Regulation of paraxis expression and somite formation by ectoderm- and neural tube-derived signals

Co- and Triaxial Electrospinning for Stem Cell-based Bone Regeneration

Bone tissue is composed of organic minerals and cells. It has the capacity to heal for certain minor damages, but when the bone defects surpass the critical threshold, they need fixing. Bone regeneration through natural and synthetic biodegradable materials requires various steps, such as manufacturing methods and materials selection. A successful biodegradable bone graft should have a high surface area/ volume ratio, strength, and a biocompatible, porous structure capable of promoting cell adhesion, proliferation, and differentiation. Considering these requirements, the electrospinning technique is promising for creating functional nano-sized scaffolds. The multi-axial methods, such as coaxial and triaxial electrospinning, are the most popular techniques to produce double or tri-layered scaffolds, respectively. Recently, stem cell culture on scaffolds and the application of osteogenic differentiation protocols on these scaffolds have opened new possibilities in the field of biomaterials research. This review discusses an overview of the progress in coaxial and triaxial technology through biodegradable composite bone materials. The review also carefully elaborates the osteogenic differentiation using stem cells and their performance with nano-sized scaffolds.

Advances in additive manufacturing of polycaprolactone based scaffolds for bone regeneration

Critical sized bone defects are difficult to manage and currently available clinical/surgical strategies for treatment are not completely successful. Polycaprolactone (PCL) which is a biodegradable and biocompatible thermoplastic can be 3D printed using medical images into patient specific bone implants. The excellent mechanical properties and low immunogenicity of PCL makes it an ideal biomaterial candidate for 3D printing of bone implants. Though PCL suffers from the limitation of being bio-inert. Here we describe the use of PCL as a biomaterial for 3D printing for bone regeneration, and advances made in the field. The specific focus is on the different 3D printing techniques used for this purpose and various modification that can enhance bone regeneration following the development pathways. We further describe the effect of various scaffold characteristics on bone regeneration both in vitro and the translational assessment of these 3D printed PCL scaffolds in animal studies. The generated knowledge will help understand cell-material interactions of 3D printed PCL scaffolds, to further improve scaffold chemistry and design that can replicate bone developmental processes and can be translated clinically.

Genetic models for lineage tracing in musculoskeletal development, injury, and healing

Musculoskeletal development and later post-natal homeostasis are highly dynamic processes, marked by rapid structural and functional changes across very short periods of time. Adult anatomy and physiology are derived from pre-existing cellular and biochemical states. Consequently, these early developmental states guide and predict the future of the system as a whole. Tools have been developed to mark, trace, and follow specific cells and their progeny either from one developmental state to the next or between circumstances of health and disease. There are now many such technologies alongside a library of molecular markers which may be utilized in conjunction to allow for precise development of unique cell 'lineages'. In this review, we first describe the development of the musculoskeletal system beginning as an embryonic germ layer and at each of the key developmental stages that follow. We then discuss these structures in the context of adult tissues during homeostasis, injury, and repair. Special focus is given in each of these sections to the key genes involved which may serve as markers of lineage or later in post-natal tissues. We then finish with a technical assessment of lineage tracing and the techniques and technologies currently used to mark cells, tissues, and structures within the musculoskeletal system.

Sequential changes in cellular properties accompanying amniote somite formation

Somites are transient structures derived from the pre-somitic mesoderm (PSM), involving mesenchyme-to-epithelial transition (MET) where the cells change their shape and polarize. Using Scanning electron microscopy (SEM), immunocytochemistry and confocal microscopy, we study the progression of these events along the tail-to-head axis of the embryo, which mirrors the progression of somitogenesis (younger cells located more caudally). SEM revealed that PSM epithelialization is a gradual process, which begins much earlier than previously thought, starting with the dorsalmost cells, then the medial ones, and then, simultaneously, the ventral and lateral cells, before a somite fully separates from the PSM. The core (internal) cells of the PSM and somites never epithelialize, which suggests that the core cells could be 'trapped' within the somitocoele after cells at the surfaces of the PSM undergo MET. Three-dimensional imaging of the distribution of the cell polarity markers PKCζ, PAR3, ZO1, the Golgi marker GM130 and the apical marker N-cadherin reveal that the pattern of polarization is distinctive for each marker and for each surface of the PSM, but the order of these events is not the same as the progression of cell elongation. These observations challenge some assumptions underlying existing models of somite formation.

Peripheral nerve development in zebrafish requires muscle patterning by tcf15/paraxis

The vertebrate peripheral nervous system (PNS) is an intricate network that conveys sensory and motor information throughout the body. During development, extracellular cues direct the migration of axons and glia through peripheral tissues. Currently, the suite of molecules that govern PNS axon-glial patterning is incompletely understood. To elucidate factors that are critical for peripheral nerve development, we characterized the novel zebrafish mutant, stl159, that exhibits abnormalities in PNS patterning. In these mutants, motor and sensory nerves that develop adjacent to axial muscle fail to extend normally, and neuromasts in the posterior lateral line system, as well as neural crest-derived melanocytes, are incorrectly positioned. The stl159 genetic lesion lies in the basic helix-loop-helix (bHLH) transcription factor tcf15, which has been previously implicated in proper development of axial muscles. We find that targeted loss of tcf15 via CRISPR-Cas9 genome editing results in the PNS patterning abnormalities observed in stl159 mutants. Because tcf15 is expressed in developing muscle prior to nerve extension, rather than in neurons or glia, we predict that tcf15 non-cell-autonomously promotes peripheral nerve patterning in zebrafish through regulation of extracellular patterning cues. Our work underscores the importance of muscle-derived factors in PNS development.

From Bipotent Neuromesodermal Progenitors to Neural-Mesodermal Interactions during Embryonic Development

To ensure the formation of a properly patterned embryo, multiple processes must operate harmoniously at sequential phases of development. This is implemented by mutual interactions between cells and tissues that together regulate the segregation and specification of cells, their growth and morphogenesis. The formation of the spinal cord and paraxial mesoderm derivatives exquisitely illustrate these processes. Following early gastrulation, while the vertebrate body elongates, a population of bipotent neuromesodermal progenitors resident in the posterior region of the embryo generate both neural and mesodermal lineages. At later stages, the somitic mesoderm regulates aspects of neural patterning and differentiation of both central and peripheral neural progenitors. Reciprocally, neural precursors influence the paraxial mesoderm to regulate somite-derived myogenesis and additional processes by distinct mechanisms. Central to this crosstalk is the activity of the axial notochord, which, via sonic hedgehog signaling, plays pivotal roles in neural, skeletal muscle and cartilage ontogeny. Here, we discuss the cellular and molecular basis underlying this complex developmental plan, with a focus on the logic of sonic hedgehog activities in the coordination of the neural-mesodermal axis.

Cellular aspects of somite formation in vertebrates

Vertebrate segmentation, the process that generates a regular arrangement of somites and thereby establishes the pattern of the adult body and of the musculoskeletal and peripheral nervous systems, was noticed many centuries ago. In the last few decades, there has been renewed interest in the process and especially in the molecular mechanisms that might account for its regularity and other spatial-temporal properties. Several models have been proposed but surprisingly, most of these do not provide clear links between the molecular mechanisms and the cell behaviours that generate the segmental pattern. Here we present a short survey of our current knowledge about the cellular aspects of vertebrate segmentation and the similarities and differences between different vertebrate groups in how they achieve their metameric pattern. Taking these variations into account should help to assess each of the models more appropriately.

Mesenchymal-to-epithelial transitions require tissue-specific interactions with distinct laminins

Mesenchymal-to-epithelial transition (MET) converts cells from migratory mesenchymal to polarized epithelial states. Despite its importance for both normal and pathological processes, very little is known about the regulation of MET in vivo. Here we exploit midgut morphogenesis in Drosophila melanogaster to investigate the mechanisms underlying MET. We show that down-regulation of the EMT transcription factor Serpent is required for MET, but not sufficient, as interactions with the surrounding mesoderm are also essential. We find that midgut MET relies on the secretion of specific laminins via the CopII secretory pathway from both mesoderm and midgut cells. We show that secretion of the laminin trimer containing the Wingblister α-subunit from the mesoderm is an upstream cue for midgut MET, leading to basal polarization of αPS1 integrin in midgut cells. Polarized αPS1 is required for the formation of a monolayered columnar epithelium and for the apical polarization of αPS3, Baz, and E-Cad. Secretion of a distinct LamininA-containing trimer from midgut cells is required to reinforce the localization of αPS1 basally, and αPS3 apically, for robust repolarization. Our data suggest that targeting these MET pathways, in conjunction with therapies preventing EMT, may present a two-pronged strategy toward blocking metastasis in cancer.

What we can learn from embryos to understand the mesenchymal-to-epithelial transition in tumor progression

Epithelial plasticity involved the terminal and transitional stages that occur during epithelial-to-mesenchymal transition (EMT) and mesenchymal-to-epithelial transition (MET), both are essential at different stages of early embryonic development that have been co-opted by cancer cells to undergo tumor metastasis. These processes are regulated at multiple instances, whereas the post-transcriptional regulation of key genes mediated by microRNAs is gaining major attention as a common and conserved pathway. In this review, we focus on discussing the latest findings of the cellular and molecular basis of the less characterized process of MET during embryonic development, with special attention to the role of microRNAs. Although we take in consideration the necessity of being cautious when extrapolating the obtained evidence, we propose some commonalities between early embryonic development and cancer progression that can shed light into our current understanding of this complex event and might aid in the design of specific therapeutic approaches.

Epithelial-mesenchymal plasticity: emerging parallels between tissue morphogenesis and cancer metastasis

Many cells possess epithelial–mesenchymal plasticity (EMP), which allows them to shift reversibly between adherent, static and more detached, migratory states. These changes in cell behaviour are driven by the programmes of epithelial–mesenchymal transition (EMT) and mesenchymal–epithelial transition (MET), both of which play vital roles during normal development and tissue homeostasis. However, the aberrant activation of these processes can also drive distinct stages of cancer progression, including tumour invasiveness, cell dissemination and metastatic colonization and outgrowth. This review examines emerging common themes underlying EMP during tissue morphogenesis and malignant progression, such as the context dependence of EMT transcription factors, a central role for partial EMTs and the nonlinear relationship between EMT and MET.

This article is part of a discussion meeting issue ‘Contemporary morphogenesis'.

The effects of lead on the development of somites in chick embryos (Gallus gallus domesticus) under in vitro conditions: a histological study

Lead (Pb) is one of the most abundant toxic metals in the environment that can cause a variety of harmful effects. During embryonic development of vertebrates, somites are temporary organs that give rise to skeletal muscle, cartilage, tendon, endothelial cells, and dermis. In this study, we investigated the effects of lead on the development of somites and their derivatives in chick embryos under in vitro conditions. For this propose, fertilized eggs of Gallus gallus domesticus were incubated until they reached the stage of 15-20 somites. The somites and notochord were isolated and treated with different concentrations of lead acetate (500, 1000, 2000, and 4000 ng ml-1) for 72 h. Our results indicated that high concentrations of lead reduced the nucleus diameter, reduced the synthesis of collagen, inhibited the formation of the cartilage matrix in somite cells, and disturbed the formation and order of myotubes. In conclusion, the results of the current study for the first time indicated the disturbing effects of lead on the development of somites in the chick embryo. Our results revealed that lead disturbed the development of somites in the chick embryo, which suggested that at high concentrations it can cause a serious mortal danger to life.

Making muscle: skeletal myogenesis in vivo and in vitro

Skeletal muscle is the largest tissue in the body and loss of its function or its regenerative properties results in debilitating musculoskeletal disorders. Understanding the mechanisms that drive skeletal muscle formation will not only help to unravel the molecular basis of skeletal muscle diseases, but also provide a roadmap for recapitulating skeletal myogenesis in vitro from pluripotent stem cells (PSCs). PSCs have become an important tool for probing developmental questions, while differentiated cell types allow the development of novel therapeutic strategies. In this Review, we provide a comprehensive overview of skeletal myogenesis from the earliest premyogenic progenitor stage to terminally differentiated myofibers, and discuss how this knowledge has been applied to differentiate PSCs into muscle fibers and their progenitors in vitro.

PAPC couples the segmentation clock to somite morphogenesis by regulating N-cadherin-dependent adhesion

Vertebrate segmentation is characterized by the periodic formation of epithelial somites from the mesenchymal presomitic mesoderm (PSM). How the rhythmic signaling pulse delivered by the segmentation clock is translated into the periodic morphogenesis of somites remains poorly understood. Here, we focused on the role of paraxial protocadherin (PAPC/Pcdh8) in this process. We showed that in chicken and mouse embryos, PAPC expression is tightly regulated by the clock and wavefront system in the posterior PSM. We observed that PAPC exhibits a striking complementary pattern to N-cadherin (CDH2), marking the interface of the future somite boundary in the anterior PSM. Gain and loss of function of PAPC in chicken embryos disrupted somite segmentation by altering the CDH2-dependent epithelialization of PSM cells. Our data suggest that clathrin-mediated endocytosis is increased in PAPC-expressing cells, subsequently affecting CDH2 internalization in the anterior compartment of the future somite. This in turn generates a differential adhesion interface, allowing formation of the acellular fissure that defines the somite boundary. Thus, periodic expression of PAPC in the anterior PSM triggers rhythmic endocytosis of CDH2, allowing for segmental de-adhesion and individualization of somites.

Time course and side-by-side analysis of mesodermal, pre-myogenic, myogenic and differentiated cell markers in the chicken model for skeletal muscle formation

The chicken is a well-established model for amniote (including human) skeletal muscle formation because the developmental anatomy of chicken skeletal muscle matches that of mammals. The accessibility of the chicken in the egg as well as the sequencing of its genome and novel molecular techniques have raised the profile of this model. Over the years, a number of regulatory and marker genes have been identified that are suited to monitor the progress of skeletal myogenesis both in wildtype and in experimental embryos. However, in the various studies, differing markers at different stages of development have been used. Moreover, contradictory results on the hierarchy of regulatory factors are now emerging, and clearly, factors need to be able to cooperate. Thus, a reference paper describing in detail and side-by-side the time course of marker gene expression during avian myogenesis is needed. We comparatively analysed onset and expression patterns of the key markers for the chicken immature paraxial mesoderm, for muscle-competent cells, for cells committed to myogenesis and for cells entering terminal differentiation. We performed this analysis from stages when the first paraxial mesoderm is being laid down to the stage when mesoderm formation comes to a conclusion. Our data show that, although the sequence of marker gene expression is the same at the various stages of development, the timing of the expression onset is quite different. Moreover, marker gene expression in myogenic cells being deployed from the dorsomedial and ventrolateral lips of the dermomyotome is different from those being deployed from the rostrocaudal lips, suggesting different molecular programs. Furthermore, expression of Myosin Heavy Chain genes is overlapping but different along the length of a myotube. Finally, Mef2c is the most likely partner of Mrf proteins, and, in contrast to the mouse and more alike frog and zebrafish fish, chicken Mrf4 is co-expressed with MyoG as cells enter terminal differentiation.

Impaired cytoskeletal arrangements and failure of ventral body wall closure in chick embryos treated with rock inhibitor (Y-27632)

AIM

Rho-associated kinase (ROCK) signaling regulates numerous fundamental developmental processes during embryogenesis, primarily by controlling actin-cytoskeleton assembly and cell contractility. ROCK knockout mice exhibit a ventral body wall defect (VBWD) phenotype due to disorganization of actin filaments at the umbilical ring. However, the exact molecular mechanisms leading to VBWD still remain unclear. Improper somitogenesis has been hypothesized to contribute to failure of VBW closure. We designed this study to investigate the hypothesis that administration of ROCK inhibitor (Y-27632) disrupts cytoskeletal arrangements in morphology during early chick embryogenesis, which may contribute to the development of VBWD.

METHODS

At 60 h incubation, chick embryos were explanted into shell-less culture and treated with 50 µL of vehicle for controls (n = 33) or 50 µL of 500 µM of Y-27632 for the experimental group (Y-27, n = 56). At 8 h post-treatment, RT-PCR was performed to evaluate mRNA levels of N-cadherin, E-cadherin and connexin43. Immunofluorescence confocal microscopy was performed to analyze the expression and distribution of actin, vinculin and microtubules in the neural tube and somites. A further cohort of embryos was treated in ovo by dropping 50 µL of vehicle or 50 µL of different concentrations of Y-27632 onto the embryo and allowing development to 12 and 14 days for further assessment.

RESULTS

Gene expression levels of N-cadherin, E-cadherin and connexin43 were significantly decreased in treated embryos compared with controls (p < 0.05). Thickened actin filament bundles were recorded in the neural tube of Y-27 embryos. In somites, cells were dissociated with reduced actin distribution in affected embryos. Clumping of vinculin expression was found in the neural tube and somites, whereas reduced expression of microtubules was observed in Y-27 embryos compared with controls. At 12 and 14 days of development, affected embryos presented with an enlarged umbilical ring and herniation of abdominal contents through the defect.

CONCLUSION

ROCK inhibition alters cytoskeletal arrangement during early chick embryogenesis, which may contribute to failure of anterior body wall closure causing VBWD at later stages of development.

Role of miR-140 in embryonic bone development and cancer

Bone is increasingly viewed as an endocrine organ with key biological functions. The skeleton produces hormones and cytokines, such as FGF23 and osteocalcin, which regulate an extensive list of homoeostatic functions. Some of these functions include glucose metabolism, male fertility, blood cell production and calcium/phosphate metabolism. Many of the genes regulating these functions are specific to bone cells. Some of these genes can be wrongly expressed by other malfunctioning cells, driving the generation of disease. The miRNAs are a class of non-coding RNA molecules that are powerful regulators of gene expression by suppressing and fine-tuning target mRNAs. Expression of one such miRNA, miR-140, is ubiquitous in chondrocyte cells during embryonic bone development. Activity in cells found in the adult breast, colon and lung tissue can silence genes required for tumour suppression. The realization that the same miRNA can be both normal and detrimental, depending on the cell, tissue and time point, provides a captivating twist to the study of whole-organism functional genomics. With the recent interest in miRNAs in bone biology and RNA-based therapeutics on the horizon, we present a review on the role of miR-140 in the molecular events that govern bone formation in the embryo. Cellular pathways involving miR-140 may be reactivated or inhibited when treating skeletal injury or disorder in adulthood. These pathways may also provide a novel model system when studying cancer biology of other cells and tissues.

Vascular Calcification and Stone Disease: A New Look towards the Mechanism

Calcium phosphate (CaP) crystals are formed in pathological calcification as well as during stone formation. Although there are several theories as to how these crystals can develop through the combined interactions of biochemical and biophysical factors, the exact mechanism of such mineralization is largely unknown. Based on the published scientific literature, we found that common factors can link the initial stages of stone formation and calcification in anatomically distal tissues and organs. For example, changes to the spatiotemporal conditions of the fluid flow in tubular structures may provide initial condition(s) for CaP crystal generation needed for stone formation. Additionally, recent evidence has provided a meaningful association between the active participation of proteins and transcription factors found in the bone forming (ossification) mechanism that are also involved in the early stages of kidney stone formation and arterial calcification. Our review will focus on three topics of discussion (physiological influences-calcium and phosphate concentration-and similarities to ossification, or bone formation) that may elucidate some commonality in the mechanisms of stone formation and calcification, and pave the way towards opening new avenues for further research.

The emergence of Pax7-expressing muscle stem cells during vertebrate head muscle development

Pax7 expressing muscle stem cells accompany all skeletal muscles in the body and in healthy individuals, efficiently repair muscle after injury. Currently, the in vitro manipulation and culture of these cells is still in its infancy, yet muscle stem cells may be the most promising route toward the therapy of muscle diseases such as muscular dystrophies. It is often overlooked that muscular dystrophies affect head and body skeletal muscle differently. Moreover, these muscles develop differently. Specifically, head muscle and its stem cells develop from the non-somitic head mesoderm which also has cardiac competence. To which extent head muscle stem cells retain properties of the early head mesoderm and might even be able to switch between a skeletal muscle and cardiac fate is not known. This is due to the fact that the timing and mechanisms underlying head muscle stem cell development are still obscure. Consequently, it is not clear at which time point one should compare the properties of head mesodermal cells and head muscle stem cells. To shed light on this, we traced the emergence of head muscle stem cells in the key vertebrate models for myogenesis, chicken, mouse, frog and zebrafish, using Pax7 as key marker. Our study reveals a common theme of head muscle stem cell development that is quite different from the trunk. Unlike trunk muscle stem cells, head muscle stem cells do not have a previous history of Pax7 expression, instead Pax7 expression emerges de-novo. The cells develop late, and well after the head mesoderm has committed to myogenesis. We propose that this unique mechanism of muscle stem cell development is a legacy of the evolutionary history of the chordate head mesoderm.

Coming together is a beginning: the making of an intervertebral disc

The intervertebral disc (IVD) is a complex fibrocartilaginous structure located between the vertebral bodies that allows for movement and acts as a shock absorber in our spine for daily activities. It is composed of three components: the nucleus pulposus (NP), annulus fibrosus, and cartilaginous endplate. The characteristics of these cells are different, as they produce specific extracellular matrix (ECM) for tissue function and the niche in supporting the differentiation status of the cells in the IVD. Furthermore, cell heterogeneities exist in each compartment. The cells and the supporting ECM change as we age, leading to degenerative outcomes that often lead to pathological symptoms such as back pain and sciatica. There are speculations as to the potential of cell therapy or the use of tissue engineering as treatments. However, the nature of the cells present in the IVD that support tissue function is not clear. This review looks at the origin of cells in the making of an IVD, from the earliest stages of embryogenesis in the formation of the notochord, and its role as a signaling center, guiding the formation of spine, and in its journey to become the NP at the center of the IVD. While our current understanding of the molecular signatures of IVD cells is still limited, the field is moving fast and the potential is enormous as we begin to understand the progenitor and differentiated cells present, their molecular signatures, and signals that we could harness in directing the appropriate in vitro and in vivo cellular responses in our quest to regain or maintain a healthy IVD as we age.

Somitogenesis: From somite to skeletal muscle

Myogenesis is controlled by an elaborate system of extrinsic and intrinsic regulatory mechanisms in all development stages. The aim of this review is to provide an overview of the different stages of myogenesis and muscle differentiation in mammals, starting from somitogenesis and analysis of the different portions that constitute the mature somite. Particular attention was paid to regulatory genes, in addition to mesodermal stem cells, which represent the earliest elements of myogenesis. Finally, the crucial role of growth factors, molecules of vital importance in contractile regulation, hormones and their function in skeletal muscle differentiation, growth and metabolism, and the role played by central nervous system, are discussed.

Evolutionarily conserved morphogenetic movements at the vertebrate head-trunk interface coordinate the transport and assembly of hypopharyngeal structures

The vertebrate head–trunk interface (occipital region) has been heavily remodelled during evolution, and its development is still poorly understood. In extant jawed vertebrates, this region provides muscle precursors for the throat and tongue (hypopharyngeal/hypobranchial/hypoglossal muscle precursors, HMP) that take a stereotype path rostrally along the pharynx and are thought to reach their target sites via active migration. Yet, this projection pattern emerged in jawless vertebrates before the evolution of migratory muscle precursors. This suggests that a so far elusive, more basic transport mechanism must have existed and may still be traceable today.

Here we show for the first time that all occipital tissues participate in well-conserved cell movements. These cell movements are spearheaded by the occipital lateral mesoderm and ectoderm that split into two streams. The rostrally directed stream projects along the floor of the pharynx and reaches as far rostrally as the floor of the mandibular arch and outflow tract of the heart. Notably, this stream leads and engulfs the later emerging HMP, neural crest cells and hypoglossal nerve. When we (i) attempted to redirect hypobranchial/hypoglossal muscle precursors towards various attractants, (ii) placed non-migratory muscle precursors into the occipital environment or (iii) molecularly or (iv) genetically rendered muscle precursors non-migratory, they still followed the trajectory set by the occipital lateral mesoderm and ectoderm. Thus, we have discovered evolutionarily conserved morphogenetic movements, driven by the occipital lateral mesoderm and ectoderm, that ensure cell transport and organ assembly at the head–trunk interface.

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At the vertebrate head–trunk interface, all tissues engage in stereotype cell movements.

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A ventrally–rostrally directed stream of cells leads along the floor of the pharynx to the developing jaw and outflow tract of the heart.

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The cell movements are spearheaded by the lateral mesoderm and surface ectoderm; muscle precursors for throat and tongue muscles (hypopharyngeal muscles); neural crest cells and outgrowing axons of the hypoglossal nerve follow.

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Hypopharyngeal muscle precursors follow the trajectory set by the lateral mesoderm and ectoderm, even when challenged with ectopic attractants or when rendered non-migratory.

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The newly discovered cell movements are the likely ground state for cell transport and organ assembly at the head–trunk interface before actively migrating muscle precursors evolved in “bony” (osteichthyan) vertebrates.

Regulation of mesenchymal-to-epithelial transition by PARAXIS during somitogenesis

BACKGROUND

Dynamic alterations in cell shape, migration, and adhesion play a central role in tissue morphogenesis during embryonic development and congenital disease. The mesenchymal-to-epithelial transition that occurs during vertebrate somitogenesis is required for proper patterning of the axial musculoskeletal system. Somitic MET is initiated in the presomitic mesoderm by PARAXIS-dependent changes in cell adhesion, cell polarity, and the composition of the extracellular matrix. However, the target genes downstream of the transcription factor PARAXIS remain poorly described.

RESULTS

A genome-wide comparison of gene expression in the anterior presomitic mesoderm and newly formed somites of Paraxis(-/-) embryos resulted in a set of deregulated genes enriched for factors associated with extracellular matrix and cytoskeletal organization and cell-cell and cell-ECM adhesion. The greatest change in expression was seen in fibroblast activation protein alpha (Fap), encoding a dipeptidyl peptidase capable of increasing fibronectin and collagen fiber organization in extracellular matrix. Further, downstream genes in the Wnt and Notch signaling pathways were downregulated, predicting that PARAXIS participates in positive feedback loops in both pathways.

CONCLUSIONS

These data demonstrate that PARAXIS initiates and stabilizes somite epithelialization by integrating signals from multiple pathways to control the reorganization of the ECM, cytoskeleton, and adhesion junctions during MET.

Scoliosis and segmentation defects of the vertebrae

The vertebral column derives from somites, which are transient paired segments of mesoderm that surround the neural tube in the early embryo. Somites are formed by a genetic mechanism that is regulated by cyclical expression of genes in the Notch, Wnt, and fibroblast growth factor (FGF) signaling pathways. These oscillators together with signaling gradients within the presomitic mesoderm help to set somitic boundaries and rostral-caudal polarity that are essential for the precise patterning of the vertebral column. Disruption of this mechanism has been identified as the cause of severe segmentation defects of the vertebrae in humans. These segmentation defects are part of a spectrum of spinal disorders affecting the skeletal elements and musculature of the spine, resulting in curvatures such as scoliosis, kyphosis, and lordosis. While the etiology of most disorders with spinal curvatures is still unknown, genetic and developmental studies of somitogenesis and patterning of the axial skeleton and musculature are yielding insights into the causes of these diseases.

An evo-devo view on the origin of the backbone: evolutionary development of the vertebrae

Vertebral columns are a group of diverse axial structures that define the vertebrates and provide supportive, locomotive, protective, and other important functions. The embryonic origin of the first vertebral element in this subphylum, the lamprey arcualia, has remained a puzzle for more than a century although much developmental and genetic progress has been made. The comparative approach is a very powerful tool for studying vertebrate morphological variation and understanding how the novel structures were generated during evolution. Here, I first briefly describe the vertebral structures and their developmental processes in major taxa, and then analyze the most recently published data on the basal vertebrates. Finally, an ontogenetic and phylogenetic origin is proposed. The lamprey may have already evolved a sclerotome, which gave rise to arcualia ontogenetically; whole genome duplications likely promoted the establishment of sclerotomal core genetic program by gene co-options.

Avian somitogenesis: translating time and space into pattern

Vertebrates have a metameric bodyplan that is based on the presence of paired somites. Somites develop from the segmental plate in a cranio-caudal sequence. At the same time, new material is added from Hensen's node, the primitive streak and the tailbud. In this way, the material residing in the segmental plate remains constant and comprises 12 prospective somites on each side. Prospective segment borders are not yet determined in the caudal segmental plate. Prior to segmentation, the cranial segmental plate undergoes epithelialization, which is controlled by signals from the neural tube and ectoderm. The bHLH transcription factor Paraxis is critically involved in this process. Formation of a new somite from the cranial end of the segmental plate is a highly controlled process involving complex cell movements in relation to each other. Hox genes specify regional identity of the somites and their derivatives. In the chicken a transposition of thoracic into cervical vertebrae has occurred as compared to the mouse. Transcription factors of the bHLH and homeodomain type also specify the cranio-caudal polarity and that of particular cell groups within the somites. According to segmentation models, somitogenesis is under the control of a "segmentation clock" in combination with a morphogen gradient. This hypothesis has recently found support from molecular data, especially the cycling expression of genes such as cHairy1 and Lunatic Fringe, which depend on the Notch/Delta pathway of signal transduction. FGF8 has been described to be distributed along a cranio-caudal gradient. The first oscillating gene described shown to be independent of Notch is Axin2, encoding a negative regulator of the canonical Wnt pathway and a target of Wnt3a. Wnt3a and Axin2 show a similar distribution as FGF8 with high levels in the tailbud. The chick embryo has recently become accessible to molecular approaches such as overexpression by electroporation and RNA interference which can be expected to help elucidating some of the still open questions concerning somitogenesis.

From segment to somite: segmentation to epithelialization analyzed within quantitative frameworks

One of the most visually striking patterns in the early developing embryo is somite segmentation. Somites form as repeated, periodic structures in pairs along nearly the entire caudal vertebrate axis. The morphological process involves short- and long-range signals that drive cell rearrangements and cell shaping to create discrete, epithelialized segments. Key to developing novel strategies to prevent somite birth defects that involve axial bone and skeletal muscle development is understanding how the molecular choreography is coordinated across multiple spatial scales and in a repeating temporal manner. Mathematical models have emerged as useful tools to integrate spatiotemporal data and simulate model mechanisms to provide unique insights into somite pattern formation. In this short review, we present two quantitative frameworks that address the morphogenesis from segment to somite and discuss recent data of segmentation and epithelialization.

Redefining the role of ectoderm in somitogenesis: a player in the formation of the fibronectin matrix of presomitic mesoderm

The absence of ectoderm impairs somite formation in cultured presomitic mesoderm (PSM) explants, suggesting that an ectoderm-derived signal is essential for somitogenesis. Here we show in chick that the standard enzymatic treatments used for explant isolation destroy the fibronectin matrix surrounding the anterior PSM, which fails to form somites when cultured for 6 hours. By contrast, explants isolated with collagenase retain their fibronectin matrix and form somites under identical culture conditions. The additional presence of ectoderm enhances somite formation, whereas endoderm has no effect. Furthermore, we show that pancreatin-isolated PSM explants cultured in fibronectin-supplemented medium, form significantly more somites than control explants. Interestingly, ectoderm is the major producer of fibronectin (Fn1) transcripts, whereas all but the anterior-most region of the PSM expresses the fibronectin assembly receptor, integrin alpha5 (Itga5). We thus propose that the ectoderm-derived fibronectin is assembled by mesodermal alpha5beta1 integrin on the surface of the PSM. Finally, we demonstrate that inhibition of fibronectin fibrillogenesis in explants with ectoderm abrogates somitogenesis. We conclude that a fibronectin matrix is essential for morphological somite formation and that a major, previously unrecognised role of ectoderm in somitogenesis is the synthesis of fibronectin.

Mesenchymal to epithelial transition in development and disease

Cellular plasticity is fundamental to embryonic development. The importance of cellular transitions in development is first apparent during gastrulation when the process of epithelial to mesenchymal transition transforms polarized epithelial cells into migratory mesenchymal cells that constitute the embryonic and extraembryonic mesoderm. It is now widely accepted that this developmental pathway is exploited in various disease states, including cancer progression. The loss of epithelial characteristics and the acquisition of a mesenchymal-like migratory phenotype are crucial to the development of invasive carcinoma and metastasis. However, given the morphological similarities between primary tumour and metastatic lesions, it is likely that tumour cells re-activate certain epithelial properties through a mesenchymal to epithelial transition (MET) at the secondary site, although this is yet to be proven. MET is also an essential developmental process and has been extensively studied in kidney organogenesis and somitogenesis. In this review we describe the process of MET, highlight important mediators, and discuss their implication in the context of cancer progression.

Compartmentalised expression of Delta-like 1 in epithelial somites is required for the formation of intervertebral joints

Background

Expression of the mouse Delta-like 1 (Dll1) gene in the presomitic mesoderm and in the caudal halves of somites of the developing embryo is required for the formation of epithelial somites and for the maintenance of caudal somite identity, respectively. The rostro-caudal polarity of somites is initiated early on within the presomitic mesoderm in nascent somites. Here we have investigated the requirement of restricted Dll1 expression in caudal somite compartments for the maintenance of rostro-caudal somite polarity and the morphogenesis of the axial skeleton. We did this by overexpressing a functional copy of the Dll1 gene throughout the paraxial mesoderm, in particular in anterior somite compartments, during somitogenesis in transgenic mice.

Results

Epithelial somites were generated normally and appeared histologically normal in embryos of two independent Dll1 over-expressing transgenic lines. Gene expression analyses of rostro-caudal marker genes suggested that over-expression of Dll1 without restriction to caudal compartments was not sufficient to confer caudal identity to rostral somite halves in transgenic embryos. Nevertheless, Dll1 over-expression caused dysmorphologies of the axial skeleton, in particular, in morphological structures that derive from the articular joint forming compartment of vertebrae. Accordingly, transgenic animals exhibited missing or reduced intervertebral discs, rostral and caudal articular processes as well as costal heads of ribs. In addition, the midline of the vertebral column did not develop normally. Transgenic mice had open neural arches and split vertebral bodies with ectopic pseudo-growth plates. Endochondral bone formation and ossification in the developing vertebrae were delayed.

Conclusion

The mice overexpressing Dll1 exhibit skeletal dysmorphologies that are also evident in several mutant mice with defects in somite compartmentalisation. The Dll1 transgenic mice demonstrate that vertebral dysmorphologies such as bony fusions of vertebrae and midline vertebral defects can occur without apparent changes in somitic rostro-caudal marker gene expression. Also, we demonstrate that the over-expression of the Dll1 gene in rostral epithelial somites is not sufficient to confer caudal identity to rostral compartments. Our data suggest that the restricted Dll1 expression in caudal epithelial somites may be particularly required for the proper development of the intervertebral joint forming compartment.

Amniote somite derivatives

Somites are segments of paraxial mesoderm that give rise to a multitude of tissues in the vertebrate embryo. Many decades of intensive research have provided a wealth of data on the complex molecular interactions leading to the formation of various somitic derivatives. In this review, we focus on the crucial role of the somites in building the body wall and limbs of amniote embryos. We give an overview on the current knowledge on the specification and differentiation of somitic cell lineages leading to the development of the vertebral column, skeletal muscle, connective tissue, meninges, and vessel endothelium, and highlight the importance of the somites in establishing the metameric pattern of the vertebrate body.

Regulation of scapula development

The scapula is a component of the shoulder girdle. Its structure has changed greatly during evolution. For example, in humans it is a large quite flat triangular bone whereas in chicks it is a long blade like structure. In this review we describe the mechanisms that control the formation of the scapula. To assimilate our understanding regarding the development of the scapula blade we start by addressing the issue concerning the origin of the scapula. Experiments using somite extirpation, chick-quail cell marking system and genetic cell labelling techniques in a variety of species have suggested that the scapula had its origin in the somites. For example we have shown in the chick that the scapula blade originates from the somite, while the cranial part, which articulates with the upper limb, is derived from the somatopleure of the forelimb field. In the second and third part of the review we discuss the compartmental origin of this bone and the signalling molecules that control the scapula development. It is very interesting that the scapula blade originates from the dorsal compartment, dermomyotome, which has been previously been associated as a source of muscle and dermis, but not of cartilage. Thus, the development of the scapula blade can be considered a case of dermomyotomal chondrogenesis. Our results show that the dermomyotomal chondrogenesis differ from the sclerotomal chondrogenesis. Firstly, the scapula precursors are located in the hypaxial domain of the dermomyotome, from which the hypaxial muscles are derived. The fate of the scapula precursors, like the hypaxial muscle, is controlled by ectoderm-derived signals and BMPs from the lateral plate mesoderm. Ectoderm ablation and inhibition of BMP activity interfers the scapula-specific Pax1 expression and scapula blade formation. However, only somite cells in the cervicothoracic transition region appear to be committed to form scapula. This indicates that the intrinsic segment specific information determines the scapula forming competence of the somite cells. Taken together, we conclude that the scapula forming cells located within the hypaxial somitic domain require BMP signals derived from the somatopleure and as yet unidentified signals from ectoderm for activation of their coded intrinsic segment specific chondrogenic programme. In the last part we discuss the new data that provides evidence that neural crest contributes for the development of the scapula.

The molecular setup of the avian head mesoderm and its implication for craniofacial myogenesis

The head mesoderm is the mesodermal tissue on either side of the brain, from forebrain to hindbrain levels, and gives rise to the genuine head muscles. Its relatedness to the more posterior paraxial mesoderm, the somites, which generate the muscles of the trunk, is conversely debated. To gain insight into the molecular setup of the head mesoderm, its similarity or dissimilarity to the somitic mesoderm, and the implications of its setup for the progress of muscle formation, we investigated the expression of markers (1) for mesoderm segmentation and boundary formation, (2) for regional specification and somitogenesis and (3) for the positive and negative control of myogenic differentiation. We show that the head mesoderm is molecularly distinct from somites. It is not segmented; even the boundary to the first somite is ill-defined. Importantly, the head mesoderm lacks the transcription factors driving muscle differentiation while genes suppressing differentiation and promoting cell proliferation are expressed. These factors show anteroposteriorly and dorsoventrally regionalised but overlapping expression. Notably, expression extends into the areas that actively contribute to the heart, overlapping with the expression of cardiac markers.

Regulation of ectodermal Wnt6 expression by the neural tube is transduced by dermomyotomal Wnt11: a mechanism of dermomyotomal lip sustainment

Ectodermal Wnt6 plays an important role during development of the somites and the lateral plate mesoderm. In the course of development, Wnt6expression shows a dynamic pattern. At the level of the segmental plate and the epithelial somites, Wnt6 is expressed in the entire ectoderm overlying the neural tube, the paraxial mesoderm and the lateral plate mesoderm. With somite maturation, expression becomes restricted to the lateral ectoderm covering the ventrolateral lip of the dermomyotome and the lateral plate mesoderm. To study the regulation of Wnt6 expression, we have interfered with neighboring signaling pathways. We show that Wnt1 and Wnt3a signaling from the neural tube inhibit Wnt6 expression in the medial surface ectoderm via dermomyotomal Wnt11. We demonstrate that Wnt11 is an epithelialization factor acting on the medial dermomyotome, and present a model suggesting Wnt11 and Wnt6 as factors maintaining the epithelial nature of the dorsomedial and ventrolateral lips of the dermomyotome, respectively,during dermomyotomal growth.

Neural tube derived signals and Fgf8 act antagonistically to specify eye versus mandibular arch muscles

Recent knockout experiments in the mouse generated amazing craniofacial skeletal muscle phenotypes. Yet none of the genes could be placed into a molecular network, because the programme to control the development of muscles in the head is not known. Here we show that antagonistic signals from the neural tube and the branchial arches specify extraocular versus branchiomeric muscles. Moreover, we identified Fgf8 as the branchial arch derived signal. However, this molecule has an additional function in supporting the proliferative state of myoblasts, suppressing their differentiation, while a further branchial arch derived signal, namely Bmp7, is an overall negative regulator of head myogenesis.

Oscillations of the snail genes in the presomitic mesoderm coordinate segmental patterning and morphogenesis in vertebrate somitogenesis

The segmented body plan of vertebrate embryos arises through segmentation of the paraxial mesoderm to form somites. The tight temporal and spatial control underlying this process of somitogenesis is regulated by the segmentation clock and the FGF signaling wavefront. Here, we report the cyclic mRNA expression of Snail 1 and Snail 2 in the mouse and chick presomitic mesoderm (PSM), respectively. Whereas Snail genes' oscillations are independent of NOTCH signaling, we show that they require WNT and FGF signaling. Overexpressing Snail 2 in the chick embryo prevents cyclic Lfng and Meso 1 expression in the PSM and disrupts somite formation. Moreover, cells mis-expressing Snail 2 fail to express Paraxis, remain mesenchymal, and are thereby inhibited from undergoing the epithelialization event that culminates in the formation of the epithelial somite. Thus, Snail genes define a class of cyclic genes that coordinate segmentation and PSM morphogenesis.

The Neural Tube Is Required to Maintain Primary Segmentation in the Sclerotome

Primary segmentation in vertebrates is considered to be an intrinsic property of the presomitic paraxial mesoderm controlled by a number of interconnected oscillating signals. Re-segmentation, in contrast, has been shown to depend on signals from the axial structures. Here we report the requirement of the neural tube for maintenance but not formation of primary segmentation in chick embryos. Unilateral removal of the neural tube, next to the anterior presomitic mesoderm, caused disturbed development of the neural arches and the spinous processes. But already 24 h postsurgery, the sclerotome showed loss of primary segmentation in the craniocaudal axis. Cells strongly expressing twist and not showing any segmentation were located dorsomedially between the remaining left half of the neural tube and the right side dermomyotome, which frequently was truncated medially.

Establishment of the epaxial–hypaxial boundary in the avian myotome

Trunk skeletal muscles are segregated into dorsomedial epaxial and ventrolateral hypaxial muscles, separated by a myoseptum. In amniotes, they are generated from a transient structure, the dermomyotome, which lays down muscle, namely the myotome underneath. However, the dermomyotome and myotome are dorsoventrally continuous, with no morphologically defined epaxial-hypaxial boundary. The transcription factors En1 and Sim1 have been shown to molecularly subdivide the amniote dermomyotome, with En1 labeling the epaxial dermomyotome and Sim1 the hypaxial counterpart. Here, we demonstrate that En1 and Sim1 expression persists in cells leaving the dermomyotome, superimposing the expression boundary onto muscle and skin. En1-expressing cells colonize the myotome initially from the rostral and caudal lips, and slightly later, directly from the de-epithelializing dermomyotomal center. En1 expression in the myotome is concomitant with the appearance of Fgfr4/Pax7-expressing mitotically active myoblasts. This finding suggests that Fgfr4+/Pax7+/En1+ cells carry their expression with them when entering the myotome. Furthermore, it suggests that the epaxial-hypaxial boundary of the myotome is established through the late arising, mitotically active myoblasts.

beta-Catenin-dependent Wnt signalling controls the epithelial organisation of somites through the activation of paraxis

The regulation of cell adhesion in epithelia is a fundamental process governing morphogenesis in embryos and a key step in the progression of invasive cancers. Here, we have analysed the molecular pathways controlling the epithelial organisation of somites. Somites are mesodermal epithelial structures of vertebrate embryos that undergo several changes in cell adhesion during early embryonic life. We show that Wnt6 in the ectoderm overlaying the somites, but not Wnt1 in the neighbouring neural tube, is the most likely candidate molecule responsible for the maintenance of the epithelial structure of the dorsal compartment of the somite: the dermomyotome. We characterised the signalling pathway that mediates Wnt6 activity. Our experiments suggest that the Wnt receptor molecule Frizzled7 probably transduces the Wnt6 signal. Intracellularly, this leads to the activation of the beta-catenin/LEF1-dependent pathway. Finally, we demonstrate that the bHLH transcription factor paraxis, which was previously shown to be a major player in the epithelial organisation of somites, is a target of the beta-catenin signal. We conclude that beta-catenin activity, initiated by Wnt6 and mediated by paraxis, is required for the maintenance of the epithelial structure of somites.

Mesenchymal-to-epithelial transition during somitic segmentation: a novel approach to studying the roles of Rho family GTPases in morphogenesis

During early development in vertebrates, cells change their shapes dramatically both from epithelial to mesenchymal and also from mesenchymal to epithelial, enabling the body to form complex tissues and organs. Using somitogenesis as a novel model, Rho family GTPases have recently been shown to play essential and differential roles in individual cell behaviors in actual developing embryos. Levels of Cdc42 activity provide a binary switch wherein high Cdc42 levels allow the cells to remain mesenchymal, while low Cdc42 levels produce epithelialization. Rac1 activity needs to be precisely controlled for proper epithelialization through the bHLH transcription factor Paraxis. Somitogenesis is expected to serve as an excellent model with which one can understand how the functions of developmental genes are resolved into the morphogenetic behavior of individual cells.

RNAi-induced targeted silencing of developmental control genes during chicken embryogenesis

The RNA interference technique is a powerful tool to understand gene function. Intriguingly, RNA interference cannot only be used for cells in vitro, but also in living organisms. Here, we have adapted the method for use in the chick embryo. However, this technique is limited by the uncertainty in predicting the RNAi transfection efficiency and site in the embryo. Hence, we elaborated a modified vector system, pEGFP-shRNA, which can coexpress enhanced green fluorescent protein (EGFP) and short hairpin RNA (shRNA) simultaneously to facilitate analysis of gene silencing in chicken embryos. We tested the silencing of two highly conserved genes (cAxin2, cParaxis), which play crucial roles in chicken embryonic developmental processes. For each target gene, four to five small DNA inserts, each of them encoding one shRNA, were selected and cloned individually to the vector downstream of the Pol III promoter (either human H1 or U6 promoter), which shared with highly conserved motifs in human and chicken. The pEGFP-shRNA constructs were electroporated into the neural tube or somites. After subsequent re-incubation of 24 h, the EGFP expression, with green fluorescent signal, indicated the transfected regions in the neural tube or somites. The EGFP expressing embryos were further submitted into the process of in situ hybridization for examination of the silencing effects. The results show that the EGFP signal in transfected areas correlated with the silencing of the target genes (cAxin2, cParaxis). The cAxin2 expression was inhibited by shRNAs of either targeting the RGS domain or the DAX domain coding region. The cParaxis mRNA level in transgenic somites and the related migratory myogenic population was also reduced. The results suggest that our novel dual expression EGFP-shRNA system opens a new possibility to study gene function in a convenient and efficient way.

Control of the segmentation process by graded MAPK/ERK activation in the chick embryo

The regular spacing of somites during vertebrate embryogenesis involves a dynamic gradient of FGF signaling that controls the timing of maturation of cells in the presomitic mesoderm (PSM). How the FGF signal is transduced by PSM cells is unclear. Here, we first show that the FGF gradient is translated into graded activation of the extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) pathway along the PSM in the chicken embryo. Using in ovo electroporation of PSM cells, we demonstrate that constitutive activation of ERK signaling in the PSM blocks segmentation by preventing maturation of PSM cells, thus phenocopying the overexpression of FGF8. Conversely, inhibition of ERK phosphorylation mimics a loss of function of FGF signaling in the PSM. Interestingly, video microscopy analysis of cell movements shows that ERK regulates the motility of PSM cells, suggesting that the decrease of cell movements along the PSM enables mesenchymal PSM cells to undergo proper segmentation. Together, our data demonstrate that ERK is the effector of the gradient of FGF in the PSM that controls the segmentation process.

A novel signal induces a segmentation fissure by acting in a ventral-to-dorsal direction in the presomitic mesoderm

We describe here a novel inductive action that operates during somitic segmentation in chicken embryos. We previously reported that the posterior border cells located at a next-forming boundary in the anterior end of the presomitic mesoderm (PSM) exhibit an inductive activity that acts on the anterior cells to cause the formation of a somitic fissure (Sato, Y., Yasuda, K., Takahashi, Y., 2002. Morphological boundary forms by a novel inductive event mediated by Lunatic fringe and Notch during somitic segmentation. Development 129, 3633-3644). In this study, we have found a second inductive action along the dorso-ventral (D-V) axis during fissure formation. When relocated into a non-segmenting region of PSM, the ventral-most cells taken from the presumptive boundary are sufficient to induce an ectopic fissure in host cells. The ventrally derived signal acts in a ventral-to-dorsal direction but not ventrally, regardless of where the ventral cells are placed. This directional signaling is governed, at least in part, by the signal-receiving cells of the PSM, which we found to be polarized along the D-V axis, and also by intimate cell-cell interactions. Finally, we have observed that morphological segmentation is able to rearrange the anterior and posterior regionalization of individual somites. These findings suggest that discrete unidirectional signals along both the antero-posterior and the D-V axes act coordinately to achieve the formation of the intersomitic fissure, and also that fissure formation is important for the fine-tuning of A-P regionalization in individual somites.