Transcriptional interferences at the Hoxa4/Hoxa5 locus: importance of correct Hoxa5 expression for the proper specification of the axial skeleton

We have previously described a Hoxa5 mutant mouse line in which specification of axial identity is perturbed and viability is markedly reduced. In the present study, we assay the Hoxa5 mutation in different genetic backgrounds and carry out a complete analysis of skeletal transformations. Although Hoxa5 is expressed over a large domain during embryogenesis, homeotic transformations of the axial skeleton are confined between cervical vertebra C3 and thoracic vertebra T2, which corresponds to the specific expression domain of the major Hoxa5 transcript. Loss of Hoxa5 function also affects the formation of the acromion in the appendicular skeleton. Disruption of the adjacent Hoxa4 gene leads to similar homeotic transformations of the cervicothoracic vertebrae. To discriminate the respective role of each gene, we generated transheterozygous animals carrying inactivated Hoxa4 and Hoxa5 alleles on different chromosomes. Compound heterozygous mutants exhibit homeotic transformations in the cervicothoracic transition region more reminiscent to those observed in Hoxa5 homozygous mutants. Although the Hoxa5 mutation does not significantly affect Hoxa4 expression, the pattern of Hoxa5 expression is impaired in cis by the Hoxa4 mutation, specifically in the cervicothoracic region of the prevertebral column. The expression of Hoxa5 in this particular domain is also perturbed by the Hoxa5 mutation itself, raising the possibility of regional autoregulation. Altogether, these results demonstrate the crucial role of Hoxa5 in the specification of the cervical and upper thoracic region of the skeleton and establish the importance of its correct expression for the proper patterning of the embryo.

Cartilaginous development of the human craniovertebral junction as visualised by a new three-dimensional computer reconstruction technique

Serial transverse histological sections of the human craniovertebral junction (CVJ) of 4 normal human embryos (aged 45 to 58 d) and of a fetus (77 d) were used to create 3-dimensional computer models of the CVJ. The main components modelled included the chondrified basioccipital, atlas and axis, notochord, the vertebrobasilar complex and the spinal cord. Chondrification of the component parts of CVJ had already begun at 45 d (Stage 18). The odontoid process appeared to develop from a short eminence of the axis forming a third occipital condyle with the caudal end of the basioccipital. The cartilaginous anterior arch of C1 appeared at 50-53 d (Stages 20-21). Neural arches of C1 and C2 showed gradual closure, but there was still a wide posterior spina bifida in the oldest reconstructed specimen (77 d fetus). The position of the notochord was constant throughout. The normal course of the vertebral arteries was already established and the chondrified vertebral foramina showed progressive closure. The findings confirm that the odontoid process is not derived solely from the centrum of C1 and that there is a 'natural basilar invagination' of C2 during normal embryonic development. On the basis of the observed shape and developmental pattern of structures of the cartilaginous human CVJ, we suggest that certain pathologies are likely to originate during the chondrification phase of development.

Hox group 3 paralogous genes act synergistically in the formation of somitic and neural crest-derived structures

Hox genes encode transcription factors that are used to regionalize the mammalian embryo. Analysis of mice carrying targeted mutations in individual and multiple Hox genes is beginning to reveal a complex network of interactions among these closely related genes which is responsible for directing the formation of spatially restricted tissues and structures. In this report we present an analysis of the genetic interactions between all members of the third paralogous group, Hoxa3, Hoxb3, and Hoxd3. Previous analysis has shown that although mice homozygous for loss-of-function mutations in either Hoxa3 or Hoxd3 have no defects in common, mice mutant for both genes demonstrate that these two genes strongly interact in a dosage-dependent manner. To complete the analysis of this paralogous gene family, mice with a targeted disruption of the Hoxb3 gene were generated. Homozygous mutants have minor defects at low penetrance in the formation of both the cervical vertebrae and the IXth cranial nerve. Analysis and comparison of all double-mutant combinations demonstrate that all three members of this paralogous group interact synergistically to affect the development of both neuronal and mesenchymal neural crest-derived structures, as well as somitic mesoderm-derived structures. Surprisingly, with respect to the formation of the cervical vertebrae, mice doubly mutant for Hoxa3 and Hoxd3 or Hoxb3 and Hoxd3 show an indistinguishable defect, loss of the entire atlas. This suggests that the identity of the specific Hox genes that are functional in a given region may not be as critical as the total number of Hox genes operating in that region.

Methanol causes posteriorization of cervical vertebrae in mice

Inhalation of methanol by pregnant mice before gestation day nine (gd 9) produces fetal skeletal alterations, principally in the cervical region. The appearance of these defects suggests homeotic shifts in segment identity, patterning, or both. To explore this possibility, detailed morphological analyses of the effects of methanol on fetal skeletal development were done. Pregnant mice were gavaged with 0, 4.0, or 5.0 g/kg methanol (MeOH) split in two doses on gd 7, the most sensitive day for induction of skeletal alterations with methanol. Dams were killed on gd 18 and the fetuses were counted, weighed, and examined externally. Fetuses were double stained with alcian blue and alizarin red for examination of cartilaginous and ossified vertebral and rib characteristics, and in selected fetuses cervical vertebrae were disarticulated for more detailed analysis. Observations indicative of methanol-induced homeotic transformations were as follows: [tabular data: see abstract volume] Examination of disarticulated vertebrae revealed foramina and other distinguishing characteristics on vertebrae anterior to those on which they normally appear. These results demonstrate that maternal methanol exposure can alter segment patterning in the developing mouse embryo, producing posteriorization of cervical vertebrae.

Targeted disruption of the Hoxb-2 locus in mice interferes with expression of Hoxb-1 and Hoxb-4

Mice with a disruption in the hoxb-2 locus were generated by gene targeting. 75% of the hoxb-2 mutant homozygotes died within 24 hours of birth. While a majority of these mice had severe sternal defects that compromised their ability to breathe, some had relatively normal sternum morphology, suggesting that one or more additional factor(s) contributed to neonatal lethality. At 3–3.5 weeks of age, half of the remaining hoxb-2 homozygotes became weak and subsequently died. All of the mutants that survived to 3 weeks of age showed marked facial paralysis similar to, but more severe than, that reported for hoxb-1 mutant homozygotes (Goddard, J. M., Rossel, M., Manley, N. R. and Capecchi, M. R. (1996) Development 122, 3217–3228). As for the hoxb-1 mutations, the facial paralysis observed in mice homozygous for the hoxb-2 mutation results from a failure to form the somatic motor component of the VIIth (facial) nerve which controls the muscles of facial expression. Features of this phenotype closely resemble the clinical signs associated with Bell's Palsy and Moebius Syndrome in humans. The sternal defects seen in hoxb-2 mutant mice are similar to those previously reported for hoxb-4 mutant mice (Ramirez-Solis, R., Zheng, H., Whiting, J., Krumlauf, R. and Bradley. A. (1993) Cell 73, 279–294). The above results suggest that the hoxb-2 mutant phenotype may result in part from effects of the hoxb-2 mutation on the expression of both hoxb-1 and hoxb-4. Consistent with this proposal, we found that the hoxb-2 mutation disrupts the expression of hoxb-1 in cis. In addition, the hoxb-2 mutation changes the expression of hoxb-4 and the hoxb-4 mutation, in turn, alters the pattern of hoxb-2 expression. Hoxb-2 and hoxb-4 appear to function together to mediate proper closure of the ventral thoracic body wall. Failure in this closure results in severe defects of the sternum.

Cervical flexion: its contribution to normal and abnormal cardiac morphogenesis

Although His in 1881 and Patten in 1922 suggested that cervical flexion could play an important role in normal cardiac morphogenesis, until recently this hypothesis has been largely neglected. The purpose of this report is to present data indicating that prevention of cervical flexion leads to double outlet right ventricle (DORV), which is unrelated to any affect on neural crest cell migration. At stage 11-12, suture material was inserted into the neural tube to prevent cervical flexion. Six out of 22 experimental embryos survived until stage 38 and 5 out of 6 had DORV. Neither abnormalities of the aortico-pulmonary septum nor interruption of the aortic arch were observed at dissection. Hemodynamic studies performed at stage 18 revealed distinctive characteristics that are inconsistent with hemodynamic studies previously reported following neural crest ablation. With respect to immunohistochemical studies using neural crest associated antigen HNK-1 antibody, normal migration of neural crest cells was noted in the outflow tract in the experimental embryos at stage 22. These hemodynamic and immunohistochemical studies suggest that insertion of suture material into the neural tube at stage 11-12 does not jeopardize neural crest migration. We propose that reduction in cervical flexion increases the distance between the future aorta and the left ventricle, which prevents the transition of intracardiac flow pattern from a serial circulation to a parallel one, leaving persistence of a "double outlet" from the right ventricle.

Targeted disruption in the mouse Hoxc-4 locus results in axial skeleton homeosis and malformation of the xiphoid process

Hoxc-4 is a mouse homeobox gene located at the 3' end of the HoxC cluster. Of the HoxC genes, Hoxc-4 is expressed in the most anterior regions of the central nervous system and prevertebral column. To investigate its role in mouse development, we have generated Hoxc-4 mutant mice by gene targeting. Mice homozygous for the Hoxc-4 mutation are viable and fertile. Analysis of the skeletal system of homozygous mutants revealed various abnormalities in the cervical and thoracic regions. The most frequent abnormality was a partial posterior homeotic transformation of the seventh cervical vertebra. Less frequently, anterior transformations of the third and eighth thoracic vertebrae were observed. Furthermore, the xiphoid process of the sternum was malformed such that it had an aperture or a fissure. Although Hoxc-4 is expressed abundantly in the central nervous system, no obvious defects were observed. These results suggest that Hoxc-4 is required for specifying cervical and thoracic vertebral identity.

Disruption of the murine homeobox gene Cdx1 affects axial skeletal identities by altering the mesodermal expression domains of Hox genes

Cdx1 is expressed along the embryonic axis from day 7.5 postcoitum until day 12, by which time the anterior limit of expression has regressed from the hindbrain level to the forelimb bud region. To assign a functional role for Cdx1 in murine embryonic development, we have inactivated the gene via homologous recombination. Viable fertile homozygous mutant mice were obtained that show anterior homeotic transformations of vertebrae. These abnormalities were concomitant with posterior shifts of Hox gene expression domains in the somitic mesoderm. The presence of putative Cdx1-binding sites in Hox gene control regions as well as in vitro transactivation of Hoxa-7 indicates a direct regulation.

Pax-1 in the development of the cervico-occipital transitional zone

The Pax-1 gene has been found to play an important role in the development of the vertebral column. The cervico-occipital transitional zone is a specialized region of the vertebral column, and malformations of this region have frequently been described in humans. The exact embryonic border between head and trunk is a matter of controversy. In order to determine a possible role of Pax-1 in the development of the cervico-occipital transitional zone we studied the expression of this gene in a series of quail embryos and murine fetuses with in situ hybridization and immunohistochemistry. Pax-1 is expressed in all somites of the embryo, including the first five occipital ones. During embryonic days 3-5 the gene is down-regulated in the caudal direction within the first five somites, whereas more caudally Pax-1 is strongly expressed in the cells of the perinotochordal tube. In 5-day-old quail embryos, the cartilaginous anlage of the basioccipital bone has developed and ther is no more expression of Pax-1 in this region. The fusion of the dens axis with the body of the axis also coincides with switching off of the Pax-1 gene. More caudally, the gene is continuously expressed in the intervertebral discs of murine embryos and therefore seems to be important for the process of resegmentation. Quail embryos do not possess permanent intervertebral discs. ¿Hyper-¿ or ¿hyposegmentation¿ defects may be explained by an over- or under-expression of Pax-1 during development. We also reinvestigated the border between the head and trunk in chick embryos by performing homotopical grafting experiments of the 5th somite between chick and quail embryos.

Specification of multiple vertebral identities by ectopically expressed Hoxb-8

We have recently generated Hoxb-8 gain-of-function mutant embryos, using a Hoxb-8 transgene driven by a retinoic acid receptor beta 2 promoter to extend the expression domain to more anterior regions of the embryo (Charité et al. [1994] Cell 78:589-601). Here we describe the phenotype in the axial skeleton of transgenic embryos. The severity of the phenotype was variable, and cervical vertebrae and the base of the skull were affected in different ways. We observed fusion of the anterior arch of the atlas to the dens of the axis, partial splitting of the vertebral body and the neural arch of the axis, and abnormal morphology of the basioccipital and exoccipital bones. The basioccipital bone projected into the atlas, sometimes fusing to the dens of the axis; the exoccipitial bones appeared to be transformed towards neural arch-like structures. A novel pattern of posterior homeotic transformations was observed, involving cervical vertebrae C3 to C7: the ventral aspect of vertebrae C5 to C7 could acquire different morphologies characteristic of more posterior vertebrae: C5 could be transformed into C6, C7, or T1, C6 into C7 or T1, and C7 into T1. Phenotypes of different severity could be arranged into a phenotypic series, starting with the transformation of C7 to T1 and involving transformation of increasingly more anterior vertebrae into increasingly more posterior identities; no vertebra acquired a more posterior morphology than that of the vertebra immediately caudal to it. Ribs appeared to be formed relatively independently of rib heads; cervical ribs (but not rib heads) could be observed as anterior as C3. The results suggest that higher levels of ectopically expressed Hoxb-8 result in specification of more posterior vertebral identities.

Compound mutants for the paralogous hoxa-4, hoxb-4, and hoxd-4 genes show more complete homeotic transformations and a dose-dependent increase in the number of vertebrae transformed

The Hox gene products are transcription factors involved in specifying regional identity along the anteroposterior body axis. In the mouse, several single mutants for Hox genes show variably penetrant, partial homeotic transformations of vertebrae at their anterior limits of expression, suggesting that compound Hox mutants might show more complete transformations with greater penetrance than the single Hox mutants. Compound mutants for the paralogous group 3 genes, hoxa-3 and hoxd-3, show deletion of a cervical vertebrae, which is not readily interpretable in terms of an alteration in regional identity. Here, we report the skeletal phenotypes of compound mutants in the group 4 Hox genes, hoxa-4, hoxb-4, and hoxd-4. Mice mutant for each of these genes were intercrossed to generate the three possible double mutant combinations and the triple mutant. In contrast to the hoxa-3, hoxd-3 double mutants, group 4 Hox compound mutants displayed clear alterations in regional identity, including a nearly complete transformation of the second cervical vertebrae toward the morphology of the first cervical vertebra in one double mutant combination. In comparing the types of homeotic transformations observed, different double mutant combinations showed different degrees of synergism. These results suggest a certain degree of functional redundancy among paralogous genes in specifying regional identity. Furthermore, there was a remarkable dose-dependent increase in the number of vertebrae transformed to a first cervical vertebra identity, including the second through the fifth cervical vertebrae in the triple mutant. Thus, these genes are required in a larger anteroposterior domain than is revealed by the single mutant phenotypes alone, such that multiple mutations in these genes result in transformations of vertebrae that are not at their anterior limit of expression.

Craniovertebral junction anomalies in inherited disorders: part of the syndrome or caused by the disorder?

Patterns of skeletal abnormality at the craniovertebral junction in the normal population and in syndromes such as Down, Morquio etc, are compared and the recent embryological data and comparative anatomy reviewed. The authors' view based on their own clinical and radiological experience is that the os odontoideum is the product of excessive movement at the time of ossification of the cartilaginous dens and is exactly analogous to the unfused Type II odontoid fracture. True hypoplasia of the odontoid peg is part of a wider segmentation defect associated with Klippel Feil, occipitalised atlas and/or basilar invagination; it is hardly ever associated with instability.

Mutations in paralogous Hox genes result in overlapping homeotic transformations of the axial skeleton: evidence for unique and redundant function

Hoxd-4 (previously known as Hox-4.2 and -5.1) is a mouse homeobox-containing gene homologous to the Drosophila homeotic gene Deformed. During embryogenesis, Hoxd-4 is expressed in the presumptive hindbrain and spinal cord, prevertebrae, and other tissues. In the adult, Hoxd-4 transcripts are expressed predominantly in the testis and kidney, and to a lesser extent in intestine and heart. To understand the role of Hoxd-4 during mouse embryogenesis, we generated Hoxd-4 mutant mice. Mice heterozygous or homozygous for the Hoxd-4 mutation exhibit homeotic transformations of the second cervical vertebrae (C2) to the first cervical vertebrae (C1) and malformations of the neural arches of C1 to C3 and of the basioccipital bone. The phenotype was incompletely penetrant and showed variable expressivity on both an F2 hybrid and 129 inbred genetic background. The mutant phenotype was detected in the cartilaginous skeleton of 14.5-day (E14.5) mutant embryos but no apparent differences were detected in the somites of E9.5 mutant embryos, suggesting that the abnormalities develop after E9.5 perhaps during or after resegmentation of the somites to form the prevertebrae. These results suggest that Hoxd-4 plays a role in conferring position information along the anteroposterior axis in the skeleton. The phenotypic similarities and differences between Hoxd-4 and previously reported Hoxa-4 and Hoxb-4 mutant mice suggest that Hox gene paralogs have both redundant and unique functions.

Formation of the cervical flexure: an experimental study on chick embryos

It has been proposed that the cervical flexure of vertebrate embryos arises from the normal morphogenesis of the heart. This hypothesis is based on experiments in which the heart tube is removed or disrupted in early chick embryos. It has been reported that, in normal atmosphere, these embryos continued normal morphogenesis except for cervical flexure formation. In the present study, we performed similar experiments. In contrast to previous work, however, only one set of our heart-deprived chick embryos was reincubated in normal air. The other sets were reincubated in oxygen-enriched air. Under normoxia, heart removal resulted not only in prevention of the cervical flexure, but also in mesenchymal defects, and in a remarkable hypoplasia of the craniocervical region. Under hyperoxia, heart-deprived embryos developed no severe mesenchymal defects and the growth of the upper body portion was more normal, with the hypoplasia confined to the cranial region. The formation of the cervical flexure was now normalized. These results show that cervical flexure formation is not directly dependent on normal morphogenesis of the heart, but does depend on a sufficient oxygen supply to the cervical region. During early development, the cranio-cervical region of a chick embryo is more sensitive to circulatory failure than the trunk.

Cervical meningoceles and myelocystoceles: a unifying hypothesis

The classification and embryogenesis of cystic cervical dysraphic lesions are discussed in the light of the authors' experience and review of the literature. It is felt that these lesions are best described as meningoceles or myelocystoceles, and the use of the term 'myelomeningocele' may be more confusing than clarifying. The authors hypothesize that the cervical meningocele and the myelocystocele are part of a spectrum of the same underlying developmental abnormality, namely limited dorsal myeloschisis, with the eventual abnormality depending on the presence or absence of associated hydromyelia.

Homeotic transformation of cervical vertebrae in Hoxa-4 mutant mice

Hoxa-4 (previously known as Hox-1.4) is a mouse homeobox-containing gene that is expressed in the presumptive hindbrain and spinal cord, prevertebrae, and other tissues during embryogenesis. To understand the role of Hoxa-4 during development, we generated Hoxa-4 mutant mice. Homozygous mutants were viable and fertile. Analysis of neonatal skeletons revealed the development of ribs on the seventh cervical vertebra at variable penetrance and expressivity. A low frequency of alterations in sternal morphogenesis was also observed. In addition, we analyzed the skeletons of transgenic mice that overexpress Hoxa-4 and found that the formation of the small rib anlagen that often develop on the seventh cervical vertebra was suppressed. Analysis of adult homozygous mutant skeletons revealed that the dorsal process normally associated with the second cervical vertebra was also found on the third cervical vertebra. These results demonstrate that Hoxa-4 plays a role in conferring positional information along the anteroposterior axis to specify the identity of the third and the seventh cervical vertebrae.

Occipitocervical segmentation in staged human embryos

Serial sections of 108 human embryos from stage 11 to stage 23 were investigated, and 33 reconstructions were prepared. The existence of 4 occipital somites was confirmed. The important developmental distinction between axial (central) and lateral components obtains in the occipital as well as in the vertebral region. The lateral occipital components begin to show dense areas as the cervical region is approached. The lateral occipital and vertebral components arise in registration with the initial sclerotomes. In both the occipital and the vertebral region the related nerves and intersegmental arteries traverse the loose areas of the sclerotomes. The axial occipital region is not segmented, whereas the cervical components develop from perinotochordal loose areas. Three complete centra (known as XYZ) develop in the atlanto-axial region, although they are related to only 2 1/2 sclerotomes and only 2 neural arches. The height of the XYZ complex equals that of 3 centra elsewhere, and not 2 1/2, as previously maintained. The experimental findings in the occipitocervical region of the chick embryo show both similarities to, as well as differences from, the data for the human embryo. A scheme showing the early development of the entire vertebral column is included.

Conservation in the Hox code during morphological evolution

The expression domains in paraxial mesoderm of the chicken embryo are described for Hoxb-3, a-4 and c-6 genes, and these are compared with published expression data for the corresponding genes in the mouse. In both species, it is found that the anterior limits of Hoxb-3 and a-4 expression lie in the upper cervical region, and the anterior limits of Hoxc-6 expression lie in the upper thoracic region. This finding is remarkable because the cervical region, or neck, of the chicken (with fourteen cervical vertebrae) is much longer than that of the mouse (seven cervical vertebrae). The results suggest that the Hox code, at least in the development of homologous axial structures, is conserved between species (Hoxb-3 and a-4, for example, being associated with an anterior cervical phenotype; Hoxc-6 being associated with an anterior thoracic phenotype). The results also suggest that an evolutionary change in body proportions is accomplished by a shift in the relative positions of Hox expression domains during embryonic development.

Ectopic expression of Hoxb-8 causes duplication of the ZPA in the forelimb and homeotic transformation of axial structures

Transgenic embryos were generated carrying a Hoxb-8 transgene under control of the mouse RAR beta 2 promoter, which extends the normal expression domain to more anterior regions of the embryo. These embryos showed mirror-image duplications in the forelimb, analogous to the duplications observed in chick in response to transplantation of a ZPA to the anterior margin of the limb bud. Examination of Sonic hedgehog, Fgf-4, and Hoxd-11 gene expression confirmed that a second ZPA had been generated at the anterior side of the limb bud. Besides other alterations, posterior homeotic transformations of axial structures were observed, involving the first spinal (Froriep's) ganglion and several cervical vertebrae.

A quantitative study on the spatial and temporal ossification patterns of vertebral centra and neural arches and their relationship to the fetal age

A double-staining technique on 37 human embryos and fetuses (crown-rump length, CRL, between 38 and 116 mm) has been performed to study the ossification patterns of the vertebral column. Different growth sequences for centra and neural arches were observed. The survey of ossified centers suggested it was possible to relate significantly their appearance with the CRL. On the basis of already known data defining the developmental age in relationship to the latter parameter, we suggest their numerical evaluation as a further parameter for the assessment of the fetal age. Therefore, we have worked out a table that may be used either to determine the normal fetal growth, or when other parameters cannot be relied upon (i.e. in morphological diseases) for this aim.

Mice with targeted disruptions in the paralogous genes hoxa-3 and hoxd-3 reveal synergistic interactions

The Hox genes encode transcription factors which mediate the formation of the mammalian body plan along the anteroposterior and appendicular axes. Paralogous Hox genes within the separate linkage groups are closely related with respect to DNA sequence and expression, suggesting that they could have at least partially redundant functions. We showed previously that mice homozygous for independent targeted disruptions in the paralogous genes hoxa-3 and hoxd-3 had no defects in common. But our current analysis of double mutants has revealed strong, dosage-dependent interactions between these genes. We report here that in hoxd-3- homozygotes the first cervical vertebra, the atlas, is homeotically transformed to the adjacent anterior structure. Unexpectedly, in double mutants, rather than observing a more extensive homeotic transformation, the entire atlas is deleted. These observations are interpreted in terms of a model in which these Hox genes differentially regulate the proliferation rates of the appropriate sets of precursor cells.

Understanding the os odontoideum

Five cases of os odontoideum and associated instability of C1-2 involving patients who underwent a posterior spinal fusion are reviewed, along with the literature. Four of the patients had Down's syndrome, and two of them were symptomatic at the time of presentation. In the two asymptomatic patients, the os odontoideum was an incidental finding on routine roentgenographic survey. The fifth patient presented with trauma and neck pain. The operation was successful in all of the patients with Down's syndrome but one, who had preoperative signs of myelopathy. Patients known to have an os odontoideum must be followed closely; if there is instability, posterior spinal fusion is recommended to avoid the risk of neurologic compromise. An understanding of the pathoanatomy of this condition is essential to recognize and properly treat these patients.