Segmentation of the vertebrate body

Morphological phenotyping after mouse whole embryo culture

Morphological phenotyping of the mouse embryo is described at neurulation stages, primarily as a guide to evaluating the outcome of whole embryo cultures between embryonic days 8.5 and 9.5. During this period, neural tube closure is initiated and progresses to completion in the cranial region. Spinal closure is still underway at the end of the culture period. The focus of this article is particularly on phenotyping that can be performed at the bench, using a stereomicroscope. This involves assessment of embryonic health, through observation and scoring of yolk sac blood circulation, measurement of developmental stage by somite counting, and determination of crown-rump length as a measure of growth. Axial rotation ("turning") can also be assessed using a simple scoring system. Neural tube closure assessment includes: 1) determining whether closure has been initiated at the Closure 1 site; 2) evaluating the complex steps of cranial neurulation including initiation at Closure sites 2 and 3, and completion of closure at the anterior and hindbrain neuropores; 3) assessment of spinal closure by measurement of posterior neuropore length. Interpretation of defects in neural tube closure requires an appreciation of, first, the stages that particular events are expected to be completed and, second, the correspondence between embryonic landmarks, for example, somite position, and the resulting adult axial levels. Detailed embryonic phenotyping, as described in this article, when combined with the versatile method of whole embryo culture, can form the basis for a wide range of experimental studies in early mouse neural development.

Spatial transcriptomics reveals a cnidarian segment polarity program in Nematostella vectensis

During early animal evolution, the emergence of axially polarized segments was central to the diversification of complex bilaterian body plans. Nevertheless, precisely how and when segment polarity pathways arose remains obscure. Here, we demonstrate the molecular basis for segment polarization in developing larvae of the sea anemone Nematostella vectensis. Utilizing spatial transcriptomics, we first constructed a 3D gene expression atlas of developing larval segments. Capitalizing on accurate in silico predictions, we identified Lbx and Uncx, conserved homeodomain-containing genes that occupy opposing subsegmental domains under the control of both bone morphogenetic protein (BMP) signaling and the Hox-Gbx cascade. Functionally, Lbx mutagenesis eliminated all molecular evidence of segment polarization at the larval stage and caused an aberrant mirror-symmetric pattern of retractor muscles (RMs) in primary polyps. These results demonstrate the molecular basis for segment polarity in a non-bilaterian animal, suggesting that polarized metameric structures were present in the Cnidaria-Bilateria common ancestor over 600 million years ago.

Spatial transcriptomics reveals a conserved segment polarity program that governs muscle patterning in Nematostella vectensis

During early animal evolution, the emergence of axially-polarized segments was central to the diversification of complex bilaterian body plans. Nevertheless, precisely how and when segment polarity pathways arose remains obscure. Here we demonstrate the molecular basis for segment polarization in developing larvae of the pre-bilaterian sea anemone Nematostella vectensis . Utilizing spatial transcriptomics, we first constructed a 3-D gene expression atlas of developing larval segments. Capitalizing on accurate in silico predictions, we identified Lbx and Uncx, conserved homeodomain-containing genes that occupy opposing subsegmental domains under the control of both BMP signaling and the Hox-Gbx cascade. Functionally, Lbx mutagenesis eliminated all molecular evidence of segment polarization at larval stage and caused an aberrant mirror-symmetric pattern of retractor muscles in primary polyps. These results demonstrate the molecular basis for segment polarity in a pre-bilaterian animal, suggesting that polarized metameric structures were present in the Cnidaria-Bilateria common ancestor over 600 million years ago.

Highlights

Nematostella endomesodermal tissue forms metameric segments and displays a transcriptomic profile similar to that observed in bilaterian mesoderm Construction of a comprehensive 3-D gene expression atlas enables systematic dissection of segmental identity in endomesoderm Lbx and Uncx , two conserved homeobox-containing genes, establish segment polarity in Nematostella The Cnidarian-Bilaterian common ancestor likely possessed the genetic toolkit to generate polarized metameric structures.

The Classification of VACTERL Association into 3 Groups According to the Limb Defect

The VACTERL association (VA) is defined as the nonrandom co-occurrence of 6 anomalies: vertebral anomalies (V), Anal atresia (A), Cardiac defects (C), Tracheo-esophageal fistula (TE), Renal defects (R), and Limb anomalies (L). The current communication presents an argument that patients with VA should be classified into three district groups based on their limb defects: VACTERL1: patients with normal limbs; VACTERL2: patients with limb anomalies other than radial ray defects of the upper limbs; and VACTERL3: patients with radial ray defects of the upper limbs. The author will demonstrate that the rationale behind the L1-3 classification in patients in VA is based on the embryogenesis of the 6 affected anatomical areas in VA. The pathogenesis of VACTERL1 is secondary to perturbations of Sonic Hedgehog (SHH) interactions. SHH signaling is known to have a major role in the normal development of the vertebrae, ano-rectal area, heart, tracheo-esophageal area, and kidney. However, SHH is not involved in the development of the radial ray; hence, patients present with no limb defects. The pathogenesis of VACTERL2 is variable depending on the type of gene mutation. The pathogenesis of VACTERL3 is related to errors in a group of proteins (namely, the proteins of the TBX5-SALL4-SALL1 loop and the FGF8-FGF10 loop/ pathway). These proteins are essential for the normal development of the radial ray and they interact in the development of the other anatomical areas of VA including the heart and kidney. Hence, VACTERL3 patients present with radial ray deficiency.

Distal spinal nerve development and divergence of avian groups

The avian transition from long to short, distally fused tails during the Mesozoic ushered in the Pygostylian group, which includes modern birds. The avian tail embodies a bipartite anatomy, with the proximal separate caudal vertebrae region, and the distal pygostyle, formed by vertebral fusion. This study investigates developmental features of the two tail domains in different bird groups, and analyzes them in reference to evolutionary origins. We first defined the early developmental boundary between the two tail halves in the chicken, then followed major developmental structures from early embryo to post-hatching stages. Differences between regions were observed in sclerotome anterior/posterior polarity and peripheral nervous system development, and these were consistent in other neognathous birds. However, in the paleognathous emu, the neognathous pattern was not observed, such that spinal nerve development extends through the pygostyle region. Disparities between the neognaths and paleognaths studied were also reflected in the morphology of their pygostyles. The ancestral long-tailed spinal nerve configuration was hypothesized from brown anole and alligator, which unexpectedly more resembles the neognathous birds. This study shows that tail anatomy is not universal in avians, and suggests several possible scenarios regarding bird evolution, including an independent paleognathous long-tailed ancestor.

Using zebrafish in systems toxicology for developmental toxicity testing

With the high cost and the long-term assessment of developmental toxicity testing in mammals, the vertebrate zebrafish has become a useful alternative model organism for high-throughput developmental toxicity testing. Zebrafish is also very favorable for the 3R perspective in toxicology; however, the methodologies used by research groups vary greatly, posing considerable challenges to integrative analysis. In this review, we discuss zebrafish developmental toxicity testing, focusing on the methods of chemical exposure, the assessment of morphological abnormalities, housing conditions and their effects on the production of healthy embryos, and future directions. Zebrafish as a systems toxicology model has the potential to elucidate developmental toxicity pathways, and to provide a sound basis for human health risk assessments.

Development of the Synarcual in the Elephant Sharks (Holocephali; Chondrichthyes): Implications for Vertebral Formation and Fusion

The synarcual is a structure incorporating multiple elements of two or more anterior vertebrae of the axial skeleton, forming immediately posterior to the cranium. It has been convergently acquired in the fossil group ‘Placodermi’, in Chondrichthyes (Holocephali, Batoidea), within the teleost group Syngnathiformes, and to varying degrees in a range of mammalian taxa. In addition, cervical vertebral fusion presents as an abnormal pathology in a variety of human disorders. Vertebrae develop from axially arranged somites, so that fusion could result from a failure of somite segmentation early in development, or from later heterotopic development of intervertebral bone or cartilage. Examination of early developmental stages indicates that in the Batoidea and the ‘Placodermi’, individual vertebrae developed normally and only later become incorporated into the synarcual, implying regular somite segmentation and vertebral development. Here we show that in the holocephalan Callorhinchus milii, uniform and regular vertebral segmentation also occurs, with anterior individual vertebra developing separately with subsequent fusion into a synarcual. Vertebral elements forming directly behind the synarcual continue to be incorporated into the synarcual through growth. This appears to be a common pattern through the Vertebrata. Research into human disorders, presenting as cervical fusion at birth, focuses on gene misexpression studies in humans and other mammals such as the mouse. However, in chondrichthyans, vertebral fusion represents the normal morphology, moreover, taxa such Leucoraja (Batoidea) and Callorhinchus (Holocephali) are increasingly used as laboratory animals, and the Callorhinchus genome has been sequenced and is available for study. Our observations on synarcual development in three major groups of early jawed vertebrates indicate that fusion involves heterotopic cartilage and perichondral bone/mineralised cartilage developing outside the regular skeleton. We suggest that chondrichthyans have potential as ideal extant models for identifying the genes involved in these processes, for application to human skeletal heterotopic disorders.

Timing embryo segmentation: dynamics and regulatory mechanisms of the vertebrate segmentation clock

All vertebrate species present a segmented body, easily observed in the vertebrate column and its associated components, which provides a high degree of motility to the adult body and efficient protection of the internal organs. The sequential formation of the segmented precursors of the vertebral column during embryonic development, the somites, is governed by an oscillating genetic network, the somitogenesis molecular clock. Herein, we provide an overview of the molecular clock operating during somite formation and its underlying molecular regulatory mechanisms. Human congenital vertebral malformations have been associated with perturbations in these oscillatory mechanisms. Thus, a better comprehension of the molecular mechanisms regulating somite formation is required in order to fully understand the origin of human skeletal malformations.

Posterior skeletal development and the segmentation clock period are sensitive to Lfng dosage during somitogenesis

The segmental structure of the axial skeleton is formed during somitogenesis. During this process, paired somites bud from the presomitic mesoderm (PSM), in a process regulated by a genetic clock called the segmentation clock. The Notch pathway and the Notch modulator Lunatic fringe (Lfng) play multiple roles during segmentation. Lfng oscillates in the posterior PSM as part of the segmentation clock, but is stably expressed in the anterior PSM during presomite patterning. We previously found that mice lacking overt oscillatory Lfng expression in the posterior PSM (Lfng(∆FCE)) exhibit abnormal anterior development but relatively normal posterior development. This suggests distinct requirements for segmentation clock activity during the formation of the anterior skeleton (primary body formation), compared to the posterior skeleton and tail (secondary body formation). To build on these findings, we created an allelic series that progressively lowers Lfng levels in the PSM. Interestingly, we find that further reduction of Lfng expression levels in the PSM does not increase disruption of anterior development. However tail development is increasingly compromised as Lfng levels are reduced, suggesting that primary body formation is more sensitive to Lfng dosage than is secondary body formation. Further, we find that while low levels of oscillatory Lfng in the posterior PSM are sufficient to support relatively normal posterior development, the period of the segmentation clock is increased when the amplitude of Lfng oscillations is low. These data support the hypothesis that there are differential requirements for oscillatory Lfng during primary and secondary body formation and that posterior development is less sensitive to overall Lfng levels. Further, they suggest that modulation of the Notch signaling by Lfng affects the clock period during development.

Sulf1 modulates BMP signaling and is required for somite morphogenesis and development of the horizontal myoseptum

Heparan sulfate proteoglycans (HSPGs) are glycosylated extracellular or membrane-associated proteins. Their unbranched heparan sulfate (HS) disaccharide chains interact with many growth factors and receptors, modifying their activity or diffusion. The pattern of HS sulfation can be altered by the enzymes Sulf1 and Sulf2, secreted extracellular 6-O endosulfatases, which remove specific sulfate groups from HS. Modification by Sulf enzymes changes the binding affinity of HS for protein such as ligands and receptors, affecting growth factor gradients and activities. The precise expression of these sulfatases are thought to be necessary for normal development. We have examined the role of the sulf1 gene in trunk development of zebrafish embryos. sulf1 is expressed in the developing trunk musculature and as well as in midline structures such as the notochord, floorplate and hypochord. Knockdown of sulf1 with antisense morpholinos results in poor differentiation of the somitic trunk muscle, loss of the horizontal myoseptum, lack of pigmentation along the mediolateral stripe, and improper migration of the lateral line primordium. sulf1 knockdown results in a decrease in the number of Pax7-expressing dermomyotome cells, particularly along the midline where the horizontal myoseptum develops. It also leads to decreased sdf1/cxcl12 expression along the mediolateral trunk musculature. Both the Pax7 and cxcl12 expression can be restored by inhibition pharmacological inhibition of BMP signaling, which also restores formation of the myoseptum, fast muscle development, and pigmentation patterning. Lateral line migration and neuromast deposition depend on sdf1/cxcl12 and FGF signaling respectively, both of which are disrupted in sulf1 morphants. Pharmacological activation of FGF signaling can rescue the spacing of neuromast deposition in these fish. Together this data indicate that sulf1 plays a crucial role in modulating both BMP and FGF signaling along the developing myoseptum to coordinate the morphogenesis of trunk musculature, associated pigment cells, and lateral line neuromasts.

Loss of abdominal muscle in Pitx2 mutants associated with altered axial specification of lateral plate mesoderm

Sequence specific transcription factors (SSTFs) combinatorially define cell types during development by forming recursively linked network kernels. Pitx2 expression begins during gastrulation, together with Hox genes, and becomes localized to the abdominal lateral plate mesoderm (LPM) before the onset of myogenesis in somites. The somatopleure of Pitx2 null embryos begins to grow abnormally outward before muscle regulatory factors (MRFs) or Pitx2 begin expression in the dermomyotome/myotome. Abdominal somites become deformed and stunted as they elongate into the mutant body wall, but maintain normal MRF expression domains. Subsequent loss of abdominal muscles is therefore not due to defects in specification, determination, or commitment of the myogenic lineage. Microarray analysis was used to identify SSTF families whose expression levels change in E10.5 interlimb body wall biopsies. All Hox9-11 paralogs had lower RNA levels in mutants, whereas genes expressed selectively in the hypaxial dermomyotome/myotome and sclerotome had higher RNA levels in mutants. In situ hybridization analyses indicate that Hox gene expression was reduced in parts of the LPM and intermediate mesoderm of mutants. Chromatin occupancy studies conducted on E10.5 interlimb body wall biopsies showed that Pitx2 protein occupied chromatin sites containing conserved bicoid core motifs in the vicinity of Hox 9-11 and MRF genes. Taken together, the data indicate that Pitx2 protein in LPM cells acts, presumably in combination with other SSTFs, to repress gene expression, that are normally expressed in physically adjoining cell types. Pitx2 thereby prevents cells in the interlimb LPM from adopting the stable network kernels that define sclerotomal, dermomyotomal, or myotomal mesenchymal cell types. This mechanism may be viewed either as lineage restriction or specification.

Neuroembryology and functional anatomy of craniofacial clefts

The master plan of all vertebrate embryos is based on neuroanatomy. The embryo can be anatomically divided into discrete units called neuromeres so that each carries unique genetic traits. Embryonic neural crest cells arising from each neuromere induce development of nerves and concomitant arteries and support the development of specific craniofacial tissues or developmental fields. Fields are assembled upon each other in a programmed spatiotemporal order. Abnormalities in one field can affect the shape and position of developing adjacent fields. Craniofacial clefts represent states of excess or deficiency within and between specific developmental fields. The neuromeric organization of the embryo is the common denominator for understanding normal anatomy and pathology of the head and neck. Tessier's observational cleft classification system can be redefined using neuroanatomic embryology. Reassessment of Tessier's empiric observations demonstrates a more rational rearrangement of cleft zones, particularly near the midline. Neuromeric theory is also a means to understand and define other common craniofacial problems. Cleft palate, encephaloceles, craniosynostosis and cranial base defects may be analyzed in the same way.

Cell lineages and early patterns of embryonic CNS vascularization

First steps of blood vessel formation and patterning in the central nervous system (CNS) of higher vertebrates are presented. Corresponding to the regional diversity of the embryonic CNS (unsegmented spinal cord vs segmented brain anlagen) and its surroundings (segmented trunk vs unsegmented head mesoderm, neural crest-derived mesenchyme), cells of different origins contribute to the endothelial and mural cell populations. The autonomous migratory potential of endothelial cells is guided by attractive and repulsive clues. Nevertheless, a common pattern in both spinal cord and forebrain vascularization appears, with primary ventral vascular sprouts supplying the periventricular vascular plexus of the neural tube, whereas dorsolateral sprouts appear later.

Shaping skeletal growth by modular regulatory elements in the Bmp5 gene

Cartilage and bone are formed into a remarkable range of shapes and sizes that underlie many anatomical adaptations to different lifestyles in vertebrates. Although the morphological blueprints for individual cartilage and bony structures must somehow be encoded in the genome, we currently know little about the detailed genomic mechanisms that direct precise growth patterns for particular bones. We have carried out large-scale enhancer surveys to identify the regulatory architecture controlling developmental expression of the mouse Bmp5 gene, which encodes a secreted signaling molecule required for normal morphology of specific skeletal features. Although Bmp5 is expressed in many skeletal precursors, different enhancers control expression in individual bones. Remarkably, we show here that different enhancers also exist for highly restricted spatial subdomains along the surface of individual skeletal structures, including ribs and nasal cartilages. Transgenic, null, and regulatory mutations confirm that these anatomy-specific sequences are sufficient to trigger local changes in skeletal morphology and are required for establishing normal growth rates on separate bone surfaces. Our findings suggest that individual bones are composite structures whose detailed growth patterns are built from many smaller lineage and gene expression domains. Individual enhancers in BMP genes provide a genomic mechanism for controlling precise growth domains in particular cartilages and bones, making it possible to separately regulate skeletal anatomy at highly specific locations in the body.

Every bone in the skeleton has a specific shape and size. These characteristic features must be under separate genetic control, because individual bones can undergo striking morphological changes in different species. Researchers have long postulated that the morphology of individual bones arises from the local activity of many separate growth domains around each bone's surface. Differential growth within such domains could modify size, curvature, and formation of specific processes. Here, we show that local growth domains around individual bones are controlled by independent regulatory sequences in bone morphogenetic protein (BMP) genes. We identify multiple regulatory sequences in the Bmp5 gene that control expression in particular bones, rather than all bones. We show that some of these elements are remarkably specific for individual subdomains around the surface of individual bones. Finally, we show that local BMP signaling is necessary and sufficient to trigger highly localized growth patterns in ribs and nasal cartilages. These results suggest that the detailed pattern of growth of individual skeletal structures is encoded in part by multiple regulatory sequences in BMP genes. Gain and loss of anatomy-specific sequences in BMP genes may provide a flexible genomic mechanism for modifying local skeletal anatomy during vertebrate evolution.

Role of nitric oxide in chick embryonic organogenesis and dysmorphogenesis

BACKGROUND

Nitric oxide (NO), produced by the nitric oxide synthase family of enzymes, mediates multiple signaling functions, and when unchecked, NO causes pathological damage. Exposure of embryos to a variety of teratogens, including carbon monoxide (CO), has been shown to increase reactive intermediates, such as NO, and recent work showed that either the excess or absence of NO caused morphological defects. While endogenous NO is known to regulate many adult tissues, its role during embryonic organogenesis and/or in mediating responses to teratogen exposure has not been explored.

METHODS

We have examined here the presence of NO during normal chick embryonic organogenesis, and investigated the teratogenicity of NO through the application of sodium nitroprusside (SNP), which mimics NO overproduction, and NG-monomethyl-L-arginine (L-NMMA), which inhibits endogenous NOS activity.

RESULTS

Topical treatment with SNP or L-NMMA for 18 h resulted in morphological defects, specifically in the neural tube and somites, which corresponded to sites of altered apoptosis. The location of NO was histochemically correlated with the observed morphological defects. Coadministration of SNP or L-NMMA with CO showed functional coregulation and interaction between NO and CO in chick embryonic development.

CONCLUSIONS

Our results showed that regulation of NO is essential for normal axial development, that sites of altered NO expression correlate to those of altered apoptosis and dysmorphogenesis, and that CO coadministration resulted in a rectification of normal NO expression. Collectively, these results suggest that alteration in endogenous NO/CO signaling is responsible, at least in part, for the observed NO-induced teratogenesis.

Molecular clocks underlying vertebrate embryo segmentation: A 10-year-old hairy-go-round

Segmentation of the vertebrate embryo body is a fundamental developmental process that occurs with strict temporal precision. Temporal control of this process is achieved through molecular segmentation clocks, evidenced by oscillations of gene expression in the unsegmented presomitic mesoderm (PSM, precursor tissue of the axial skeleton) and in the distal limb mesenchyme (limb chondrogenic precursor cells). The first segmentation clock gene, hairy1, was identified in the chick embryo PSM in 1997. Ten years later, chick hairy2 expression unveils a molecular clock operating during limb development. This review revisits vertebrate embryo segmentation with special emphasis on the current knowledge on somitogenesis and limb molecular clocks. A compilation of human congenital disorders that may arise from deregulated embryo clock mechanisms is presented here, in an attempt to reconcile different sources of information regarding vertebrate embryo development. Challenging open questions concerning the somitogenesis clock are presented and discussed, such as When?, Where?, How?, and What for? Hopefully the next decade will be equally rich in answers.

The vertebrate segmentation clock and its role in skeletal birth defects

The segmental structure of the vertebrate body plan is most evident in the axial skeleton. The regulated generation of somites, a process called somitogenesis, underlies the vertebrate body plan and is crucial for proper skeletal development. A genetic clock regulates this process, controlling the timing of somite development. Molecular evidence for the existence of the segmentation clock was first described in the expression of Notch signaling pathway members, several of which are expressed in a cyclic fashion in the presomitic mesoderm (PSM). The Wnt and fibroblast growth factor (FGF) pathways have also recently been linked to the segmentation clock, suggesting that a complex, interconnected network of three signaling pathways regulates the timing of somitogenesis. Mutations in genes that have been linked to the clock frequently cause abnormal segmentation in model organisms. Additionally, at least two human disorders, spondylocostal dysostosis (SCDO) and Alagille syndrome (AGS), are caused by mutations in Notch pathway genes and exhibit vertebral column defects, suggesting that mutations that disrupt segmentation clock function in humans can cause congenital skeletal defects. Thus, it is clear that the correct, cyclic function of the Notch pathway within the vertebrate segmentation clock is essential for proper somitogenesis. In the future, with a large number of additional cyclic genes recently identified, the complex interactions between the various signaling pathways making up the segmentation clock will be elucidated and refined.

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.

Expression of homeotic genes Hoxa3, Hoxb3, Hoxd3 and Hoxc4 is decreased in the lungs but not in the hearts of adriamycin-exposed mice

Exposure of rat and mouse embryos to adriamycin (doxorubicin chlorhydrate) induces esophageal atresia (EA) and VACTERL association. Sonic hedgehog (Shh) and Gli2/Gli3 pathways are involved in these conditions and knockout mice for homeotic Hox genes Hoxa3, Hoxb3, Hoxc3, Hoxc4 and Hoxa5 show phenotypes with some of the associated VACTERL features. This study aims at evaluating the possible influence of Hoxa3, Hoxb3, Hoxd3 and Hoxc4 as upstream regulators of this complex signalling. Pregnant mice were exposed either to 4 mg/kg of adriamycin (EA group) or vehicle (controls) on embryonic days 7.5 and 8.5. Embryos were recovered at four endpoints (E12.5-E15.5) and randomly assigned for immunohistochemical or molecular biology studies. Lungs and hearts were separately harvested and processed for Hoxa3, Hoxb3, Hoxd3 and Hoxc4 quantitative RT-PCR measurements. Antibodies for Hoxa3, Hoxb3 and Hoxd3 proteins were used for immunohistochemical studies. RT-PCR studies showed a drastic and statistically significant decrease of the four genes in the lungs of EA mice when compared to controls, with a slight recovery from E15.5. Hearts of both groups showed a similar expression of all the genes throughout gestation. Control embryos expressed the hox3 paralogous genes in heart, skin, foregut derivatives and their surrounding mesoderm through E12.5-E15.5 whereas adriamycin-exposed embryos showed a severe decrease in expression of these three proteins in the same tissues but not in the heart. Adriamycin drastically reduced the expression of Hoxa3, Hoxb3, Hoxd3 and Hoxc4 in mice embryonic lungs. Their expression in the heart did not seem to be influenced by adriamycin in this experimental setting.

Abnormal rib number in childhood malignancy: implications for the scoliosis surgeon

STUDY DESIGN

Retrospective review.

OBJECTIVE

To determine if rib anomalies are present in pediatric malignancies in the United States.

SUMMARY OF BACKGROUND DATA

Scoliosis surgeons view radiographs of the entire spine, counting the number of ribs. A European study noted that rib anomalies were more common in certain malignancies. We wished to determine if this is also true in the United States. If so, the potential for screening, early detection of malignancy, and a better understanding of tumor biology is possible.

METHODS

A retrospective review of 218 children with malignancy and a control group of 200 children with polytrauma or suspected child abuse was performed. Chest radiographs were reviewed to determine the number of ribs, and the presence of rib anomalies. 24 ribs was considered normal, <24 or >24 was considered abnormal. P < 0.05 was considered significant.

RESULTS

The average age was 6.8 +/- 5.5 years and number of ribs was 23.8 +/- 0.6. Rib number was normal in 86.8%. There were significant differences between the malignancy and control groups in age (control, 5.7 +/- 5.1 years; malignancy, 7.8 +/- 5.7 years, P = 0.00007), rib number (control, 23.9 +/- 0.5; malignancy, 23.7 +/- 0.7, P = 0.001), and normal/abnormal rib counts (control, 92% normal; malignancy, 82% normal, P = 0.003). In the malignant group, 50% had a lymphoproliferative malignancy, 33% a solid tumor, and 17.0% a neural tumor. Neural malignancies had a higher incidence of rib abnormalities compared with lymphoproliferative or solid malignancies (P = 0.01). Relative to the control group, those with a neural and lymphoproliferative malignancy were 6.23 (95% CI, 2.7-14.5) and 2.0 (95% CI, 1.0-4.1) times more likely to have an abnormal rib count.

CONCLUSIONS

Homeobox genes, important in vertebral and rib sequencing, are abnormally expressed in many different malignancies. This association is a question of great interest. What is the potential for rib number being used as a predictor of childhood malignancy? Can these findings be expanded to adults? These questions require further research. The association noted in this study is interesting but should not yet be used to alarm parents regarding an increased risk of malignancy in their children.

Krox20 is down-regulated following triazole in vitro embryonic exposure: a polycompetitor-based assay

This study was conducted in order to analyse gene-expression alterations in rat embryos following exposure to triazoles, using an easy-handling approach. Triazole derivatives have been shown to alter the morphology of cranio-facial structures and to induce abnormalities in hindbrain patterning and neural crest cell migration. Specification of hindbrain segments is regulated by retinoic acid and the hox code. Krox20 was chosen as molecular marker for its specific distribution in the anterior neural tube. In fact, this zinc-finger protein is expressed in rhombomere 3 and 5. Mis-regulation of Krox20 levels have shown to induce severe alterations in the correct patterning of the rhomboencephalon and the derived structures. In order to analyse Krox20 mRNA levels in rat embryos exposed in vitro to the triazole derivative triadimefon, a semi-quantitative approach utilising the competitive RT-PCR was chosen. A lambda phage-based plasmid construct that could compete with target and internal standard gene at the same time during enzymatic reaction was generated. Results were confirmed by real-time RT-PCR analysis on the same samples. Our data show a down-regulation of Krox20 transcript levels after exposure to the triazole derivative, implying a key role of this molecule in the pathogenic pathway induced by triazole exposure.

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.

Dual origin of avian lymphatics

The earliest signs of the lymphatic vascular system are the lymph sacs, which develop adjacent to specific embryonic veins. It has been suggested that sprouts from the lymph sacs form the complete lymphatic vascular system. We have studied the origin of the jugular lymph sacs (JLS), the dermal lymphatics and the lymph hearts of avian embryos. In day 6.5 embryos, the JLS is an endothelial-lined sinusoidal structure. The lymphatic endothelial cells (LECs) stain (in the quail) positive for QH1 antibody and soybean agglutinin. As early as day 4, the anlagen of the JLS can be recognized by their Prox1 expression. Prox1 is found in the jugular section of the cardinal veins, and in scattered cells located in the dermatomes along the cranio-caudal axis and in the splanchnopleura. In the quail, such cells are positive for Prox1 and QH1. In the jugular region, the veins co-express the angiopoietin receptor Tie2. Quail-chick-chimera studies show that the peripheral parts of the JLS form by integration of cells from the paraxial mesoderm. Intra-venous application of DiI-conjugated acetylated low-density lipoprotein into day 4 embryos suggests a venous origin of the deep parts of the JLS. Superficial lymphatics are directly derived from the dermatomes, as shown by dermatome grafting. The lymph hearts in the lumbo-sacral region develop from a plexus of Prox1-positive lymphatic capillaries. Both LECs and muscle cells of the lymph hearts are of somitic origin. In sum, avian lymphatics are of dual origin. The deep parts of the lymph sacs are derived from adjacent veins, the superficial parts of the JLS and the dermal lymphatics from local lymphangioblasts.

Negative feedback loop formed by Lunatic fringe and Hes7 controls their oscillatory expression during somitogenesis

Recent studies show that the cyclic expression of Lfng in the presomitic mesoderm (PSM) is controlled at the transcription level by Notch signal through the CBF1 binding site for activation and periodic repression on the Lfng promoter. Here we provide genetic evidence that the oscillatory expression is controlled by a negative feedback mechanism. We also show that Hes7, another cyclically expressed protein, can bind to the N-boxes on both Lfng and its own promoters and repress their activity. In addition, we demonstrate that the 3' untranslated region (3'-UTR) is important for rapid degradation of Lfng mRNA.

Bifid ribs and unusual vertebral anomalies diagnosed in an anatomical specimen. Gorlin syndrome?

A hitherto unknown combination of multiple bifid ribs, as seen in Gorlin syndrome (GS), interpedicular fusion and apparent malsegmentation of vertebral laminae at various upper thoracic levels was found in the skeleton of a newborn infant. This specific combination of anomalies is also seen in the mouse open brain (opb) mutant. Since the genes involved in GS (Patched2) and opb (rab23) both play an essential role in the hedgehog signaling pathway, it is likely that the cause of the anomalies presented here is to be sought in impaired functioning of this pathway.

The scoliosis (sco) mouse: a new allele of Pax1

We describe the spontaneous mutant mouse scoliosis (sco) that carries a new allele of Pax1 (un-i, undulated intermediate). The Pax1(un-i) allele is lacking the 5'-flanking region and exon 1 to 4 which is mapped to nt -2636 to -640 and -272 to 4271 of the Pax1 gene. Homozygous mice show a mild form of the known phenotypes of other Pax1 mutants. Adult mice have a lumbar scoliosis and kinky tails. In homozygous embryos the skeleton ossifies early, ossification centers of the vertebral bodies are fused with the ossification centers of the pedicles. Neural arches and spinous processes are underdeveloped but the pedicles and transverse processes are overdeveloped which is in contrast to other Pax1 mutants. In the scapula, the acromion is missing and the deltoid tuberosity of the proximal humerus is shortened and thickened. Among the inner organs the thymus development is affected. In late embryos, the thymus is small and thymocyte numbers are reduced. T-cell development from CD4- and CD8- double negative (DN) to CD4+ and CD8+ double positive (DP) is decelerated. The percentage of CD90+ cells is also reduced but in contrast to other Pax1 mutants no alteration of the expression level of the CD90 (Thy-1) could be found.

B-cell translocation gene 2 (Btg2) regulates vertebral patterning by modulating bone morphogenetic protein/smad signaling

Btg2 is a primary p53 transcriptional target gene which may function as a coactivator-corepressor and/or an adaptor molecule that modulates the activities of its interacting proteins. We have generated Btg2-null mice to elucidate the in vivo function of Btg2. Btg2-null mice are viable and fertile but exhibit posterior homeotic transformations of the axial vertebrae in a dose-dependent manner. Consistent with its role in vertebral patterning, Btg2 is expressed in the presomitic mesoderm, tail bud, and somites during somitogenesis. We further provide biochemical evidence that Btg2 interacts with bone morphogenetic protein (BMP)-activated Smads and enhances the transcriptional activity of BMP signaling. In view of the genetic evidence that reduced BMP signaling causes posteriorization of the vertebral pattern, we propose that the observed vertebral phenotype in Btg2-null mice is due to attenuated BMP signaling.

Zic4, a zinc-finger transcription factor, is expressed in the developing mouse nervous system

Zic genes comprise a family of transcription factors, characterized by the presence of a zinc-finger domain containing two cysteines and two histidines (C2-H2). Whereas the embryonic expression patterns of Zic1, 2, 3, and 5 have been described in detail, Zic4 has not yet received close attention. We studied the expression of Zic4 by in situ hybridization during mouse embryogenesis. Zic4 mRNA was first detected at low intensity at embryonic day (E) 9 and, by E10.5, expression was up-regulated in the dorsal midline of the forebrain with a strong, expanded expression domain at the boundary between the diencephalon and telencephalon, the septum, and the lamina terminalis. The choroid plexus of the third ventricle expresses Zic4, as does the dorsal part of the spinal neural tube, excluding the roof plate. The dorsal sclerotome and the dorsomedial lip of the dermomyotome also express Zic4 whereas dorsal root ganglia are negative. At E12.5, Zic4 continues to be expressed in the midline of the forebrain and in the dorsal spinal neural tube. Postnatally, Zic4 is expressed in the granule cells of the postnatal day 2 cerebellum, and in the periventricular thalamus and anterior end of the superior colliculus. We conclude that Zic4 has an expression pattern distinct from, but partly overlapping with, other members of the Zic gene family.

Wnt 6 regulates the epithelialisation process of the segmental plate mesoderm leading to somite formation

In higher vertebrates, the paraxial mesoderm undergoes a mesenchymal to epithelial transformation to form segmentally organised structures called somites. Experiments have shown that signals originating from the ectoderm overlying the somites or from midline structures are required for the formation of the somites, but their identity has yet to be determined. Wnt6 is a good candidate as a somite epithelialisation factor from the ectoderm since it is expressed in this tissue. In this study, we show that injection of Wnt6-producing cells beneath the ectoderm at the level of the segmental plate or lateral to the segmental plate leads to the formation of numerous small epithelial somites. Ectopic expression of Wnt6 leads to sustained expression of markers associated with the epithelial somites and reduced or delayed expression of markers associated with mesenchymally organised somitic tissue. More importantly, we show that Wnt6-producing cells are able to rescue somite formation after ectoderm ablation. Furthermore, injection of Wnt6-producing cells following the isolation of the neural tube/notochord from the segmental plate was able to rescue somite formation at both the structural (epithelialisation) and molecular level, as determined by the expression of marker genes like Paraxis or Pax-3. We show that Wnts are indeed responsible for the epithelialisation of somites by applying Wnt antagonists, which result in the segmental plate being unable to form somites. These results show that Wnt6, the only known member of this family to be localised to the chick paraxial ectoderm, is able to regulate the development of epithelial somites and that cellular organisation is pivotal in the execution of the differentiation programmes. We propose a model in which the localisation of Wnt6 and its antagonists regulates the process of epithelialisation in the paraxial mesoderm.

Neural tube programming and craniofacial cleft formation. I. The neuromeric organization of the head and neck

This review presents a brief synopsis of neuromeric theory. Neuromeres are developmental units of the nervous system with specific anatomic content. Outlying each neuromere are tissues of ectoderm, mesoderm and endoderm that bear an anatomic relationship to the neuromere in three basic ways. This relationship is physical in that motor and sensory connections exist between a given neuromeric level and its target tissues. The relationship is also developmental because the target cells exit during gastrulation precisely at that same level. Finally the relationship is chemical because the genetic definition of a neuromere is shared with those tissues with which it interacts. The model developed by Puelles and Rubenstein is used to describe the neuroanatomy of the neuromeres. Although important details of the model are currently being refined it has immediate clinical relevance for practicing clinicians because it permits us to understand many pathologic states as relationships between the brain and the surrounding tissues. Relationships between the processes of neurulation and gastrulation have been presented to demonstrate the manner in which neuromeric anatomy is established in the embryo. We are now in a position to describe in detail the static anatomic structures that result from this system. The neuromeric 'map' of craniofacial bones, dermis, dura, muscles, and fascia will be the subject of the next part of this series.

Morphogenesis of embryonic CNS vessels

This chapter focuses on the morphology of blood vessel formation in and around the early central nervous system (CNS, i.e., brain and spinal cord) of avian embryos. We discuss cell lineages, proliferation and interactions of endothelial cells, pericytes and smooth muscle cells, and macrophages. Due to space limitations, we can not review the molecular control of CNS angiogenesis, but refer the reader to other chapters in this book and to recent publications on the assembly of the vasculature (1,2).

Contribution of somitic cells to the avian ribs

The traditional view that all parts of the ribs originate from the sclerotome of the thoracic somites has recently been challenged by an alternative view suggesting that only the proximal rib derives from the sclerotome, while the distal rib arises from regions of the dermomyotome. In view of this continuing controversy and to learn more about the cell interactions during rib morphogenesis, this study aimed to reveal the precise contributions made by somitic cells to the ribs and associated tissues of the thoracic cage. A replication-deficient lacZ-encoding retrovirus was utilized to label cell populations within distinct regions of somites 19-26 in stage 13-18 chick embryos. Analysis of the subsequent contributions made by these cells revealed that the thoracic somites are the sole source of cells for the ribs. More precisely, it is the sclerotome compartment of the somites that contributes cells to both the proximal and distal elements of the ribs, confirming the traditional view of the origin of the ribs. Results also indicate that the precursor cells of the ribs and intercostal muscles are intimately associated within the somite, a relationship that may be essential for proper rib morphogenesis. Finally, the data from this study also show that the distal ribs are largely subject to resegmentation, although cell mixing may occur at the most sternal extremities.

Clock regulatory elements control cyclic expression of Lunatic fringe during somitogenesis

Somitogenesis requires a segmentation clock and Notch signaling. Lunatic fringe (Lfng) expression in the presomitic mesoderm (PSM) cycles in the posterior PSM, is refined in the segmenting somite to the rostral compartment, and is required for segmentation. We identify distinct cis-acting regulatory elements for each aspect of Lfng expression. Fringe clock element 1 (FCE1) represents a conserved 110 bp region that is necessary to direct cyclic Lfng RNA expression in the posterior PSM. Mutational analysis of E boxes within FCE1 indicates a potential interplay of positive and negative transcriptional regulation by cyclically expressed bHLH proteins. A separable Lfng regulatory region directs expression to the prospective rostral aspect of the condensing somite. These independent Lfng regulatory cassettes advance a molecular framework for deciphering somite segmentation.

Expression of apoptosis-inducing factor during early neural differentiation in the chick embryo

The distribution of apoptosis-inducing factor (AIF) immunoreactivity has been studied in the developing somites and nervous system of the chick embryo at embryonic day 4. AIF was found to be expressed primarily in the cytoplasm of cells of the ventral motor roots, at the points of their insertion into the neural tube. Co-localization of mitochondrial AIF immunoreactivity with the epitopes recognized by the monoclonal antibodies HNK-1 and 1E8 suggests that the AIF may be present in Schwann cell precursors as well as in nerve fibres. AIF immunoreactivity was not observed in either cell bodies in the neural tube, or in the somitic tissue surrounding the ventral roots. The results are consistent with the hypothesis that AIF may be involved in neuronal cell death during development, and that target-derived neuronal survival factors may act by controlling AIF activity.

Histological examinations of bone and cartilage in the axial skeleton of human triploidy fetuses

The aim of this study was to examine histologically bone and cartilage in vertebral corpora of axial skeletons of eight human triploid fetuses, gestational ages 14-25 weeks, CRL 100-200 mm. The results were compared to earlier studies on vertebral development in trisomies 21, 18, 13, and to normal corpora development. After radiography in frontal and lateral projections, the vertebral column was sectioned into cervical, thoracic and lumbar segments, decalcified, dehydrated, and embedded in paraffin. The blocks were serially sectioned in the vertical plane and stained with Toluidine blue and Alcian blue/van Gieson. The radiographic characteristics of the vertebral corpora observed in frontal and lateral projection varied from small cleft vertebral corpora to fusions between the individual corpora. Histological examination of the vertebral corpora confirmed the abnormal pattern of ossification seen radiographically. As a new finding abnormal metachromasia of the ground substance was observed in the cartilage. Marked borderlines were registered in the cartilage between regions with differences in metachromasia. These borderlines were similar but more extensive than borderlines observed previously in trisomies 21, 18 and 13.

Regulation of Epha4 expression in paraxial and lateral plate mesoderm by ectoderm-derived signals

Somitogenesis in all vertebrates involves a mesenchymal to epithelial transition of segmental plate cells. Such a transition involves cells altering their morphology and their adhesive properties. The Eph family of receptor tyrosine kinases has been postulated to regulate cytoskeletal organization. In this study, we show that a receptor belonging to this family, EphA4, is expressed in the segmental plate in a region where cells are undergoing changes in cell shape as a prelude to epithelialization. We have identified the ectoderm covering the somites and the midline ectoderm as sources of signals capable of inducing EphA4. Loss of EphA4 results in cells of irregular morphology and somites fail to form. We also show that when somites fail to develop, expression of EphA4 in the lateral plate is also lost. We suggest that signaling occurs between the somites and the lateral plate mesoderm and provide evidence that retinoic acid is involved in this communication.

The effects of collagen synthesis inhibitory drugs on somitogenesis and myogenin expression in cultured chick and mouse embryos

The role of fibrillar collagen on myogenic differentiation has previously been studied in tissue culture cell lines but has not been studied in situ. We treated cultured chick and mouse embryos with collagen synthesis inhibitors to determine the role of fibrillar collagen on somitogenesis and on myogenic differentiation in vivo. Stage 12 chick embryos and 8.7 dpc mouse embryos were cultured in control medium or a range of concentrations of the collagen synthesis inhibitors ethyl-3,4-dihydroxybenzoate (EDHB) or cis-hydroxy-proline (CHP). Chick embryos were cultured for 24 h and mouse embryos were cultured for 30 h. Both collagen synthesis inhibitors produced a range of somite abnormalities including formation of fewer and irregular somites in both chick and mouse at high drug concentrations, as well as formation of double somites in EDHB-treated chick embryos. Examination of EDHB-treated mouse embryos by scanning electron microscopy demonstrated a dosage-dependent loss of fibrillar collagen and associated extracellular matrix. Expression of myogenin in EDHB-treated mouse embryos, examined by whole-mount in situ hybridization, was suppressed at higher dosage levels. This study suggests that inhibition of fibrillar collagen production and/or loss of fibrillar collagen in the developing avian and mammalian embryo results in abnormal somite formation and perturbed myogenic differentiation.

Programmed cell death in the developing somites is promoted by nerve growth factor via its p75(NTR) receptor

Neurotrophins control neuron number during development by promoting the generation and survival of neurons and by regulating programmed neuronal death. In the latter case, the cell death induced by nerve growth factor (NGF) in the developing chick retina is mediated by p75(NTR), the common neurotrophin receptor (J. M. Frade, A. Rodriguez-Tebar, and Y.-A. Barde, 1996, Nature 383, 166-168). Here we show that NGF also induces the programmed death of paraxial mesoderm cells in the developing somites. Both NGF and p75(NTR) are expressed in the somites of chick embryos at the time and the place of programmed cell death. Moreover, neutralizing the activity of endogenous NGF with a specific blocking antibody, or antagonizing NGF binding to p75(NTR) by the application of human NT-4/5, reduces the levels of apoptotic cell death in both the sclerotome and the dermamyotome by about 50 and 70%, respectively. Previous data have shown that Sonic hedgehog is necessary for the survival of differentiated somite cells. Consistent with this, Sonic hedgehog induces a decrease of NGF mRNA in somite explant cultures, thus showing the antagonistic effect of NGF and Sonic hedgehog with respect to somite cell survival. The regulation of programmed cell death by NGF/p75(NTR) in a mesoderm-derived tissue demonstrates the capacity of neurotrophins and their receptors to influence critical developmental processes both within and outside of the nervous system.

The developmental fate of the rostral/caudal half of a somite for vertebra and rib formation: experimental confirmation of the resegmentation theory using chick-quail chimeras

To determine whether resegmentation of somites forms the axial skeleton, we traced the development of the rostral and the caudal half of a somite during skeletogenesis in chick-quail chimeras by replacing the rostral or caudal half of a newly formed chick somite with that of a quail somite. The rostral half-somite transplant formed the caudal half of the vertebral body, the entire spinous process and the distal rib, while the caudal half-somite transplant formed the rostral half of vertebral body, the rostral half of spinous process, the vertebral arch, the transverse process and the entire rib. These findings confirm the resegmentation theory except the spinous process and the distal rib.

Molecular and cellular biology of avian somite development

Much of our understanding of early vertebrate embryogenesis derives from experimental work done with the chick embryo. Studies of the avian somite have played a key role in elucidating the developmental history of this important structure, the source of most muscle and bone in the organism. Here we review the development of the avian somite including morphological and molecular data on the origin of paraxial mesoderm, maturation of the segmental plate, specification and formation of somite compartments, and somite cell differentiation into cartilage and skeletal muscle.