Mechanisms of vertebrate segmentation

Antagonistic regulation of homeologous uncx.L and uncx.S genes orchestrates myotome and sclerotome differentiation in the evolutionarily divergent vertebral column of Xenopus laevis

In anurans, the vertebral column diverges widely from that of other tetrapods; yet the molecular mechanisms underlying its morphogenesis remain largely unexplored. In this study, we investigate the role of the homeologous uncx.L and uncx.S genes in the vertebral column morphogenesis of the allotetraploid frog Xenopus laevis. We initiated our study by cloning the uncx orthologous genes in the anuran Xenopus and determining their spatial expression patterns using in situ hybridization. Additionally, we employed gain-of-function and loss-of-function approaches through dexamethasone-inducible uncx constructs and antisense morpholino oligonucleotides, respectively. Comparative analysis of the messenger RNA sequences of homeologous uncx genes revealed that the uncx.L variant lacks the eh1-like repressor domain. Our spatial expression analysis indicated that in the presomitic mesoderm and somites, the transcripts of uncx.L and uncx.S are located in overlapping domains. Alterations in the function of uncx genes significantly impact the development and differentiation of the sclerotome and myotome, resulting in axial skeleton malformations. Our findings suggest a scenario where the homeologous genes uncx.L and uncx.S exhibit antagonistic functions during somitogenesis. Specifically, uncx.S appears to be crucial for sclerotome development and differentiation, while uncx.L primarily influences myotome development. Postallotetraploidization, the uncx.L gene in X. laevis evolved to lose its eh1-like repressor domain, transforming into a "native dominant negative" variant that potentially competes with uncx.S for the same target genes. Finally, the histological analysis revealed that uncx.S expression is necessary for the correct formation of pedicles and neural arch of the vertebrae, and uncx.L is required for trunk muscle development.

Keeping development on time: Insights into post-transcriptional mechanisms driving oscillatory gene expression during vertebrate segmentation

Biological time keeping, or the duration and tempo at which biological processes occur, is a phenomenon that drives dynamic molecular and morphological changes that manifest throughout many facets of life. In some cases, the molecular mechanisms regulating the timing of biological transitions are driven by genetic oscillations, or periodic increases and decreases in expression of genes described collectively as a "molecular clock." In vertebrate animals, molecular clocks play a crucial role in fundamental patterning and cell differentiation processes throughout development. For example, during early vertebrate embryogenesis, the segmentation clock regulates the patterning of the embryonic mesoderm into segmented blocks of tissue called somites, which later give rise to axial skeletal muscle and vertebrae. Segmentation clock oscillations are characterized by rapid cycles of mRNA and protein expression. For segmentation clock oscillations to persist, the transcript and protein molecules of clock genes must be short-lived. Faithful, rhythmic, genetic oscillations are sustained by precise regulation at many levels, including post-transcriptional regulation, and such mechanisms are essential for proper vertebrate development. This article is categorized under: RNA Export and Localization > RNA Localization RNA Turnover and Surveillance > Regulation of RNA Stability Translation > Regulation.

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.

Ca2+ as a coordinator of skeletal muscle differentiation, fusion and contraction

Muscle regeneration is essential for vertebrate muscle homeostasis and recovery after injury. During regeneration, muscle stem cells differentiate into myocytes, which then fuse with pre-existing muscle fibres. Hence, differentiation, fusion and contraction must be tightly regulated during regeneration to avoid the disastrous consequences of premature fusion of myocytes to actively contracting fibres. Cytosolic calcium (Ca²⁺ ), which is coupled to both induction of myogenic differentiation and contraction, has more recently been implicated in the regulation of myocyte-to-myotube fusion. In this viewpoint, we propose that Ca²⁺ -mediated coordination of differentiation, fusion and contraction is a feature selected in the amniotes to facilitate muscle regeneration.

The vertebrate Embryo Clock: Common players dancing to a different beat

Vertebrate embryo somitogenesis is the earliest morphological manifestation of the characteristic patterned structure of the adult axial skeleton. Pairs of somites flanking the neural tube are formed periodically during early development, and the molecular mechanisms in temporal control of this early patterning event have been thoroughly studied. The discovery of a molecular Embryo Clock (EC) underlying the periodicity of somite formation shed light on the importance of gene expression dynamics for pattern formation. The EC is now known to be present in all vertebrate organisms studied and this mechanism was also described in limb development and stem cell differentiation. An outstanding question, however, remains unanswered: what sets the different EC paces observed in different organisms and tissues? This review aims to summarize the available knowledge regarding the pace of the EC, its regulation and experimental manipulation and to expose new questions that might help shed light on what is still to unveil.

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.

Homology of process: developmental dynamics in comparative biology

Comparative biology builds up systematic knowledge of the diversity of life, across evolutionary lineages and levels of organization, starting with evidence from a sparse sample of model organisms. In developmental biology, a key obstacle to the growth of comparative approaches is that the concept of homology is not very well defined for levels of organization that are intermediate between individual genes and morphological characters. In this paper, we investigate what it means for ontogenetic processes to be homologous, focusing specifically on the examples of insect segmentation and vertebrate somitogenesis. These processes can be homologous without homology of the underlying genes or gene networks, since the latter can diverge over evolutionary time, while the dynamics of the process remain the same. Ontogenetic processes like these therefore constitute a dissociable level and distinctive unit of comparison requiring their own specific criteria of homology. In addition, such processes are typically complex and nonlinear, such that their rigorous description and comparison requires not only observation and experimentation, but also dynamical modelling. We propose six criteria of process homology, combining recognized indicators (sameness of parts, morphological outcome and topological position) with novel ones derived from dynamical systems modelling (sameness of dynamical properties, dynamical complexity and evidence for transitional forms). We show how these criteria apply to animal segmentation and other ontogenetic processes. We conclude by situating our proposed dynamical framework for homology of process in relation to similar research programmes, such as process structuralism and developmental approaches to morphological homology.

Etiopathogenesis of adolescent idiopathic scoliosis: Review of the literature and new epigenetic hypothesis on altered neural crest cells migration in early embryogenesis as the key event

Adolescent idiopathic scoliosis (AIS) affects 2-3% of children. Numerous hypotheses on etiologic/causal factors of AIS were investigated, but all failed to identify therapeutic targets and hence failed to offer a cure. Therefore, currently there are only two options to minimize morbidity of the patients suffering AIS: bracing and spinal surgery. From the beginning of 1960th, spinal surgery, both fusion and rod placement, became the standard of management for progressive adolescent idiopathic spine deformity. However, spinal surgery is often associated with complications. These circumstances motivate AIS scientific community to continue the search for new etiologic and causal factors of AIS. While the role of the genetic factors in AIS pathogenesis was investigated intensively and universally recognized, these studies failed to nominate mutation of a particular gene or genes combination responsible for AIS development. More recently epigenetic factors were suggested to play causal role in AIS pathogenesis. Sharing this new approach, we investigated scoliotic vertebral growth plates removed during vertebral fusion (anterior surgery) for AIS correction. In recent publications we showed that cells from the convex side of human scoliotic deformities undergo normal chondrogenic/osteogenic differentiation, while cells from the concave side acquire a neuronal phenotype. Based on these facts we hypothesized that altered neural crest cell migration in early embryogenesis can be the etiological factor of AIS. In particular, we suggested that neural crest cells failed to migrate through the anterior half of somites and became deposited in sclerotome, which in turn produced chondrogenic/osteogenic-insufficient vertebral growth plates. To test this hypothesis we conducted experiments on chicken embryos with arrest neural crest cell migration by inhibiting expression of Paired-box 3 (Pax3) gene, a known enhancer and promoter of neural crest cells migration and differentiation. The results showed that chicken embryos treated with Pax3 siRNA (microinjection into the neural tube, 44 h post-fertilization) progressively developed scoliotic deformity during maturation. Therefore, this analysis suggests that although adolescent idiopathic scoliosis manifests in children around puberty, the real onset of the disease is of epigenetic nature and takes place in early embryogenesis and involves altered neural crest cells migration. If these results confirmed and further elaborated, the hypothesis may shed new light on the etiology and pathogenesis of AIS.

A developmental perspective of homology and evolutionary novelty

The development and evolution of multicellular body plans is complex. Many distinct organs and body parts must be reproduced at each generation, and those that are traceable over long time scales are considered homologous. Among the most pressing and least understood phenomena in evolutionary biology is the mode by which new homologs, or "novelties" are introduced to the body plan and whether the developmental changes associated with such evolution deserve special treatment. In this chapter, we address the concepts of homology and evolutionary novelty through the lens of development. We present a series of case studies, within insects and vertebrates, from which we propose a developmental model of multicellular organ identity. With this model in hand, we make predictions regarding the developmental evolution of body plans and highlight the need for more integrative analysis of developing systems.

Chiari malformation and atlantoaxial instability: problems of co-existence

BACKGROUND

Association of Chiari malformation and atlantoaxial subluxation varies. There is a complex relationship between the two, bony and soft tissue pathologies.

METHODS

This is a review of various articles available from the literature on the management of Chiari and its association with atlantoaxial instability.

RESULTS

We have an experience of operating on 86 cases of paediatric atlantoaxial subluxation, of which 12 had Chiari malformation diagnosed preoperatively (13.95%). Of the 76 children with Chiari malformations operated on by us, 11 had associated atlantoaxial subluxation diagnosed on imaging (14.47%).

CONCLUSIONS

Re-alignment and reduction with fixation may be effective in achieving decompression in cases where reduction is possible from posterior approach. In these cases, posterior fixation is all that is required. If reduction is not possible from posterior and there is "fixed" ventral compression, anterior decompression needs to be combined with posterior fixation. In most cases, direct posterior decompression is warranted.

The Neural Crest: A Remarkable Model System for Studying Development and Disease

Neural crest cells are the embryonic precursors of most neurons and all glia of the peripheral nervous system, pigment cells, some endocrine components, and connective tissue of the head, face, neck, and heart. Following induction, crest cells undergo an epithelial to mesenchymal transition that enables them to migrate along specific pathways culminating in their phenotypic differentiation. Researching this unique embryonic population has revealed important understandings of basic biological and developmental principles. These principles are likely to assist in clarifying the etiology and help in finding strategies for the treatment of neural crest diseases, collectively termed neurocristopathies. The progress achieved in neural crest research is made feasible thanks to the continuous development of species-specific in vivo and in vitro paradigms and more recently the possibility to produce neural crest cells and specific derivatives from embryonic or induced pluripotent stem cells. All of the above assist us in elucidating mechanisms that regulate neural crest development using state-of-the art cellular, molecular, and imaging approaches.

Neural crest and the patterning of vertebrate craniofacial muscles

Patterning of craniofacial muscles overtly begins with the activation of lineage-specific markers at precise, evolutionarily conserved locations within prechordal, lateral, and both unsegmented and somitic paraxial mesoderm populations. Although these initial programming events occur without influence of neural crest cells, the subsequent movements and differentiation stages of most head muscles are neural crest-dependent. Incorporating both descriptive and experimental studies, this review examines each stage of myogenesis up through the formation of attachments to their skeletal partners. We present the similarities among developing muscle groups, including comparisons with trunk myogenesis, but emphasize the morphogenetic processes that are unique to each group and sometimes subsets of muscles within a group. These groups include branchial (pharyngeal) arches, which encompass both those with clear homologues in all vertebrate classes and those unique to one, for example, mammalian facial muscles, and also extraocular, laryngeal, tongue, and neck muscles. The presence of several distinct processes underlying neural crest:myoblast/myocyte interactions and behaviors is not surprising, given the wide range of both quantitative and qualitative variations in craniofacial muscle organization achieved during vertebrate evolution.

Surgical Treatment of Thoracolumbar Segmental Spinal Dysgenesis: Optimal Type of Fusion

OBJECTIVES

We sought to evaluate long-term results of surgical treatment of thoracolumbar segmental spinal dysgenesis (SSD).

METHODS

We analyzed 8 patients with thoracolumbar SSD treated in our institution. Each case was evaluated for specific clinical and radiologic criteria and types and outcomes of treatment.

RESULT

The average age of primary surgery was 3.4 years (median 3.4 years, range 1.7-7 years). The average correction of kyphosis was 49.3° (mean 45°, from 25°-75°) and scoliosis 10.6° (mean 10°, from 0°-25°). Average follow-up time was 3.2 years (mean 2.6 years, from 1.8-5.6 years). Neurologic improvement was also achieved in all patients. The Japanese Orthopaedic Association scale score (Benzel's modification) was increased by 2.5 points on average (mean 2.5 points, from 2-5 points). One patient had complications: pseudarthrosis and rod fracture followed by refusion.

CONCLUSIONS

Our treatment strategy provides favorable deformity correction and neurologic improvement. It is limited by immature vertebral structures in newborns and infants, who should be carefully monitored from birth with braces until they reach the age when a fixing tool can be used.

Cell Sorting and Noise-Induced Cell Plasticity Coordinate to Sharpen Boundaries between Gene Expression Domains

A fundamental question in biology is how sharp boundaries of gene expression form precisely in spite of biological variation/noise. Numerous mechanisms position gene expression domains across fields of cells (e.g. morphogens), but how these domains are refined remains unclear. In some cases, domain boundaries sharpen through differential adhesion-mediated cell sorting. However, boundaries can also sharpen through cellular plasticity, with cell fate changes driven by up- or down-regulation of gene expression. In this context, we have argued that noise in gene expression can help cells transition to the correct fate. Here we investigate the efficacy of cell sorting, gene expression plasticity, and their combination in boundary sharpening using multi-scale, stochastic models. We focus on the formation of hindbrain segments (rhombomeres) in the developing zebrafish as an example, but the mechanisms investigated apply broadly to many tissues. Our results indicate that neither sorting nor plasticity is sufficient on its own to sharpen transition regions between different rhombomeres. Rather the two have complementary strengths and weaknesses, which synergize when combined to sharpen gene expression boundaries.

In many developing systems, chemical gradients control the formation of segmental domains of gene expression, specifying distinct domains that go on to form different tissues and structures, in a concentration-dependent manner. These gradients are noisy however, raising the question of how sharply delineated boundaries between distinct segments form. It is crucial that developing systems be able to cope with stochasticity and generate well-defined boundaries between different segmented domains. Previous work suggests that cell sorting and cellular plasticity help sharpen boundaries between segments. However, it remains unclear how effective each of these mechanisms is and what their role in sharpening may be. Motivated by recent experimental observations, we construct a hybrid stochastic model to investigate these questions. We find that neither mechanism is sufficient on its own to sharpen boundaries between different segments. Rather, results indicate each has its own strengths and weaknesses, and that they work together synergistically to promote the development of precise, well defined segment boundaries. Formation of segmented rhombomeres in the zebrafish hindbrain, which later form different components of the central nervous system, is a motivating case for this study.

Surgical planning and innervation in pontine gaze palsy with ipsilateral esotropia

PURPOSE

To discuss surgical intervention strategies among patients with horizontal gaze palsy with concurrent esotropia.

METHODS

Five consecutive patients with dorsal pontine lesions are presented. Each patient had horizontal gaze palsy with symptomatic diplopia as a consequence of esotropia in primary gaze and an anomalous head turn to attain single binocular vision.

RESULTS

Clinical findings in the first 2 patients led us to presume there was complete loss of rectus muscle function from rectus muscle palsy. Based on this assumption, medial rectus recessions with simultaneous partial vertical muscle transposition (VRT) on the ipsilateral eye of the gaze palsy and recession-resection surgery on the contralateral eye were performed, resulting in significant motility limitation. Sequential recession-resection surgery without simultaneous VRT on the 3rd patient created an unexpected motility improvement to the side of gaze palsy, an observation differentiating rectus muscle palsy from paresis. Recession combined with VRT approach in the esotropic eye was abandoned on subsequent patients. Simultaneous recession-resection surgery without VRT in the next 2 patients resulted in alleviation of head postures, resolution of esotropia, and also substantial motility improvements to the ipsilateral hemifield of gaze palsy without limitations in adduction and vertical deviations.

CONCLUSIONS

Ocular misalignment and abnormal head posture as a result of conjugate gaze palsy can be successfully treated by basic recession-resection surgery, with the advantage of increasing versions to the ipsilateral side of the gaze palsy. Improved motility after surgery presumably represents paresis, not "paralysis," with residual innervation in rectus muscles.

Building the backbone: the development and evolution of vertebral patterning

The segmented vertebral column comprises a repeat series of vertebrae, each consisting of two key components: the vertebral body (or centrum) and the vertebral arches. Despite being a defining feature of the vertebrates, much remains to be understood about vertebral development and evolution. Particular controversy surrounds whether vertebral component structures are homologous across vertebrates, how somite and vertebral patterning are connected, and the developmental origin of vertebral bone-mineralizing cells. Here, we assemble evidence from ichthyologists, palaeontologists and developmental biologists to consider these issues. Vertebral arch elements were present in early stem vertebrates, whereas centra arose later. We argue that centra are homologous among jawed vertebrates, and review evidence in teleosts that the notochord plays an instructive role in segmental patterning, alongside the somites, and contributes to mineralization. By clarifying the evolutionary relationship between centra and arches, and their varying modes of skeletal mineralization, we can better appreciate the detailed mechanisms that regulate and diversify vertebral patterning.

Paraxis is required for somite morphogenesis and differentiation in Xenopus laevis

BACKGROUND

In most vertebrates, the segmentation of the paraxial mesoderm involves the formation of metameric units called somites through a mesenchymal-epithelial transition. However, this process is different in Xenopus laevis because it does not form an epithelial somite. Xenopus somitogenesis is characterized by a complex cells rearrangement that requires the coordinated regulation of cell shape, adhesion, and motility. The molecular mechanisms that control these cell behaviors underlying somite formation are little known. Although the Paraxis has been implicated in the epithelialization of somite in chick and mouse, its role in Xenopus somite morphogenesis has not been determined.

RESULTS

Using a morpholino and hormone-inducible construction approaches, we showed that both gain and loss of function of paraxis affect somite elongation, rotation and alignment, causing a severe disorganization of somitic tissue. We further found that depletion or overexpression of paraxis in the somite led to the downregulation or upregulation, respectively, of cell adhesion expression markers. Finally, we demonstrated that paraxis is necessary for the proper expression of myotomal and sclerotomal differentiation markers.

CONCLUSIONS

Our results demonstrate that paraxis regulates the cell rearrangements that take place during the somitogenesis of Xenopus by regulating cell adhesion. Furthermore, paraxis is also required for somite differentiation.

Multiple roles of timing in somite formation

During development, vertebrate embryos produce serially repeated elements, the somites, on each side of the midline. These generate the vertebral column, skeletal musculature and dermis. They form sequentially, one pair at a time, from mesenchymal tissue near the tail. Somite development is a complex process. The embryo must control the number, size, and timing of somite formation, their subdivision into functional regions along three axes, regional identity such that somites develop in a region-specific way, and interactions with neighbouring tissues that coordinate them with nearby structures. Here we discuss many timing-related mechanisms that contribute to set up the spatial pattern.

Surgical treatment of congenital thoracolumbar spondyloptosis in a 2-year-old child with vertebral column resection and posterior-only circumferential reconstruction of the spine column: case report

Spondyloptosis refers to complete dislocation of a vertebral body onto another. The L5-S1 level is frequently affected. As this condition is rare, few published reports describing its clinical features and surgical outcomes exist, especially in the pediatric patient population. The authors report the presentation, pathological findings, and radiographic studies of a 2-year-old girl who presented to Texas Children's Hospital with a history since birth of progressive spastic paraparesis. Preoperative CT and MRI showed severe spinal cord compression associated with T11-12 spondyloptosis. The patient underwent a single-stage posterior approach for complete resection of the dysplastic vertebral bodies at the apex of the spinal deformity with reconstruction and stabilization of the vertebral column using a titanium expandable cage and pedicle screws. At the 12-month follow-up, the patient remained neurologically stable without any radiographic evidence of instrumentation failure or loss of alignment. To the best of the authors' knowledge, there have been only 2 other children with congenital thoracolumbar spondyloptosis treated with the above-described strategy. The authors describe their case and review the literature to discuss the aggregate clinical features, surgical strategies, and operative outcomes for congenital thoracolumbar spondyloptosis.

Currarino syndrome: report of five consecutive patients

INTRODUCTION

The Currarino syndrome is regarded as a developmental disorder based on its recognized etiological heterogeneity. This syndrome is thought to result from abnormal separation of the neuroectoderm from the endoderm. Our aim was to report the neurosurgical management of Currarino syndrome in children and adults and to describe what clinician could do if the Currarino triad was suspected.

CASE REPORTS

We present five cases of Currarino triad who underwent surgical intervention. All patients had sacral bony deformity, anorectal malformations, and anterior sacral meningocele. A 40-year-old-male had chronic constipation. He was incidentally diagnosed with Currarino syndrome. A 19-year-old-female suffered from a slight weakness in lower extremities and urinary incontinence. Her past medical history was remarkable for anal atresia. The other three cases were children.

CONCLUSION

When an anterior sacral meningocele is encountered, Currarino syndrome should be taken into consideration. Although it is rarity, the Currarino syndrome might be one of the causes of chronic constipation. Endoscopic or endoscope-assisted surgery via a posterior sacral route can be feasible for treatment of some of the patients with anterior sacral meningocele. Anterior meningocele pouch associated with Currarino syndrome will regresses over time following transdural ligation of its neck.

Embryology, classification, and surgical management of bony malformations of the craniovertebral junction

The embryology of the bony craniovertebral junction (CVJ) is reviewed with the purpose of explaining the genesis and unusual configurations of the numerous congenital malformations in this region. Functionally, the bony CVJ can be divided into a central pillar consisting of the basiocciput and dental pivot; and a two-tiered ring revolving round the central pivot, comprising the foramen magnum rim and occipital condyles above, and the atlantal ring below. Embryologically, the central pillar and the surrounding rings descend from different primordia, and accordingly, developmental anomalies at the CVJ can also be segregated into those affecting the central pillar and the surrounding rings, respectively. A logical classification of this seemingly unwieldy group of malformations is thus possible based on their ontogenetic lineage, morbid anatomy, and clinical relevance. Representative examples of the main constituents of this classification scheme are given, and their surgical treatments are selectively discussed.

Mitochondrial DNA deletions and depletion within paraspinal muscles

Aims

Although mitochondrial abnormalities have been reported within paraspinal muscles in patients with axial weakness and neuromuscular disease as well as with ageing, the basis of respiratory deficiency in paraspinal muscles is not known. This study aimed to determine the extent and basis of respiratory deficiency in paraspinal muscles from cases undergoing surgery for degenerative spinal disease and post mortem cases without a history of spinal disease, where age-related histopathological changes were previously reported.

Methods

Cervical and lumbar paraspinal muscles were obtained peri-operatively from 13 patients and from six post mortem control cases (age range 18–82 years) without a neurological disease. Sequential COX/SDH (mitochondrial respiratory chain complex IV/complex II) histochemistry was performed to identify respiratory-deficient muscle fibres (lacking complex IV with intact complex II activity). Real-time polymerase chain reaction, long-range polymerase chain reaction and sequencing were used to identify and characterize mitochondrial DNA (mtDNA) deletions and determine mtDNA copy number status. Mitochondrial respiratory chain complex subunits were detected by immunohistochemistry.

Results

The density of respiratory-deficient fibres increased with age. On average, 3.96% of fibres in paraspinal muscles were respiratory-deficient (range 0–10.26). Respiratory deficiency in 36.8% of paraspinal muscle fibres was due to clonally expanded mtDNA deletions. MtDNA depletion accounted for further 13.5% of respiratory deficiency. The profile of immunohistochemically detected subunits of complexes was similar in respiratory-deficient fibres with and without mtDNA deletions or mtDNA depletion.

Conclusions

Paraspinal muscles appeared to be particularly susceptible to age-related mitochondrial respiratory chain defects. Clonally expanded mtDNA deletions and focal mtDNA depletion may contribute towards the development of age-related postural abnormalities.

Molecular basis of the clinical features of Al-Awadi-Raas-Rothschild (limb/pelvis/uterus-hypoplasia/aplasia) syndrome (AARRS) and Fuhrmann syndrome

This paper reviews the molecular basis of the clinical features of Al-Awadi-Raas-Rothschild (limb/pelvis/uterus-hypoplasia-aplasia) (AARRS) syndrome and Fuhrmann syndrome. Human WNT7A mutations are also reviewed. Based on this review, these mutations will be classified into two main groups of phenotypes: Fuhrmann and AARRS phenotypes in which there is partial and complete loss of WNT7A functions, respectively.

Three cases of bipartition of the atlas

BACKGROUND CONTEXT

A bipartite atlas is a rare coincidental finding, and it is reported in only 0.1% of the general population. It is a congenital disorder characterized by incomplete fusion of the anterior and the posterior arches of C1, and it is important to differentiate it from a Jefferson fracture.

STUDY DESIGN/SETTING

Case report and literature review.

PURPOSE

To report three cases of patients with bipartition of the atlas with a focus on imaging. To review the literature on these fusion defects, the embryologic basis, and the differentiation from a Jefferson fracture.

METHODS

We report three cases of patients with a bipartite atlas as a coincidental finding in a trauma setting. The bipartite atlas was assessed by multidetector computed tomography (CT). The first case, for example, describes a 36-year-old patient who was struck by a moped. The CT of the skull showed a bipartite atlas as an additional finding. The embryologic development of C1 is reviewed and also the imaging features and the management. Furthermore, a CT image of a Jefferson fracture is provided for comparison.

RESULTS

The CT scans of the three patients show midline clefts of the anterior and the posterior arches of C1 with similar imaging features: smooth margins lined by cortical bone and no lateral offset. The patients had no neurological symptoms relating to the C1 abnormality, and no follow-up was performed. The clefts at level C1 are the result of the failure of three ossification centers to fuse properly. Anterior and posterior clefts are caused by hypoplasia of the hypochordal bow and lateral parts of the C1 sclerotome, respectively. Because of the risk of instability, assessing atlantoaxial stability is advised. However, patients usually have no symptoms and require no specific treatment.

CONCLUSIONS

A bipartite atlas is a rare congenital abnormality, caused by a failure of anterior and lateral ossification centers to fuse. It needs to be differentiated from a Jefferson fracture in a trauma setting. It usually requires no specific treatment.

Neural crest and somitic mesoderm as paradigms to investigate cell fate decisions during development

The dorsal domains of the neural tube and somites are transient embryonic epithelia; they constitute the source of neural crest progenitors that generate the peripheral nervous system, pigment cells and ectomesenchyme, and of the dermomyotome that develops into myocytes, dermis and vascular cells, respectively. Based on the variety of derivatives produced by each type of epithelium, a classical yet still highly relevant question is whether these embryonic epithelia are composed of homogeneous multipotent progenitors or, alternatively, of subsets of fate-restricted cells. Growing evidence substantiates the notion that both the dorsal tube and the dermomyotome are heterogeneous epithelia composed of multipotent as well as fate-restricted precursors that emerge as such in a spatio-temporally regulated manner. Elucidation of the state of commitment of the precedent progenitors is of utmost significance for deciphering the mechanisms that regulate fate segregation during embryogenesis. In addition, it will contribute to understanding the nature of well documented neural crest-somite interactions shown to modulate the timing of neural crest cell emigration, their segmental migration, and myogenesis.

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.

Control of neural crest cell behavior and migration: Insights from live imaging

Neural crest cells (NCCs) are a remarkable, dynamic group of cells that travel long distances in the embryo to reach their target sites. They are responsible for the formation of craniofacial bones and cartilage, neurons and glia in the peripheral nervous system, and pigment cells. Live imaging of NCCs as they traverse the embryo has been critical to increasing our knowledge of their biology. NCCs exhibit multiple behaviors and communicate with each other and their environment along each step of their journey. Imaging combined with molecular manipulations has led to insights into the mechanisms controlling these behaviors. In this review, we highlight studies that have used live imaging to provide novel insight into NCC migration and discuss how continued use of such techniques can advance our understanding of NCC biology.

Embryology and bony malformations of the craniovertebral junction

BACKGROUND

The embryology of the bony craniovertebral junction (CVJ) is reviewed with the purpose of explaining the genesis and unusual configurations of the numerous congenital malformations in this region. Functionally, the bony CVJ can be divided into a central pillar consisting of the basiocciput and dental pivot and a two-tiered ring revolving round the central pivot, comprising the foramen magnum rim and occipital condyles above and the atlantal ring below. Embryologically, the central pillar and the surrounding rings descend from different primordia, and accordingly, developmental anomalies at the CVJ can also be segregated into those affecting the central pillar and those affecting the surrounding rings, respectively.

DISCUSSION

A logical classification of this seemingly unwieldy group of malformations is thus possible based on their ontogenetic lineage, morbid anatomy, and clinical relevance. Representative examples of the main constituents of this classification scheme are given, and their surgical treatments are selectively discussed.

Mechanosensilla in the adult abdomen of Drosophila: engrailed and slit help to corral the peripheral sensory axons into segmental bundles

The abdomen of adult Drosophila bears mechanosensory bristles with axons that connect directly to the CNS, each hemisegment contributing a separate nerve bundle. Here, we alter the amount of Engrailed protein and manipulate the Hedgehog signalling pathway in clones of cells to study their effects on nerve pathfinding within the peripheral nervous system. We find that high levels of Engrailed make the epidermal cells inhospitable to bristle neurons; sensory axons that are too near these cells are either deflected or fail to extend properly or at all. We then searched for the engrailed-dependent agent responsible for these repellent properties. We found slit to be expressed in the P compartment and, using genetic mosaics, present evidence that Slit is the responsible molecule. Blocking the activity of the three Robo genes (putative receptors for Slit) with RNAi supported this hypothesis. We conclude that, during normal development, gradients of Slit protein repel axons away from compartment boundaries - in consequence, the bristles from each segment send their nerves to the CNS in separated sets.

Congenital scoliosis - Quo vadis?

Congenital spinal vertebral anomalies can present as scoliosis or kyphosis or both. The worldwide prevalence of the vertebral anomalies is 0.5-1 per 1000 live births. Vertebral anomalies can range from hemi vertebrae (HV) which may be single or multiple, vertebral bar with or without HV, block vertebrae, wedge shaped or butterfly vertebrae. Seventy per cent of congenital vertebral anomalies result in progressive deformities. The risk factors for progression include: type of defect, site of defect (junctional regions) and patient's age at the time of diagnosis. The key to success in managing these spinal deformities is early diagnosis and anticipation of progression. One must intervene surgically to halt the progression of deformity and prevent further complications associated with progressive deformity. Planning for surgery includes a preoperative MRI scan to rule out spinal anomalies such as diastematomyelia. The goals of surgical treatment for congenital spinal deformity are to achieve a straight growing spine, a normal standing sagittal profile, and a short fusion segment. The options of surgery include in situ fusion, convex hemi epiphysiodesis and hemi vertebra excision. These basic surgical procedures can be combined with curve correction, instrumentation and short segment fusion. Most surgeons prefer posterior (only) surgery for uncomplicated HV excision and short segment fusion. These surgical procedures can be performed through posterior, anterior or combined approaches. The advocates of combined approaches suggest greater deformity correction possibilities with reduced incidence of pseudoarthrosis and minimize crankshaft phenomenon. We recommend posterior surgery for curves involving only an element of kyphosis or modest deformity, whereas combined anterior and posterior approach is indicated for large or lordotic deformities. In the last decade, the use of growing rods and vertebral expandable prosthetic titanium rib has improved the armamentarium of the spinal surgeon in dealing with certain difficult congenital spinal deformities. The goal of growing rod treatment is to provide simultaneous deformity correction and allow for continued spinal growth. Once maximal spinal growth has been achieved, definitive fusion and instrumentation is performed.

Two patients with proatlas segmentation malformation

A 58-year-old female and an 18-year-old male patient had progressive spastic quadriparesis of 10 years and 6 months duration, respectively. Proatlas segmentation malformation (PSM) was confirmed using three-dimensional (3D) reconstructive CT scans and MRI. Surgical procedures in one patient involved anterior decompression via a transoropharyngeal approach, cranial traction, and posterior occipital-cervical fixation and fusion. His postoperative neurological status had improved remarkably, with imaging showing good realignment of the occipito-atlanto-axial complex with comfortable decompression of the cervico-medulla junction and relief of syringomyelia. MRI and 3D-CT scans are the definitive diagnostic tools for PSM, and appropriate aggressive surgical intervention should be undertaken.

Congenital scoliosis: etiology and associations

STUDY DESIGN

Literature review.

OBJECTIVE

To provide a current overview of congenital scoliosis and associated conditions.

SUMMARY OF BACKGROUND DATA

The etiology of congenital scoliosis is unknown. A variety of factors have been implicated in the development of vertebral abnormalities. These factors provide clues to the origin of congenital scoliosis.

METHODS

A search of PubMed, using the keywords congenital scoliosis, etiology, and genetics was performed.

RESULTS

Environmental factors, genetics, vitamin deficiency, chemicals, and drugs, singly or in combination, have all been implicated in the development of vertebral abnormalities. Whatever the cause, the physiologic injury occurs early in the embryologic period, well before the development of cartilage and bone. The resulting defects can lead to full or partial fusion or lack of development of the vertebrae, which, in turn, can cause a curvature that, may be progressive during the growth of the child.

CONCLUSION

The origin of congenital scoliosis may be environmental, genetic, or a combination of factors. Research on these various factors continues. Early identification and management of concomitant defects can improve the patient's quality of life.

Congenital dislocated spine: implications for orthopaedic management

BACKGROUND

Congenital dislocation of the spine (CDS) is a rare malformation due to a developmental failure of the spine and the spinal cord at a single spinal level. New embryological and physiopathological findings define CDS as an autonomous entity compared with multilevel pathologies. The severity of CDS neurological outcome requires its treatment by experienced pediatric spine surgeons in a pediatric specialty hospital. This report aims to propose a comprehensive orthopaedic management strategy and operative technique of CDS in 6 new patients.

METHODS

The records of patients treated at our institution for congenital anomalies of the spine were reviewed in a retrospective study. Inclusion criteria were extracted from the actual context of new embryologic theories: single level involvement; sudden mainly sagittal vertebral displacement with anterior translation of the entire cranial vertebral column on the caudal vertebrae ("step-off sign"); underlying spinal malformation at a clearly distinct level; spinal cord intact both cranial and caudal to the malformation; possibility of malformed aspect of the 2 involved vertebrae. Demographic data, family, and clinical history were collected. Complete set of plain radiographs and modern imaging computed tomography and magnetic resonance imaging were analyzed.

RESULTS

Six children treated between 1993 and 2007 have been classified as CDS. The mean follow-up to date of the 4 patients alive after the last corrective surgery is 9.8 years (range, 1-14.6 years). All patients alive have at follow-up solid stable fusion and no progression of spinal deformity. Two of the patients are independent walkers.

CONCLUSIONS

The neurological involvement of CDS if present initially is the consequence of an associated spinal cord malformation without mechanical factor. Adaptation of the therapeutic strategy may avoid secondary neurological damage. Parents should be counseled as soon as the diagnosis is made, the obstetrical and postnatal orthopaedic management has to be adapted. Stabilization of the spine including very early cast immobilization and an early instrumented decompression-stabilization with circumferential fusion in 1 stage is required.

Extensive molecular differences between anterior- and posterior-half-sclerotomes underlie somite polarity and spinal nerve segmentation

Background

The polarization of somite-derived sclerotomes into anterior and posterior halves underlies vertebral morphogenesis and spinal nerve segmentation. To characterize the full extent of molecular differences that underlie this polarity, we have undertaken a systematic comparison of gene expression between the two sclerotome halves in the mouse embryo.

Results

Several hundred genes are differentially-expressed between the two sclerotome halves, showing that a marked degree of molecular heterogeneity underpins the development of somite polarity.

Conclusion

We have identified a set of genes that warrant further investigation as regulators of somite polarity and vertebral morphogenesis, as well as repellents of spinal axon growth. Moreover the results indicate that, unlike the posterior half-sclerotome, the central region of the anterior-half-sclerotome does not contribute bone and cartilage to the vertebral column, being associated instead with the development of the segmented spinal nerves.

REMNANTS OF OCCIPITAL VERTEBRAE

OBJECTIVE

Developmental remnants around the foramen magnum, or proatlas segmentation abnormalities, have been recorded in postmortem studies but very rarely in a clinical setting. Because of their rarity, the pathological anatomy has been misunderstood, and treatment has been fraught with failures. The objectives of this prospective study were to understand the correlative anatomy, pathology, and embryology and to recognize the clinical presentation and gain insights on the treatment and management.

METHODS

Our craniovertebral junction (CVJ) database started in 1977 and comprises 5200 cases. This prospective study has retrieval capabilities. Neurodiagnostic studies changed with the evolution of imaging. Seventy-two patients were recognized as having symptomatic proatlas segmentation abnormalities.

RESULTS

Ventral bony masses from the clivus or medial occipital condyle occurred in 66% (44/72), lateral or anterolateral compressive masses in 37% (27 of 72 patients), and dorsal bony compression in 17% (12 of 72 patients). Hindbrain herniation was associated in 33%. The age at presentation was 3 to 23 years. Motor symptoms occurred in 72% (52 of 72 patients); palsies in Cranial Nerves IX, X, and XII in 33% (24 of 72 patients); and vertebrobasilar symptoms in 25% (18 of 72 patients). Trauma precipitated symptoms in 55% (40 of 72 patients). The best definition of the abnormality was demonstrated by 3-dimensional computed tomography combined with magnetic resonance imaging. Treatment was aimed at decompression of the pathology and stabilization.

CONCLUSION

Remnants of the occipital vertebrae around the foramen magnum were recognized in 72 of 5200 CVJ cases (7.2%). Magnetic resonance imaging with 3-dimensional computed tomography of the CVJ provides the best definition and understanding of the lesions. Brainstem myelopathy and lower cranial nerve deficits are common clinical presentations in the first and second decades of life. Treatment is aimed at decompression of the pathology and CVJ stabilization.

Craniocervical developmental anatomy and its implications

INTRODUCTION

Congenital and developmental osseous abnormalities and anomalies that affect the craniocervical junction complex can result in neural compression and vascular compromise and can manifest itself with abnormal cerebrospinal fluid dynamics. An understanding of the development of the craniocervical junction is essential to recognize the pathological abnormalities.

MATERIALS AND METHODS

Atlas assimilation, segmentation failures, os odontoideum, basilar invagination, and the various syndromes that affect the craniocervical junction have been analyzed. The natural history provides an added insight into its treatment.

RESULTS

Proatlas segmentation abnormalities surrounded the foramen magnum and the posterior arch of C1. Hindbrain herniation was associated in 33 of the 90 children involved. Spastic quadriparesis presented in 80% and lower cranial nerve abnormalities in 33%. Vertebrobasilar dysfunction was observed in 40% and trauma presentation seen in 60% of individuals. Atlas assimilation was present in 550 individuals who were evaluated for craniovertebral junction abnormalities. Hindbrain herniation occurred in 38%. Segmentation failure of C2 and C3 vertebrae compounded the abnormal dynamics resulting in atlantoaxial instability. This was a reducible instability with formation of pannus around the odontoid process until it became irreducible at approximately 14 years of age. Unilateral atlas assimilation caused torticollis in children. Os odontoideum was investigated regarding craniocervical trauma at a young age.

CONCLUSION

The conclusion was that os odontoideum was associated with an unrecognized fracture in children below the age of 5 with a previously normal odontoid structure as observed in our series. Atlas and axis abnormalities were reviewed in this series. This large database has provided an understanding of the natural history of many entities and allowed treatment protocols to be established that have stood the test of time.

Compartment-specific transcription factors orchestrate angiogenesis gradients in the embryonic brain

Prevailing notions of cerebral vascularization imply that blood vessels sprout passively into the brain parenchyma from pial vascular plexuses to meet metabolic needs of growing neuronal populations. Endothelial cells, building blocks of blood vessels, are thought to be homogeneous in the brain with respect to their origins, gene expression patterns and developmental mechanisms. These current notions that cerebral angiogenesis is regulated by local environmental signals contrast with current models of cell-autonomous regulation of neuronal development. Here we demonstrate that telencephalic angiogenesis in mice progresses in an orderly, ventral-to-dorsal gradient regulated by compartment-specific homeobox transcription factors. Our data offer new perspectives on intrinsic regulation of angiogenesis in the embryonic telencephalon, call for a revision of the current models of telencephalic angiogenesis and support novel roles for endothelial cells in brain development.

Misty somites, a maternal effect gene identified by transposon-mediated insertional mutagenesis in zebrafish that is essential for the somite boundary maintenance

Somite boundary formation is crucial for segmentation of vertebrate somites and vertebrae and skeletal muscle morphogenesis. Previously, we developed a Tol2 transposon-mediated gene trap method in zebrafish. In the present study, we aimed to isolate transposon insertions that trap maternally-expressed genes. We found that homozygous female fish carrying a transposon insertion within a maternally-expressed gene misty somites (mys) produced embryos that showed obscure somite boundaries at the early segmentation stage (12-13 hpf). The somite boundaries became clear and distinct after this period and the embryos survived to adulthood. This phenotype was rescued by expression of mys cDNA in the homozygous adults, confirming that it was caused by a decreased mys activity. We analyzed a role of the mys gene by using morpholino oligonucleotides (MOs). The MO-injected embryo exhibited severer phenotypes than the insertional mutant probably because the mys gene was partially active in the insertional mutant. The MO-injected embryo also showed the obscure somite boundary phenotype. Fibronectin and phosphorylated FAK at the intersomitic regions were accumulated at the boundaries at this stage, but, unlike wild type embryos, somitic cells adjacent to the boundaries did not undergo epithelialization, suggesting that Mys is required for epithelialization of the somitic cells. Then in the MO-injected embryos, the boundaries once became clear and distinct, but, in the subsequent stages, disappeared, resulting in abnormal muscle morphogenesis. Accumulation of Fibronectin and phosphorylated FAK observed in the initial stage also disappeared. Thus, Mys is crucial for maintenance of the somite boundaries formed at the initial stage. To analyze the mys defect at the cellular level, we placed cells dissociated from the MO-injected embryo on Fibronectin-coated glasses. By this cell spreading assay, we found that the mys-deficient cells reduced the activity to form lamellipodia on Fibronectin while FAK was activated in these cells. Thus, we demonstrate that a novel gene misty somites is essential for epithelialization of the somitic cells and maintenance of the somite boundary. Furthermore, Mys may play a role in a cellular pathway leading to lamellipodia formation in response to the Fibronectin signaling. We propose that the Tol2 transposon mediated gene trap method is powerful to identify a novel gene involved in vertebrate development.

Coordinated action of N-CAM, N-cadherin, EphA4, and ephrinB2 translates genetic prepatterns into structure during somitogenesis in chick

During gastrulation in vertebrates, mesenchymal cells at the anterior end of the presomitic mesoderm (PSM) periodically compact, transiently epithelialize and detach from the posterior PSM to form somites. In the prevailing clock-and-wavefront model of somitogenesis, periodic gene expression, particularly of Notch and Wnt, interacts with an FGF8-based thresholding mechanism to determine cell fates. However, this model does not explain how cell determination and subsequent differentiation translates into somite morphology. In this paper, we use computer simulations of chick somitogenesis to show that experimentally-observed temporal and spatial patterns of adhesive N-CAM and N-cadherin and repulsive EphA4-ephrinB2 pairs suffice to reproduce the complex dynamic morphological changes of somitogenesis in wild-type and N-cadherin (-/-) chick, including intersomitic separation, boundary-shape evolution and sorting of misdifferentiated cells across compartment boundaries. Since different models of determination yield the same, experimentally-observed, distribution of adhesion and repulsion molecules, the patterning is independent of the details of this mechanism.

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.

Molecular basis for skeletal variation: insights from developmental genetic studies in mice

Skeletal variations are common in humans, and potentially are caused by genetic as well as environmental factors. We here review molecular principles in skeletal development to develop a knowledge base of possible alterations that could explain variations in skeletal element number, shape or size. Environmental agents that induce variations, such as teratogens, likely interact with the molecular pathways that regulate skeletal development.

Congenital Abnormalities of the Thoracic and Lumbar Spine

Congenital spinal anomalies entail a wide spectrum of conditions that share in common some form of error during embryogenesis. Congenital disorders of the spine may not always be readily apparent at birth; they can present as a deformity with growth or with clinical signs of neurologic dysfunction early or later as an adolescent or adult. In this article the authors briefly summarize the embryology of the spine, which provides a background for understanding the pathophysiology of congenital spinal lesions. The discussion entails spine embryology and the developmental abnormalities commonly seen in the thoracolumbar spine.