A homeotic transformation is generated in the rostral branchial region of the head by disruption of Hoxa-2, which acts as a selector gene

Gastrulation and homeobox genes in chick embryos

We review the early stages of chick embryogenesis, in particular the formation of the hypoblast, and the ingression of endoderm and mesoderm through the primitive streak. The formation of a trilaminar embryo during gastrulation is accompanied by the specification of body axes. The first axis is already present in the unfertilized egg and runs from the cytoplasmatic animal to the yolk rich vegetal pole. Already within the uterus a second axis conveys bilateral symmetry to the embryo. It extends from a dorsal/anterior to a ventral/posterior position. These axial poles segregate during gastrulation to form the classical coordinates, a dorsal-ventral and an anterior-posterior axis. The establishment of axes is accompanied by the expression of specific combinations of homeobox genes during gastrulation in the chick, as in other metazoa. We review the avian specific information and compare it with findings in other species. A combinatorial homeobox code for the specification of identities during development is discussed.

The EphA4 and EphB1 receptor tyrosine kinases and ephrin-B2 ligand regulate targeted migration of branchial neural crest cells

BACKGROUND

During vertebrate head development, neural crest cells migrate from hindbrain segments to specific branchial arches, where they differentiate into distinct patterns of skeletal structures. The rostrocaudal identity of branchial neural crest cells appears to be specified prior to migration, so it is important that they are targeted to the correct destination. In Xenopus embryos, branchial neural crest cells segregate into four streams that are adjacent during early stages of migration. It is not known what restricts the intermingling of these migrating cell populations and targets them to specific branchial arches. Here, we investigated the role of Eph receptors and ephrins-mediators of cell-contact-dependent interactions that have been implicated in neuronal pathfinding-in this targeted migration.

RESULTS

Xenopus EphA4 and EphB1 are expressed in migrating neural crest cells and mesoderm of the third arch, and third plus fourth arches, respectively. The ephrin-B2 ligand, which interacts with these receptors, is expressed in the adjacent second arch neural crest and mesoderm. Using truncated receptors, we show that the inhibition of EphA4/EphB1 function leads to abnormal migration of third arch neural crest cells into second and fourth arch territories. Furthermore, ectopic activation of these receptors by overexpression of ephrin-B2 leads to scattering of third arch neural crest cells into adjacent regions. Similar disruptions occur when the expression of ephrin-B2 or truncated receptors is targeted to the neural crest.

CONCLUSIONS

These data indicate that the complementary expression of EphA4/EphB1 receptors and ephrin-B2 is involved in restricting the intermingling of third and second arch neural crest and in targeting third arch neural crest to the correct destination. Together with previous work showing that Eph receptors and ligands mediate neuronal growth cone repulsion, our findings suggest that similar mechanisms are used for neural crest and axon pathfinding.

Musculoskeletal patterning in the pharyngeal segments of the zebrafish embryo

The head skeleton and muscles of the zebrafish develop in a stereotyped pattern in the embryo, including seven pharyngeal arches and a basicranium underlying the brain and sense organs. To investigate how individual cartilages and muscles are specified and organized within each head segment, we have examined their early differentiation using Alcian labeling of cartilage and expression of several molecular markers of muscle cells. Zebrafish larvae begin feeding by four days after fertilization, but cartilage and muscle precursors develop in the pharyngeal arches up to 2 days earlier. These chondroblasts and myoblasts lie close together within each segment and differentiate in synchrony, perhaps reflecting the interdependent nature of their patterning. Initially, cells within a segment condense and gradually become subdivided into individual dorsal and ventral structures of the differentiated arch. Cartilages or muscles in one segment show similar patterns of condensation and differentiation as their homologues in another, but vary in size and shape in the most anterior (mandibular and hyoid) and posterior (tooth-bearing) arches, possibly as a consequence of changes in the timing of their development. Our results reveal a segmental scaffold of early cartilage and muscle precursors and suggest that interactions between them coordinate their patterning in the embryo. These data provide a descriptive basis for genetic analyses of craniofacial patterning.

Developmental patterning and evolution of the mammalian viscerocranium: genetic insights into comparative morphology

The vertebrate cranium is generally classified into the neurocranium and the viscerocranium. The latter is derived from the neural crest and so is the prechordal portion of the neurocranium. A view we favor considers the prechordal neurocranium as the premandibular component of the viscerocranium, and the vertebrate skull to consist of the neural crest-derived viscerocranium and the mesodermal neurocranium. Of these developmental units, only the viscerocranium appears to have completely segmented metamerical organization. The Hox code which is known to function in specification of the viscerocranium does not extend rostrally into the mandibular and premandibular segments. By genetic manipulation of rostrally expressed non-Hox homeobox genes, the patterning mechanism of the head is now demonstrated to be more complicated than isomorphic registration of the Hox code to pharyngeal arches. The phenotype by haplo-insufficiency of Otx2 gene, in particular, implies the premandibular cranium shares a common specification mechanism with the mandibular arch. Our interpretation of the metamerical plan of the viscerocranium offers a new scheme of molecular codes associated with the vertebrate head evolution.

Genetic analysis of craniofacial development in the vertebrate embryo

Every cartilage and bone in the vertebrate skeleton has a precise shape and position. The head skeleton develops in the embryo from the neural crest, which emigrates from the neural ectoderm and forms the skull and pharyngeal arches. Recent genetic data from mice and zebrafish suggest that cells in the pharyngeal segments are specified by positional information in at least two dimensions, Hox genes along the anterior-posterior axis and other homeobox genes along the dorsal-ventral axis within a segment. Many zebrafish and human mutant phenotypes indicate that additional genes are required for the development of groups of adjacent pharyngeal arches and for patterning along the mediolateral axis of the skull. The complementary genetic approaches in humans, mice and fish reveal networks of genes that specify the complex morphology of the head skeleton along a relatively simple set of coordinates.

Role of the Dlx homeobox genes in proximodistal patterning of the branchial arches: mutations of Dlx-1, Dlx-2, and Dlx-1 and -2 alter morphogenesis of proximal skeletal and soft tissue structures derived from the first and second arches

The Dlx homeobox gene family is expressed in a complex pattern within the embryonic craniofacial ectoderm and ectomesenchyme. A previous study established that Dlx-2 is essential for development of proximal regions of the murine first and second branchial arches. Here we describe the craniofacial phenotype of mice with mutations in Dlx-1 and Dlx-1 and -2. The skeletal and soft tissue analyses of mice with Dlx-1 and Dlx-1 and -2 mutations provide additional evidence that the Dlx genes regulate proximodistal patterning of the branchial arches. This analysis also elucidates distinct and overlapping roles for Dlx-1 and Dlx-2 in craniofacial development. Furthermore, mice lacking both Dlx-1 and -2 have unique abnormalities, including the absence of maxillary molars. Dlx-1 and -2 are expressed in the proximal and distal first and second arches, yet only the proximal regions are abnormal. The nested expression patterns of Dlx-1, -2, -3, -5, and -6 provide evidence for a model that predicts the region-specific requirements for each gene. Finally, the Dlx-2 and Dlx-1 and -2 mutants have ectopic skull components that resemble bones and cartilages found in phylogenetically more primitive vertebrates.

Nasal and pharyngeal abnormalities caused by the mouse goosecoid gene mutation

The Goosecoid (gsc) gene is a homeobox-containing gene expressed first in the gastrula, and later during organogenesis in development. The gsc gene transcript is found in the first and second branchial arches, frontonasal mass in its late phase of expression. We have previously shown that targeted mutation of the mouse gsc gene leads to neonatal death and craniofacial defects. In this study, we performed histological studies on craniofacial phenotypes in order to elucidate the processes underlying the neonatal death of gsc mutant mice. We found that gsc mutant mice have aplastic nasal cavities and lack the Sinus Paranasalis. We also showed that secretory olfactory glands in the basal layers are aplastic. This is suggested to be essential defects for olfaction. gsc mutant mice also show several pharyngeal phenotypes, including defects in the pharyngeal muscles and the pharyngeal mucosa. It is therefore suggested that mutant mice develop lethal gastro-intestinal phenotypes caused by defects in breathing and sucking of milk as a consequence of these craniofacial disorders. These results should help elucidating the molecular genetic programs essential to the neonatal development of mammals.

Retinoic acid disturbs mouse middle ear development in a stage-dependent fashion

The mammalian middle ear contains a chain of three ossicles, the malleus, incus, and stapes, that transmit into the inner ear the vibrations produced in the tympanic membrane by aerial sound. I show here that retinoic acid interferes with the formation of the middle ear in a stage-specific fashion. The malformations produced are derived from a retinoic acid-induced inhibition of the formation and/or migration of the cranial neural crest that generates the middle ear skeletal elements and not from a respecification of neural crest identity to more posterior fates. I have used these effects of retinoic acid to analyze the temporal sequence of events involved in the morphogenesis of the middle ear. I show that the middle ear bones develop from several primordia that originate in a typical temporal sequence from Day 8 plus 4.5 hr to Day 8 plus 7.5 hr of pregnancy. Moreover, interactions between adjacent bones are not required for their proper formation. My results also suggest a Hoxa-2-mediated patterning of the otic capsule in the region where the oval window is located.

Perinatal lethality and multiple craniofacial malformations in MSX2 transgenic mice

MSX2 is a homeodomain transcription factor that has been implicated in craniofacial morphogenesis on the basis of its expression pattern during mouse development and the finding of a missense mutation (P148H) in humans affected with Boston-type craniosynostosis. We have generated transgenic mice carrying a 34 kb DNA fragment encompassing a human MSX2 gene encoding either wild-type or mutant (P148H) MSX2. Inheritance of either transgene resulted in perinatal lethality and multiple craniofacial malformations of varying severity, including mandibular hypoplasia, cleft secondary palate, exencephaly, and median facial cleft, which are among the severe craniofacial malformations observed in humans. Transgenic mice also manifested aplasia of the interparietal bone and decreased ossification of the hyoid. Transgene-induced malformations involved cranial neural-crest derivatives, were characterized by a deficiency of tissue, and were similar to malformations associated with embryonic exposure to ethanol or retinoic acid, teratogens that cause increased cell death. Together with previous observations implicating MSX2 expression in developmentally-programmed cell death, these results suggest that wild-type levels of MSX2 activity may establish a balance between survival and apoptosis of neural crest-derived cells required for proper craniofacial morphogenesis.

Cranial anomaly of homozygous rSey rat is associated with a defect in the migration pathway of midbrain crest cells

Craniofacial development of vertebrates depends largely on neural crest contribution and each subdomain of the crest-derived ectomesenchyme follows its specific genetic control. The rat small eye (rSey) involves a mutation in the Pax-6 gene and the external feature of rSey homozygous embryos exhibits craniofacial defects in ocular and frontonasal regions. In order to identify the mechanism of craniofacial development, we examined the cranial morphology and migration of cephalic crest cells in rSey embryos. The chondrocranial defects of homozygous rSey embryos primarily consisted of spheno-orbital and ethmoidal anomalies. The former defects appeared to be brought about by the lack of the eye. In the ethmoid region, the nasal septum and the derivative of the medial nasal prominence were present, while the rest of the nasal capsule, as well as the nasal and lachrymal bones, were totally absent except for a pair of cartilaginous rods in place of the nasal capsule. This suggests that the primary cranial defect is restricted to the lateral nasal prominence derivatives. Dil labeling revealed the abnormal migration of crest cells specifically from the anterior midbrain to the lateral nasal prominence in homozygous rSey embryos. Pax-6 was not expressed in the crest cells but was strongly expressed in the frontonasal ectoderm. To determine whether or not this migratory defect actually resides in environmental cues, normal midbrain crest cells from wild-type embryos were labeled with Dil and were orthotopically injected into host rSey embryos. Migration of the donor crest cells into the lateral nasal prominence was abnormal in homozygous host embryos, while they migrated normally in wild-type or heterozygous embryos. Therefore, the cranial defects in rSey homozygous embryos are due to inappropriate substrate for crest cell migration towards the lateral nasal prominence, which consistently explains the cranial morphology of homozygous rSey embryos.

In vivo functional analysis of the Hoxa-1 3′ retinoic acid response element (3′RARE)

Retinoids are essential for normal development and both deficiency and excess of retinoic acid (RA) are teratogenic. Retinoic acid response elements (RAREs) have been identified in Hox gene promoters suggesting that endogenous retinoids may be involved in the direct control of Hox gene patterning functions. In order to test this hypothesis, we have mutated the Hoxa-1 3′RARE using the Cre-loxP targeting strategy, and studied its functional role during mouse development. We find that this enhancer plays an important role in the early establishment of the Hoxa-1 anterior expression boundary in the neural plate. This early disturbance in Hoxa-1 activation results in rhombomere and cranial nerve abnormalities reminiscent of those obtained in the Hoxa-1 total knockout, although their severity and penetrance are lower, thus providing strong evidence for direct control of Hox gene function by retinoids during normal development. Interestingly, we also find that the Hoxa-1 expression response to RA treatment is not entirely controlled by the RARE, suggesting the existence of other retinoid-induced factors mediating the Hoxa-1 response to RA and/or the presence of additional RAREs. Interestingly, although the RARE is not required for the spatiotemporal control of colinear expression of the Hoxa genes, it is absolutely required for correct Hoxa-2 expression in rhombomere 5.

Hoxb-8 gain-of-function transgenic mice exhibit alterations in the peripheral nervous system

To understand the developmental role of Hoxb-8, this relatively 5' Hoxb gene was ectopically expressed in embryonic regions where only more 3' Hox genes are normally expressed. Hoxb-8 coding sequences driven by a retinoic acid receptor beta2 promoter fragment were introduced in the mouse germ line by pronuclear injection. The promoter was chosen with the aim to extend rostrally the expression domain of the gene in neurectoderm and mesoderm at the time of development when Hox gene expression domains are being established. Embryos developing from DNA-injected zygotes, and from transgenic mouse lines were analyzed. Pattern alterations were observed in transgenic embryos, some of which involved the peripheral nervous system. Spinal ganglia in the mouse are first detectable around embryonic day 9.5. By day 11.5, the first of these ganglia (C1, Froriep's ganglion) has degenerated in the mouse and other amniotes. In contrast, this first ganglion did persist in the Hoxb-8 gain-of-function transgenic mice. We have started to take advantage of the phenotype of transgenic versus wild-type embryos to understand the mechanisms underlying the ontogeny and degeneration of Froriep's ganglion in wild-type mice, and the role of Hoxb-8 in C1 maintenance in transgenic embryos. The present work describes a morphological, histological and immunocytological analysis of both the degenerating and the permanent C1, and a preliminary characterization of the axonal extensions from the transgenic C1. We discuss the methodology of generating gain-of-function transgenic mice to study the genetics of pattern formation along the antero-posterior axis, and the usefulness of analyzing these particular Hoxb-8 transgenic embryos to understand some aspects of the ontogenesis and development of the upper cervical dorsal root ganglia.

Embryonic neural chimeras in the study of vertebrate brain and head development

Construction of neural chimeras between quail and chick embryos has been employed since 1969 when the unique nucleolar structure of the quail nucleus and its use to devise a cell marking technique by associating quail and chick cells in ovo were described in the "Bulletin Biologique de la France et de la Belgique." This method was first applied to the ontogeny of the neural crest, a structure whose development involves extensive cell migration, and, since 1984, to that of the central nervous system (CNS). This chapter highlights some of the most significant findings provided by this approach concerning the CNS, such as (i) demonstration of the common origin of the floor plate and notochord from a group of cells localized in the "organizer", i.e., Hensen's node, and the way in which these two structures become positioned respectively within and under the neural tube during gastrulation and neurulation in Amniotes; (ii) the neural crest origin of the skull vault and the facial and hypobranchial skeleton. This means that the mesodermal contribution to the skull is limited to the occipital and otic regions and extends only to the rostral limit of the notochord. A correlation can be drawn between the development of the telencephalon and the mesectodermally derived skull in the vertebrate phylum; (iii) demonstration that the midbrain-hindbrain junction, at the stage of the encephalic vesicles, acts as an organizing center for tectal and cerebellar structures. This function was correlated with the activity of several developmental genes, thus providing insight into their function during neurogenesis; (iv) the pattern of morphogenetic movements and cell migration taking place in defined brain-to-be areas, as well as the origin of various cell types of nervous tissues; and (v) a new avenue for studying brain localization of either behavioral traits or genetically encoded brain disorders.

Abnormal patterning of the aortic arch arteries does not evoke cardiac malformations

Ablation of the cardiac neural crest results in abnormal development of the aortic arch arteries leading to altered patterning of the great arteries. The cardiac outflow tract is also affected after neural crest ablation because normally a subset of neural crest cells migrates from the pharyngeal region to form the outflow septum. Using neural crest ablation, it has not been possible to separate the occurrence of aortic arch maldevelopment from cardiac outflow tract dysmorphogenesis. In order to determine whether normal aortic arch artery development is a prerequisite for normal outflow tract development, we have used a combination of antisense treatment with backtransplantation of cardiac neural folds to produce abnormal patterning of the aortic arch arteries. Paralogous groups of Hox messages with their anterior expression domains in pharyngeal arches 3, 4 and 6 were targeted. Antisense targeted to paralogous group 3 Hox message caused aortic arch 3 located within the pharyngeal arch to regress in a manner similar to aortic arch 2, while antisense targeted to paralogous group 5 Hox message caused the appearance of an additional pharyngeal arch containing a novel and completely independent aortic arch artery. Antisense treatment targeting paralogous group 4 Hox message led to no detectable cardiovascular phenotype in the first 6 days of development. While regression of arch artery 3 was associated with abnormal branching patterns of the aorta and pulmonary trunk, this did not involve abnormal separation of the aorta and pulmonary trunks, the semilunar valves or the subvalvular region of the outflow tract. Because none of these changes in pharyngeal or aortic arch artery development was accompanied by abnormal development of the cardiac outflow tract, it appears that normal patterning of the aortic arch arteries is not a prerequisite for normal heart development. Using reverse transcription polymerase chain reaction (RT-PCR) we were unable to detect changes in any of the Hox messages except group 4, thus, using this particular experimental strategy, we are unable to demonstrate or refute that expression of hox genes by cardiac neural crest cells controls aortic arch patterning. Development of the cardiac outflow tract was normal in each instance. This suggests that abnormal aortic arch patterning does not necessarily lead to cardiac malformations.

Heterochrony and neotenic salamanders: possible clues for understanding the animal development and evolution

A synthesis of developmental genetics with evolutionary genetics is now making possible to understand significant evolutionary changes in multicellular organisms. The key concept for unifying the two must be heterochrony. Heterochrony causes evolutionary modifications due to changes in timing and/or rate of development. The heterochrony is conventionally categorized into three patterns as neoteny (retardation in somatic development), progenesis (acceleration in gonadal development), and direct development (acceleration in somatic development, resulting in lack of larval or tadpole stages). A lot of species showing neoteny are known in urodeles, but not in anurans. Neotenic urodeles are also divided into three categories; permanent or obligate, "inducible" obligate and facultative neotenies. Hynobius retardatus, a specific population of which had been reported to show neoteny but is believed to be extinct at present, has become to be used for experimental analysis of heterochronic expression of several adult characters during its ontogeny. Gonadal maturation and a transition of globin subunits from larval to adult types have been shown to occur independently on the morphological metamorphosis in H. retardatus. Mechanisms underlying the heterochrony, including morphogenetic clock, heterochronic genes in Drosophila and C. elegans, temporal colinearity in Hox gene complex in mice, and atavistic transformation induced by altered expression of Hox genes are discussed in terms of current molecular biology.

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.

Paralogous Hox genes: function and regulation

The Hox homeobox gene family plays a pivotal role in regulating patterning and axial morphogenesis in vertebrates. Molecular characterization of the four Hox clusters has shown that they are evolutionarily related with respect to sequence, organization, and expression, suggesting they arose by duplication and divergence. Transgenic analysis has clearly demonstrated the functional roles of individual genes in a broad range of embryonic tissues, and in compound mutants has addressed the issues of cooperativity and redundancy. There is an emerging picture of the cis-regulatory elements underlying Hox expression, and for the 3' members of the clusters there is a considerable degree of conservation between paralogous genes with respect to their functional roles and regulatory control.

The regeneration of the cephalic neural crest, a problem revisited: the regenerating cells originate from the contralateral or from the anterior and posterior neural fold

The mesencephalic and rhombencephalic levels of origin of the hypobranchial skeleton (lower jaw and hyoid bone) within the neural fold have been determined at the 5-somite stage with a resolution corresponding to each single rhombomere, by means of the quail-chick chimera technique. Expression of certain Hox genes (Hoxa-2, Hoxa-3 and Hoxb-4) was recorded in the branchial arches of chick and quail embryos at embryonic days 3 (E3) and E4. This was a prerequisite for studying the regeneration capacities of the neural crest, after the dorsal neural tube was resected at the mesencephalic and rhombencephalic level. We found first that excisions at the 5-somite stage extending from the midmesencephalon down to r8 are followed by the regeneration of neural crest cells able to compensate for the deficiencies so produced. This confirmed the results of previous authors who made similar excisions at comparable (or older) developmental stages. When a bilateral excision was followed by the unilateral homotopic graft of the dorsal neural tube from a quail embryo, thus mimicking the situation created by a unilateral excision, we found that the migration of the grafted unilateral neural crest (quail-labelled) is bilateral and compensates massively for the missing crest derivatives. The capacity of the intermediate and ventral neural tube to yield neural crest cells was tested by removing the chick rhombencephalic neural tube and replacing it either uni- or bilaterally with a ventral tube coming from a stage-matched quail. No neural crest cells exited from the ventral neural tube but no deficiency in neural crest derivatives was recorded. Crest cells were found to regenerate from the ends of the operated region. This was demonstrated by grafting fragments of quail neural fold at the extremities of the excised territory. Quail neural crest cells were seen migrating longitudinally from both the rostral and caudal ends of the operated region and filling the branchial arches located inbetween. Comparison of the behaviour of neural crest cells in this experimental situation with that showed by their normal fate map revealed that crest cells increase their proliferation rate and change their migratory behaviour without modifying their Hox code.

Regulation of striatal D1A dopamine receptor gene transcription by Brn-4

Brn-4 is a member of the POU transcription factor family and is expressed in the central nervous system. In this study, we addressed whether Brn-4 regulates expression of the D1A dopamine receptor gene. We found a functional Brn-4 responsive element in the intron of this gene by means of cotransfection chloramphenical acetyltransferase assays. This region contains two consensus sequences for binding of POU factors. Gel mobility-shift assays using glutathione S-transferase-Brn-4 fusion protein indicated that Brn-4 binds to these sequences. Both these sites are essential for transactivation by Brn-4 because deletion of either significantly reduced this enhancer activity. In situ hybridization revealed colocalization of Brn-4 and D1A mRNAs at the level of a single neuron in the rat striatum where this dopamine receptor is most abundantly expressed. Gel mobility-supershift assay using rat striatal nuclear extract and Brn-4 antibody confirmed the presence of Brn-4 in this brain region and its ability to bind to its consensus sequences in the D1A gene. These data suggest a functional role for Brn-4 in the expression of the D1A dopamine receptor gene both in vitro and in vivo.

Mice with targeted disruption of Hoxb-1 fail to form the motor nucleus of the VIIth nerve

Mice were generated with targeted disruptions in the hoxb-1 gene. Two separate mutations were created: the first disrupts only the homeodomain and the second inactivates the first exon as well as the homeodomain. The phenotypes associated with these two mutant alleles are indistinguishable in surviving adult mice. The predominant defect in these mutant mice is a failure to form the somatic motor component of the VIIth (facial) nerve, possibly through a failure to specify these neurons. The phenotype of hoxb-1 mutant homozygotes closely resembles features of the clinical profile associated with humans suffering from Bell's Palsy or Moebius Syndrome. These animals should therefore provide a useful animal model for these human diseases.

Rhombencephalic neural crest segmentation is preserved throughout craniofacial ontogeny

To investigate the influence of hindbrain segmentation on craniofacial patterning we have studied the long term fate of neural crest (NC) subpopulations of individual rhombomeres (r), using quail-chick chimeras. Mapping of all skeletal and muscle connective tissues developing from these small regions revealed several novel features of the cranial neural crest. First, the mandibular arch skeleton has a composite origin in which the proximal elements are r1+r2 derived, whereas more distal ones are exclusively midbrain derived. The most proximal region of the lower jaw is derived from second arch (r4) NC. Second, both the lower jaw and tongue skeleton display an organisation which precisely reflects the rostrocaudal order of segmental crest deployment from the embryonic hindbrain. Third, cryptic intraskeletal boundaries, which do not correspond to anatomical landmarks, form sharply defined interfaces between r1+r2, r4 and r6+r7 crest. Cells that survive the early apoptotic elimination of premigratory NC in r3 and r5 are restricted to tiny contributions within the 2nd arch (r4) skeleton. Fourth, a highly constrained pattern of cranial skeletomuscular connectivity was found that precisely respects the positional origin of its constitutive crest: each rhombomeric population remains coherent throughout ontogeny, forming both the connective tissues of specific muscles and their respective attachment sites onto the neuro- and viscerocranium. Finally, focal clusters of crest cells, confined to the attachment sites of branchial muscles, intrude into the otherwise mesodermal cranial base. In the viscerocranium, an equally strict, rhombomere-specific matching of muscle connective tissues and their attachment sites is found for all branchial and tongue (hypoglossal) muscles. This coherence of segmental crest populations explains how cranial skeletomuscular pattern can be implemented and conserved despite evolutionary changes in the shapes of skeletal elements.

The conserved role of Krox-20 in directing Hox gene expression during vertebrate hindbrain segmentation

Transient segmentation in the hindbrain is a fundamental morphogenetic phenomenon in the vertebrate embryo, and the restricted expression of subsets of Hox genes in the developing rhombomeric units and their derivatives is linked with regional specification. Here we show that patterning of the vertebrate hindbrain involves the direct upregulation of the chicken and pufferfish group 2 paralogous genes, Hoxb-2 and Hoxa-2, in rhombomeres 3 and 5 (r3 and r5) by the zinc finger gene Krox-20. We identified evolutionarily conserved r3/r5 enhancers that contain high affinity Krox-20. binding sites capable of mediating transactivation by Krox-20. In addition to conservation of binding sites critical for Krox-20 activity in the chicken Hoxa-2 and pufferfish Hoxb-2 genes, the r3/r5 enhancers are also characterized by the presence of a number of identical motifs likely to be involved in cooperative interactions with Krox-20 during the process of hindbrain patterning in vertebrates.

Genetics of nonsyndromic oral clefts revisited

Nonsyndromic oral clefts are among the most common birth defects, affecting approximately 1 in 1000 Caucasian newborns. In recent decades, many investigators have used genetic and epidemiologic methods to identify etiologic factors, but results have often been inconclusive or contradictory. Etiologic heterogeneity is undoubtedly a major component in these birth defects, and there may not be a single answer to this problem. Here, we describe the main features of published studies pointing out their strengths and limitations. Additionally, we give insight into current methods for detecting the presence of interaction between genetic markers and environmental exposures in the etiology of oral clefts.

Retinoic acid alters the expression of pattern-related genes in the developing rat lung

Exogenous retinoids alter pattern formation and differentiation in many developing systems, such as limb, vertebrae, and central nervous system. Many of these effects are mediated by changes in expression of patterning genes such as Hox genes and Sonic hedgehog. We have previously shown that exogenous retinoic acid, administered to the embryonic rat lung in culture alters the structural pattern of the developing lung, suppressing formation of distal lung and favoring growth of proximal tubules. To determine whether these retinoic acid-induced changes in lung development were linked to alterations in pattern-related genes, we characterized the expression of Hoxa-2, Hoxb-6, and Sonic hedgehog mRNAs in vivo and in vitro, with or without 10(-5)M retinoic acid, by in situ hybridization and quantitative polymerase chain reaction. Each of these genes demonstrated unique timing and distribution of expression that was similar in vivo and in control cultured embryonic lungs. Hoxb-6 and Sonic hedgehog mRNAs both decreased during lung development in vivo or in vitro. From the patterns of mRNA expression we propose that Hoxb-6 is involved in distal airway branching while Hoxa-2 is involved in differentiation of proximal mesenchymal derivatives and vasculogenesis in the lung. RA upregulated all three genes, changing their developmental pattern of distribution and preventing the developmental decrease in Sonic hedgehog expression. We propose that RA acts to maintain high levels of expression of these and likely other pattern-related genes in a fashion that is characteristic of the immature lung, promoting continued formation of proximal lung structures and preventing formation of typical distal lung structures of the mature lung.

The chinless mutation and neural crest cell interactions in zebrafish jaw development

During vertebrate development, neural crest cells are thought to pattern many aspects of head organization, including the segmented skeleton and musculature of the jaw and gills. Here we describe mutations at the gene chinless, chn, that disrupt the skeletal fates of neural crest cells in the head of the zebrafish and their interactions with muscle precursors. chn mutants lack neural-crest-derived cartilage and mesoderm-derived muscles in all seven pharyngeal arches. Fate mapping and gene expression studies demonstrate the presence of both undifferentiated cartilage and muscle precursors in mutants. However, chn blocks differentiation directly in neural crest, and not in mesoderm, as revealed by mosaic analyses. Neural crest cells taken from wild-type donor embryos can form cartilage when transplanted into chn mutant hosts and rescue some of the patterning defects of mutant pharyngeal arches. In these cases, cartilage only forms if neural crest is transplanted at least one hour before its migration, suggesting that interactions occur transiently in early jaw precursors. In contrast, transplanted cells in paraxial mesoderm behave according to the host genotype; mutant cells form jaw muscles in a wild-type environment. These results suggest that chn is required for the development of pharyngeal cartilages from cranial neural crest cells and subsequent crest signals that pattern mesodermally derived myocytes.

Transcription factors and the cardiac gene programme

During the past decade, major advances have been made in uncovering the mechanisms that switch genes on and off. Gene methylation and histones play an important role in gene (in)activation. Following gene activation, the initiation of transcription by RNA polymerase requires the assembly of multiple protein complexes on the promoter region of a gene. How a cell type-specific gene expression pattern can be induced is a key question in cardiovascular biology today. Members of the helix-loop-helix-family of the transcription factors play a dominant role in skeletal muscle formation. In cardiac muscle the situation is less obvious. Recent studies identified muscle transcription factors like MEF-2, TEF-1 and MNF, which are common to both the skeletal and cardiac muscle lineages. A few transcription factors, among which Nkx 2.5 and GATA-4, are expressed predominantly in the heart. The absence of master regulators in the heart points to the importance of interaction between ubiquitous factors and tissue restricted factors to initiate the cardiac gene programme and to lock these cells in their differentiated state. The recent development of murine transgenic and gene-targeting technology provides tools to study the role of mammalian transcription factors in vivo. Interesting cardiac phenotypes are found in gene targeted mice, indicating a crucial role for retinoic acid and homeobox genes in murine cardiogenesis.

Ectopic expression of Hoxa-1 in the zebrafish alters the fate of the mandibular arch neural crest and phenocopies a retinoic acid-induced phenotype

Considerable evidence has demonstrated that retinoic acid influences the formation of the primary body axis in vertebrates and that this may occur through the regulation of Hox gene expression. In this study, we show that the phenotype induced by exogenous retinoic acid in the zebrafish can also be generated by the overexpression of Hoxa-1 following injection of synthetic RNA into the fertilised egg. The isolation, sequence and expression pattern of the zebrafish Hoxa-1 gene is described. We show that exogenously applied retinoic acid causes the ectopic accumulation of Hoxa-1 message during gastrulation in the hypoblast in the head region. Overexpression of Hoxa-1 following injection of RNA causes abnormal growth of the anterior hindbrain, duplication of Mauthner neurons in rhombomere (r) 2 and fate changes of r2 mesenchymal and neurogenic neural crest. These results are discussed in terms of the role of Hoxa-1 in controlling anterior hindbrain patterning and the relationship between expression of Hoxa-1 and retinoic acid.

Hox homeodomain proteins exhibit selective complex stabilities with Pbx and DNA

Eight of the nine homeobox genes of the Hoxb locus encode proteins which contain a conserved hexapeptide motif upstream from the homeodomain. All eight proteins (Hoxb-1-Hoxb-8) bind to a target oligonucleotide in the presence of Pbx1a under conditions where minimal or no binding is detected for the Hox or Pbx1a proteins alone. The stabilities of the Hox-Pbx1a-DNA complexes vary >100-fold, with the proteins from the middle of the locus (Hoxb-5 and Hoxb-6) forming very stable complexes, while Hoxb-4, Hoxb-7 and Hoxb-8 form complexes of intermediate stability and proteins at the 3'-side of the locus (Hoxb-1-Hoxb-3) form complexes which are very unstable. Although Hox-b proteins containing longer linker sequences between the hexapeptide and homeodomains formed unstable complexes, shortening the linker did not confer complex stability. Homeodomain swapping experiments revealed that this motif does not independently determine complex stability. Naturally occurring variations within the hexapeptides of specific Hox proteins also do not explain complex stability differences. However, two core amino acids (tryptophan and methionine) which are absolutely conserved within the hexapeptide domains appear to be required for complex formation. Removal of N- and C-terminal flanking regions did not influence complex stability and the members of paralog group 4 (Hoxa-4, b-4, c-4 and d-4), which share highly conserved hexapeptides, linkers and homeodomains but different flanking regions, form complexes of similar stability. These data suggest that the structural features of Hox proteins which determine Hox-Pbx1a-DNA complex stability reside within the precise structural relationships between the homeodomain, hexapeptide and linker regions.

Common ground plans in early brain development in mice and flies

Comparing expression patterns of orthologous genes between insects and vertebrates, we have recently proposed that the ventral nerve cord in insects may correspond to the dorsal nerve cord in vertebrates. Here we show that the early development of the insect and vertebrate brain anlagen is indeed very similar. Insect and vertebrate brains express similar sets of genes in comparable areas with similar functions in the adult. In addition, early axogenesis establishes surprisingly similar patterns of axonal connectivity in both groups. We therefore propose that insect and vertebrate brains are built according to a common ground plan, and that specific areas of the insect and vertebrate brains be considered as homologous, meaning that these areas already existed, with their specific functions, in their common ancestor.

Checklist: vertebrate homeobox genes

Up to now around 170 different homeobox genes have been cloned from vertebrate genomes. A compilation of the various isolates from mouse, chick, frog, fish and man is presented in the form of a concise checklist, including the designations from the original publications. Putative homologs from different species are aligned, and key characteristics of embryonic or adult expression domains, as well as mutant phenotypes are briefly indicated.

Rhombomere of origin determines autonomous versus environmentally regulated expression of Hoxa-3 in the avian embryo

We have investigated the pattern and regulation of Hoxa3 expression in the hindbrain and associated neural crest cells in the chick embryo, using whole mount in situ hybridization in conjunction with DiI labeling of neural crest cells and microsurgical manipulations. Hoxa3 is expressed in the neural plate and later in the neural tube with a rostral border of expression corresponding to the boundary between rhombomeres (r) 4 and 5. Initial expression is diffuse and becomes sharp after boundary formation. Hoxa3 exhibits uniform expression within r5 after formation of rhombomeric borders. Cell marking experiments reveal that neural crest cells migrating caudally, but not rostrally, from r5 and caudally from r6 express Hoxa3 in normal embryo. Results from transposition experiments demonstrate that expression of Hoxa3 in r5 neural crest cells is not strictly cell-autonomous. When r5 is transposed with r4 by rostrocaudal rotation of the rhomobomeres, Hoxa3 is expressed in cells migrating lateral to transposed r5 and for a short time, in condensing ganglia, but not by neural crest within the second branchial arch. Since DiI-labeled cells from transposed r5 are present in the second arch, Hoxa3-expressing neural crest cells from r5 appear to down-regulate their Hoxa3 expression in their new environment. In contrast, when r6 is transposed to the position of r4 after boundary formation, Hoxa3 is maintained in both migrating neural crest cells and those positioned within the second branchial arch and associated ganglia. These results suggest that Hoxa3 expression is cell-autonomous in r6 and its associated neural crest. Our results suggest that neural crest cells expressing the same Hox gene are not eqivalent; they respond differently to environmental signals and exhibit distinct degrees of cell autonomy depending upon their rhombomere of origin.

Target gene identification: target specific transcriptional activation by three murine homeodomain/VP16 hybrid proteins in Saccharomyces cerevisiae

The mammalian homeodomain proteins encoded by Hox genes play an important role in embryonic development by providing positional queues which define developmental identities along the anteroposterior axis of developing organisms. These proteins bind DNA specifically through their homeodomain to sequences containing ATTA cores, and thereby are thought to exert their effect regulating downstream genes. Little is known about the specificity of binding of homeodomain proteins to their sequences and the identity of their target genes. We have developed a transcriptional activation assay in yeast which employs a homeobox/VP16 fusion gene as a transcriptional activator and a target construct in which test fragments of DNA are inserted upstream to a reporter gene. Using this assay, we compared transcriptional activation by three chimeric proteins containing the homeodomains of the mouse homeobox genes, Hoxa-5, Hoxb-6, and Hoxc-8. When tested on previously defined target sequences, strong differential specificities of activation were observed. In an effort to identify enhancers that normally respond to homeodomain transcriptional activators, random fragments of mouse genomic DNA were cloned upstream of the reporter gene. Genomic DNA fragments with distinct activation profiles were obtained and were found to share matches beyond the ATTA core with previously described enhancers. These results demonstrate that the transcriptional activation system in yeast can be used as a convenient system to detect DNA motifs which bind homeodomain proteins, and subsequently, to identify authentic target genes responsive to Hox gene proteins.

Segmental expression of Hoxa-2 in the hindbrain is directly regulated by Krox-20

The hindbrain is a segmented structure divided into repeating metameric units termed rhombomeres (r). The Hox family, vertebrate homologs of the Drosophila HOM-C homeotic selector genes, are expressed in rhombomere-restricted patterns and are believed to participate in regulating segmental identities. Krox-20, a zinc finger gene, has a highly conserved pattern of expression in r3 and r5 and is functionally required for their maintenance in mouse embryos. Krox-20 has been shown to directly regulate the Hoxb-2 gene and we wanted to determine if it was involved in regulating multiple Hox genes as a part of its functional role. Hoxa-2 is the only known paralog of Hoxb-2, and we examined the patterns of expression of the mouse Hoxa-2 gene with particular focus on r3 and r5 in wild type and Krox-20−/− mutant embryos. There was a clear loss of expression in r3, which indicated that Hoxa-2 was downstream of Krox-20. Using transgenic analysis with E. coli lacZ reporter genes we have identified and mapped an r3/r5 enhancer in the 5′ flanking region of the Hoxa-2 gene. Deletion analysis narrowed this region to an 809 bp Bg/II fragment, and in vitro binding and competition assays with bacterially expressed Krox-20 protein identified two sites within the enhancer. Mutation of these Krox-20 sites in the regulatory region specifically abolished r3/r5 activity, but did not affect neural crest and mesodermal components. This indicated that the two Krox-20 sites are required in vivo for enhancer function. Furthermore, ectopic expression of Krox-20 in r4 was able to transactivate the Hoxa 2/lacZ reporter in this rhombomere. Together our findings suggest that Krox-20 directly participates in the transcriptional regulation of Hoxa-2 during hindbrain segmentation, and is responsible for the upregulation of the r3 and r5 domains of expression of both vertebrate group 2 Hox paralogs. Therefore, the segmental phenotypes in the Krox-20 mutants are likely to reflect the role of Krox-20 in directly regulating multiple Hox genes.

Teleost HoxD and HoxA genes: comparison with tetrapods and functional evolution of the HOXD complex

In tetrapods, Hox genes are essential for the proper organization and development of axial structures. Experiments involving Hox gene inactivations have revealed their particularly important functions in the establishment of morphological transitions within metameric series such as the vertebral column. Teleost fish show a much simpler range of axial (trunk or appendicular) morphologies, which prompted us to investigate the nature of the Hox system in these lower vertebrates. Here, we show that fish have a family of Hox genes, very similar in both number and general organization, to that of tetrapods. Expression studies, carried out with HoxD and HoxA genes, showed that all vertebrates use the same general scheme, involving the colinear activation of gene expression in both space and time. Comparisons between tetrapods and fish allowed us to propose a model which accounts for the primary function of this gene family. In this model, a few ancestral Hox genes were involved in the determination of polarity in the digestive tract and were further recruited in more elaborate axial structures.

Development of the mammalian ear: coordinate regulation of formation of the tympanic ring and the external acoustic meatus

The tympanic membrane in mammals is a trilaminar structure formed by the apposition of two epithelial cell layers, along with an intervening layer of cells derived from pharyngeal arch mesenchyme. One epithelial layer is contributed by the external acoustic meatus, a derivative of the first pharyngeal cleft. The other epithelial layer is contributed by the tubotympanic recess, a derivative of the first pharyngeal pouch. We demonstrate here an absolute correlation between formation of the external acoustic meatus and formation of the tympanic ring, a first arch-derived membrane bone that anchors the tympanic membrane. Experimental loss of the tympanic ring by retinoic acid treatment, or duplication of the ring in Hoxa-2 null mutant embryos, resulted in corresponding alterations in formation of the external acoustic meatus. We suggest that the tympanic ring primordium induces formation and morphogenesis of the external acoustic meatus, and that expression of the Hoxa-2 and goosecoid genes may be involved in regulating the formation and morphogenesis of these structures.

Transformations in null mutants of Hox genes: do they represent intercalary regenerates?

In the minds of many, Hox gene null mutant phenotypes have confirmed the direct role that these genes play in specifying the pattern of vertebrate embryos. The genes are envisaged as defining discrete spatial domains and, subsequently, conferring specific segmental identities on cells undergoing differentiation along the antero-posterior axis. However, several aspects of the observed mutant phenotypes are inconsistent with this view. These include: the appearance of other, unexpected transformations along the dorsal axis; the occurrence of mirror-image duplications; and the development of anomalies outside the established domains of normal Hox gene expression. In this paper, Hox gene disruptions are shown to elicit regeneration-like responses in tissues confronted with discontinuities in axial identity. The polarities and orientations of transformed segments which emerge as a consequence of this response obey the rules of distal transformation and intercalary regeneration. In addition, the incidence of periodic anomalies suggests that the initial steps of Hox-mediated patterning occurs in Hensen's node. As gastrulation proceeds, mesoderm cell cycle kinetics impose constraints upon subsequent cellular differentiation. This results in the delayed manifestation of transformations along the antero-posterior axis. Finally, a paradigm is sketched in which temporal, rather than spatial axial determinants direct differentiation. Specific, testable predictions are made about the role of Hox genes in the establishment of segmental identity.

Transforming growth factor-beta 3 is required for secondary palate fusion

Mice lacking TGF-beta 3 exhibit an incompletely penetrant failure of the palatal shelves to fuse leading to cleft palate. The defect appears to result from impaired adhesion of the apposing medial edge epithelia of the palatal shelves and subsequent elimination of the mid-line epithelial seam. No craniofacial abnormalities were observed. This result demonstrates that TGF-beta 3 affects palatal shelf fusion by an intrinsic, primary mechanism rather than by effects secondary to craniofacial defects.

Abnormal lung development and cleft palate in mice lacking TGF-beta 3 indicates defects of epithelial-mesenchymal interaction

A broad spectrum of biological activities has been proposed for transforming growth factor-beta 3 (TGF-beta 3). To study TGF-beta 3 function in development, TGF-beta 3 null mutant mice were generated by gene-targeting. Within 20 hours of birth, homozygous TGF-beta 3-/- mice die with unique and consistent phenotypic features including delayed pulmonary development and defective palatogenesis. Unlike other null mutants with cleft palate, TGF-beta 3-/- mice lack other concomitant craniofacial abnormalities. This study demonstrates an essential function for TGF-beta 3 in the normal morphogenesis of palate and lung, and directly implicates this cytokine in mechanisms of epithelial-mesenchymal interaction.

Mouse Lbx1 and human LBX1 define a novel mammalian homeobox gene family related to the Drosophila lady bird genes

We have cloned two novel homeobox genes which are the mouse (Lbx1) and human (LBX1) homologs of the Drosophila lady bird genes. They are highly related not only within the coding region but also in 5' and 3' untranslated regions. Several amino acid residues inside and around the homeodomain, have been conserved between the mammalian Lbx genes and their Drosophila counterparts. The mouse Lbx1 gene is located on chromosome 19 (region D) and the human LBX1 gene maps to the related q24 region of chromosome 10, known as a breakpoint region in translocations t(7;10) and t(10;14) involved in T-cell leukemias. Thus, LBX1 and the protooncogene HOX11 map to a common chromosomal region, as do their Drosophila counterparts, the lady bird and 93Bal genes. The mouse Lbx1 gene is specifically expressed during embryogenesis. From 10.5 days of gestation, Lbx1 expression is detected in the central nervous system and some developing muscles. In the CNS, Lbx1 transcripts are expressed in the dorsal part of the mantle layer of the spinal cord and hindbrain, up to a sharp boundary within the developing metencephalon. Thus, Lbx1 may be inolved in spinal cord and hindbrain differentiation and/or patterning, and its restricted expression pattern could depend upon evolutionarily conserved inductive signals involving some mammalian Wnt and Pax genes, as is the case for Drosophila lady bird genes and wingless or gooseberry.

Mouse Otx2 functions in the formation and patterning of rostral head

The anterior part of the vertebrate head expresses a group of homeo box genes in segmentally restricted patterns during embryogenesis. Among these, Otx2 expression covers the entire fore- and midbrains and takes place earliest. To examine its role in development of the rostral head, a mutation was introduced into this locus. The homozygous mutants did not develop structures anterior to rhombomere 3, indicating an essential role of Otx2 in the formation of the rostral head. In contrast, heterozygous mutants displayed craniofacial malformations designated as otocephaly; affected structures appeared to correspond to the most posterior and most anterior domains of Otx expression where Otx1 is not expressed. The homo- and heterozygous mutant phenotypes suggest Otx2 functions as a gap-like gene in the rostral head where Hox code is not present. The evolutionary significance of Otx2 mutant phenotypes was discussed for the innovation of the neurocranium and the jaw.

Null mutation of Dlx-2 results in abnormal morphogenesis of proximal first and second branchial arch derivatives and abnormal differentiation in the forebrain

Genetic analysis of the development and evolution of the vertebrate head is at a primitive stage. Many homeo box genes, including the Distal-less family, are potential regulators of head development. To determine the function of Dlx-2, we generated a null mutation in mice using gene targeting. In homozygous mutants, differentiation within the forebrain is abnormal and the fate of a subset of cranial neural crest cells is respecified. The latter causes abnormal morphogenesis of the skeletal elements derived from the proximal parts of the first and second branchial arches. We hypothesize that the affected skull bones from the first arch have undergone a transformation into structures similar to those found in reptiles. These results show that Dlx-2 controls development of the branchial arches and the forebrain and suggests its role in craniofacial evolution.

Prenatal craniofacial development: new insights on normal and abnormal mechanisms

Technical advances are radically altering our concepts of normal prenatal craniofacial development. These include concepts of germ layer formation, the establishment of the initial head plan in the neural plate, and the manner in which head segmentation is controlled by regulatory (homeobox) gene activity in neuromeres and their derived neural crest cells. There is also a much better appreciation of ways in which new cell associations are established. For example, the associations are achieved by neural crest cells primarily through cell migration and subsequent cell interactions that regulate induction, growth, programmed cell death, etc. These interactions are mediated primarily by two groups of regulatory molecules: "growth factors" (e.g., FGF and TGF alpha) and the so-called steroid/thyroid/retinoic acid superfamily. Considerable advances have been made with respect to our understanding of the mechanisms involved in primary and secondary palate formation, such as growth, morphogenetic movements, and the fusion/merging phenomenon. Much progress has been made on the mechanisms involved in the final differentiation of skeletal tissues. Molecular genetics and animal models for human malformations are providing many insights into abnormal development. A mouse model for the fetal alcohol syndrome (FAS), a mild form of holoprosencephaly, demonstrates a mid-line anterior neural plate deficiency which leads to olfactory placodes being positioned too close to the mid-line, and other secondary changes. Work on animal models for the retinoic acid syndrome (RAS) shows that there is major involvement of neural crest cells. There is also major crest cell involvement in similar syndromes, apparently including hemifacial microsomia. Later administration of retinoic acid prematurely and excessively kills ganglionic placodal cells and leads to a malformation complex virtually identical to the Treacher Collins syndrome. Most clefts of the lip and/or palate appear to have a multifactorial etiology. Genetic variations in TGF alpha s, RAR alpha s, NADH dehydrogenase, an enzyme involved in oxidative metabolism, and cytochrome P-450, a detoxifying enzyme, have been implicated as contributing genetic factors. Cigarette smoking, with the attendant hypoxia, is a probable contributing environmental factor. It seems likely that few clefts involve single major genes. In most cases, the pathogenesis appears to involve inadequate contact and/or fusion of the facial prominences or palatal shelves. Specific mutations in genes for different FGF receptor molecules have been identified for achondroplasia and Crouzon's syndrome, and in a regulatory gene (Msx2) for one type of craniosynostosis. Poorly co-ordinated control of form and size of structures, or groups of structures (e.g., teeth and jaws), by regulatory genes should do much to explain the very frequent "mismatches" found in malocclusions and other dentofacial "deformities". Future directions for research, including possibilities for prevention, are discussed.

Plasticity of transposed rhombomeres: Hox gene induction is correlated with phenotypic modifications

In this study we have analysed the expression of Hoxb-4, Hoxb-1, Hoxa-3, Hoxb-3, Hoxa-4 and Hoxd-4 in the neural tube of chick and quail embryos after rhombomere (r) heterotopic transplantations within the rhombencephalic area. Grafting experiments were carried out at the 5-somite stage, i.e. before rhombomere boundaries are visible. They were preceeded by the establishment of the precise fate map of the rhombencephalon in order to determine the presumptive territory corresponding to each rhombomere. When a rhombomere is transplanted from a caudal to a more rostral position it expresses the same set of Hox genes as in situ. By contrast in many cases, if rhombomeres are transplanted from rostral to caudal their Hox gene expression pattern is modified. They express genes normally activated at the new location of the explant, as evidenced by unilateral grafting. This induction occurs whether transplantation is carried out before or after rhombomere boundary formation. Moreover, the fate of the cells of caudally transplanted rhombomeres is modified: the rhombencephalic nuclei in the graft develop according to the new location as shown for an r5/6 to r8 transplantation. Transplantation of 5 consecutive rhombomeres (i.e. r2 to r6), to the r8 level leads to the induction of Hoxb-4 in the two posteriormost rhombomeres but not in r2,3,4. Transplantations to more caudal regions (posterior to somite 3) result in some cases in the induction of Hoxb-4 in the whole transplant. Neither the mesoderm lateral to the graft nor the notochord is responsible for the induction. Thus, the inductive signal emanates from the neural tube itself, suggesting that planar signalling and predominance of posterior properties are involved in the patterning of the neural primordium.

Targeted mutation of the murine goosecoid gene results in craniofacial defects and neonatal death

The goosecoid gene encodes a homeodomain-containing protein that has been identified in a number of species and has been implicated in a variety of key developmental processes. Initially suggested to be involved in organizing the embryo during early development, goosecoid has since been demonstrated to be expressed during organogenesis-most notably in the head, the limbs and the ventrolateral body wall. To investigate the role of goosecoid in embryonic development, we have inactivated the gene by gene targeting to generate mice mutant for the goosecoid gene. Mice that are homozygous for the goosecoid mutation do not display a gastrulation phenotype and are born; however, they do not survive more than 24 hours. Analysis of the homozygotes revealed numerous developmental defects affecting those structures in which goosecoid is expressed during its second (late) phase of embryonic expression. Predominantly, these defects involve the lower mandible and its associated musculature including the tongue, the nasal cavity and the nasal pits, as well as the components of the inner ear (malleus, tympanic ring) and the external auditory meatus. Although the observed phenotype is in accordance with the late expression domains of goosecoid in wild-type embryos, we suggest that the lack of an earlier phenotype is the result of functional compensation by other genes.

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.

Cryptorchidism and homeotic transformations of spinal nerves and vertebrae in Hoxa-10 mutant mice

Homozygous mice mutated by homologous recombination for the AbdB-related Hoxa-10 gene are viable but display homeotic transformations of vertebrae and lumbar spinal nerves. Mutant males exhibit unilateral or bilateral criptorchidism due to developmental abnormalities of the gubernaculum, resulting in abnormal spermatogenesis and sterility. These results reveal an important role of Hoxa-10 in patterning posterior body regions and suggest that Hox genes are involved in specifying regional identity of both segmented and nonovertly segmented structures of the developing body.