; however, the mechanisms behind their creation are not well understood. A commonly used model for network morphogenesis proposes that sprouts develop through interactions between leader (tip) cells and follower (stalk) cells. In this description, tip cells emerge from existing structures, travel up chemoattractant gradients, and form new networks by guiding the movement of stalk cells. Such dynamics have been mathematically represented by continuum "snail-trail" models in which the tip cell flux contributes to the stalk cell proliferation rate. Although snail-trail models constitute a classical depiction of leader-follower behavior, their accuracy has yet to be evaluated in a rigorous quantitative setting. Here, we extend the snail-trail modeling framework to two spatial dimensions by introducing a novel multiplicative factor to the stalk cell rate equation, which corrects for neglected network creation in directions other than that of the migrating front. Our derivation of this factor demonstrates that snail-trail models are valid descriptions of cell dynamics when chemotaxis dominates cell movement. We confirm that our snail-trail model accurately predicts the dynamics of tip and stalk cells in an existing agent-based model (ABM) for network formation [Pillay et al., Phys. Rev. E 95, 012410 (2017)10.1103/PhysRevE.95.012410]. We also derive conditions for which it is appropriate to use a reduced, one-dimensional snail-trail model to analyze ABM results. Our analysis identifies key metrics for cell migration that may be used to anticipate when simple snail-trail models will accurately describe experimentally observed cell dynamics in network formation.

Uncovering cellular networks in branching morphogenesis using single-cell transcriptomics

Single-cell RNA-sequencing (scRNA-seq) and related technologies to identify cell types and measure gene expression in space, in time, and within lineages have multiplied rapidly in recent years. As these techniques proliferate, we are seeing an increase in their application to the study of developing tissues. Here, we focus on single-cell investigations of branching morphogenesis. Branched organs are highly complex but typically develop recursively, such that a given developmental stage theoretically contains the entire spectrum of cell identities from progenitor to terminally differentiated. Therefore, branched organs are a highly attractive system for study by scRNA-seq. First, we provide an update on advances in the field of scRNA-seq analysis, focusing on spatial transcriptomics, computational reconstruction of differentiation trajectories, and integration of scRNA-seq with lineage tracing. In addition, we discuss the possibilities and limitations for applying these techniques to studying branched organs. We then discuss exciting advances made using scRNA-seq in the study of branching morphogenesis and differentiation in mammalian organs, with emphasis on the lung, kidney, and mammary gland. We propose ways that scRNA-seq could be used to address outstanding questions in each organ. Finally, we highlight the importance of physical and mechanical signals in branching morphogenesis and speculate about how scRNA-seq and related techniques could be applied to study tissue morphogenesis beyond just differentiation.

Endothelial cells on the move: dynamics in vascular morphogenesis and disease

The vascular system is a hierarchically organized network of blood vessels that play crucial roles in embryogenesis, homeostasis and disease. Blood vessels are built by endothelial cells – the cells lining the interior of blood vessels – through a process named vascular morphogenesis. Endothelial cells react to different biomechanical signals in their environment by adjusting their behavior to: (1) invade, proliferate and fuse to form new vessels (angiogenesis); (2) remodel, regress and establish a hierarchy in the network (patterning); and (3) maintain network stability (quiescence). Each step involves the coordination of endothelial cell differentiation, proliferation, polarity, migration, rearrangements and shape changes to ensure network integrity and an efficient barrier between blood and tissues. In this review, we highlighted the relevance and the mechanisms involving endothelial cell migration during different steps of vascular morphogenesis. We further present evidence on how impaired endothelial cell dynamics can contribute to pathology.