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

Distinct temporal expression of mouse Nkx-5.1 and Nkx-5.2 homeobox genes during brain and ear development

The mouse Nkx-5.1 and Nkx-5.2 genes have been identified by sequence homology to Drosophila NK genes within the homeobox domain. Here, we report the isolation of the Nkx-5.2 cDNA and a detailed comparative analysis of the spatio-temporal expression patterns for Nkx-5.1 and Nkx-5.2 genes. Nkx-5.2 transcripts are first detected in E13.5 embryos where they colocalize with Nkx-5.1 mRNA in the developing central nervous system and the inner ear. However, the onset of Nkx-5.1 transcription begins much earlier in 10 somite stage embryos (E8.5) in the otic placode and the branchial region. Nkx-5.1 expression in the ear persists until birth, whereas in branchial arches it is transient between E8.5 to E11.5. Transcript distribution appears regionalized in the otic vesicle concentrating at the anterior and posterior margin and later at the dorsal side of the otocyst. These domains are distinct from regions expressing Pax-2 and sek, two other early markers for otic development. From E11.5 to birth several Nkx-5.1 expression domains appear in the brain between the ventral diencephalon and the myelencephalon. The same expression domains also exist for Nkx-5.2 beginning at E13.5. The regionally restricted expression pattern of both Nkx-5 genes during mouse development suggests their involvement in cell type specification of neuronal cells.

Genetically engineered mice: tools to understand craniofacial development

In this review, we provide a survey of the experimental approaches used to generate genetically engineered mice. Two specific examples are presented that demonstrate the applicability of these approaches to craniofacial development. In the first, a promoter analysis of the Msx2 gene is presented which illustrates the cis regulatory interactions that defined cell-specific gene expression. In the second, a mouse model of the human disease craniosynostosis, Boston type, has been created by misregulation of the Msx2 gene product. Finally. we present a formulary of spontaneously occurring and genetically engineered mice that exhibit defects in developmental processes affecting the craniofacial complex. The purpose of this review is to provide insight into the experimental approaches that are used to create genetically engineered mice and to impress upon the reader that genetically engineered mice are well-suited to address fundamental questions pertaining to the development maintenance, and regeneration of tissues and organs.

The role of Hoxa-3 in mouse thymus and thyroid development

Targeted disruption of Hoxa-3 results in a number of regionally restricted defects in tissues and structures derived from or patterned by mesenchymal neural crest. However, analysis of mutant embryos with injections of a carbocyanine dye or with molecular markers that label these cells indicates that neither the amount nor the migration patterns of this neural crest population are grossly affected. Therefore, it appears that the loss of Hoxa-3 affects the intrinsic capacity of this neural crest cell population to differentiate and/or to induce proper differentiation of the surrounding pharyngeal arch and pouch tissues. Hoxa-3 mutant mice are athymic and show thyroid hypoplasia. Thymus development is first evident as an expansion of mesenchymal neural crest in the posterior part of the 3rd pharyngeal pouch. Prior to this expansion, a marked reduction in pax-1 expression is observed in these cells in the mutant embryos. As pax-1 mutant mice also show thymic hypoplasia, these results suggest that Hoxa-3 may be required to maintain pax-1 expression in these cells and that the reduction of pax-1 expression is part of the athymic teleology in Hoxa-3 mutant mice. The thyroid gland is formed from the fusion of two structures of separate embryonic origin, the thyroid diverticulum, which is formed from endodermal epithelium in the floor of the pharynx, and the ultimobranchial body, formed from mesenchymal neural crest in the 4th pharyngeal pouch. Both of these sites express Hoxa-3 and are defective in mutant mice. Often a vesicle is observed in mutant mice that is exclusively composed of calcitonin-producing cells, suggesting the persistence of an ultimobranchial body. Both aspects of the thyroid phenotype show variable expressivity among mutant animals, even on the two sides of the same mutant animal. This variability suggests the presence of a compensating gene or genes, whose utilization is stochastic. A reasonable candidate for providing this compensatory function is the paralogous gene Hoxb-3.

Molecular cloning of tyrosine kinases in the early Xenopus embryo: identification of Eck-related genes expressed in cranial neural crest cells of the second (hyoid) arch

Growth factors and their receptors play an important role in controlling cellular proliferation, migration, and differentiation during vertebrate embryogenesis. We have used the reverse transcription-polymerase chain reaction to survey the repertoire of receptor tyrosine kinases (TK) expressed during early embryogenesis of Xenopus laevis. Twelve distinct Xenopus TK cDNA classes were identified among a total of 352 cDNAs screened. A single TK cDNA class has been described previously and encodes the fibroblast growth factor receptor FGFR-A1. The remaining 11 TK cDNA classes appear to encode novel genes of the FGFR, platelet-derived growth factor receptor (PDGFR), Eph, Csk, Tyk2, and Klg subfamilies. By RNase protection assays, Xenopus TK mRNAs are rare transcripts (< 10(7) mRNA molecules/embryo), and are usually found to be expressed also maternally in the embryo. Most Xenopus TK genes examined by whole-mount in situ hybridization were expressed widely in tissues derived from multiple germ layers. Two Eck-related genes, however, were found to be restricted in their expression to neural crest of the second (hyoid) arch. Our findings are consistent with the proposed function of TKs in the regulation of specification and differentiation of embryonic tissues.

Homeobox genes and growth factors in regulation of craniofacial and tooth morphogenesis

Homeobox genes encode a special group of transcription factors that regulate gene expression in the developing embryo. The so-called Hox-cluster genes were first discovered in the Drosophila (fruit fly). They specify the identity of body segments and their patterning along the anteroposterior axis. Other homeobox-containing genes appear to regulate patterning of the head and face. The function of the Msx-1 homeobox gene has been shown to be necessary for tooth development. In general, it is thought that special combinations of homeobox genes specify the patterning of individual structures. Bone morphogenetic proteins (BMPs) are growth factors belonging to the family of transforming growth factor-beta (TGF-beta). BMPs regulate bone and cartilage development, and individual BMPs have been shown to contribute to the shaping of various skeletal elements. BMPs regulate bone and dentin formation also postnatally, and they have therapeutic potential in reparative osteogenesis and odontogenesis. BMPs also act as inductive signals between tissue layers in the embryo, and they regulate the expression of several transcription factors, including homeobox-containing genes. BMP-4 has been identified as an epithelial inductive signal in tooth development. As it is produced by early dental epithelium and regulates tooth-specific gene expression in the dental mesenchyme, including Msx-1 expression, it may be an important signal for the initiation of tooth development.

Developmental roles of the retinoic acid receptors

Retinoic acid, one of the principle active metabolites of vitamin A (retinol), is believed to be essential for numerous developmental and physiological processes. Vitamin A deprivation (VAD) during development leads to numerous congenital defects. Previous studies of retinoic acid receptor (RAR) deficient mice failed to reveal any of these VAD-induced defects. This finding suggested that either the RARs are functionally redundant or that they are not critically required during development. In order to address these possibilities, we derived a number of RAR compound mutants. Unlike RAR single mutants, these compound null mutants died either in utero or shortly following birth. Histological analysis revealed essentially all of the defects characteristic of fetal VAD. A number of additional malformations, not described in previous VAD studies, were also observed. These included defects of the ocular and salivary glands and their ducts, the skeletal elements of the fore- and hindlimbs, and the cervical region of the axial skeleton. In addition, with the exception of derivatives forming within the first pharyngeal arch, most of the elements derived from mesectoderm emanating from cranial and hindbrain levels were affected. A number of these mutants also exhibited supernumerary cranial skeletal elements characteristics of the reptilian skull. A summary of the defects found in these RAR double mutants is presented.

The paired-like homeo box gene MHox is required for early events of skeletogenesis in multiple lineages

Formation of cartilage and bone involves sequential processes in which undifferentiated mesenchyme aggregates into primordial condensations that subsequently grow and differentiate, eventually forming the adult skeleton. Although much has been learned about the structural molecules that compose cartilage and bone, little is known about the nuclear factors that regulate chondrogenesis and osteogenesis. MHox is a homeo box-containing gene that is expressed in the mesenchyme of facial, limb, and vertebral skeletal precursors during mouse embryogenesis. MHox expression has been shown to require epithelial-derived signals, suggesting that MHox may regulate the epithelial-mesenchymal interactions required for skeletal organogenesis. To determine the functions of MHox, we generated a loss-of-function mutation in the MHox gene. Mice homozygous for a mutant MHox allele die soon after birth and exhibit defects of skeletogenesis, involving the loss or malformation of craniofacial, limb, and vertebral skeletal structures. The affected skeletal elements are derived from the cranial neural crest, as well as somitic and lateral mesoderm. Analysis of the mutant phenotype during ontogeny demonstrated a defect in the formation and growth of chondrogenic and osteogenic precursors. These findings provide evidence that MHox regulates the formation of preskeletal condensations from undifferentiated mesenchyme.

Evolutionary-conserved enhancers direct region-specific expression of the murine Hoxa-1 and Hoxa-2 loci in both mice and Drosophila

The HOM-C/Hox complexes are an evolutionary related family of genes that have been shown to direct region-specific development of the animal body plan. We examined in transgenic mice the DNA regulatory elements that determine the temporal and spatially restricted expression of two of the earliest and most anteriorly expressed murine genes, Hoxa-1 and Hoxa-2, which are homologues of the labial and proboscipedia genes of Drosophila. In both mouse and Drosophila, these genes have been shown to play a critical role in head development. We identified three independent enhancers which direct distinct portions of the Hoxa-1 and Hoxa-2 expression domains during early murine embryogenesis. Two enhancers mediate hindbrain-specific expression, being active in either rhombomere 2, the most anterior rhombomere expressing Hoxa-2, or in rhombomere 4, a region where Hoxa-1 and Hoxa-2 have been shown to exert critical developmental roles. The third enhancer is essential for the most extensive expression domain of Hoxa-1 and contains a retinoic acid response element. Point mutations within the retinoic acid response element abolish expression in neuroepithelium caudal to rhombomere 4, supporting a natural role for endogenous retinoids in patterning of the hindbrain and spinal cord. Analysis of the murine Hoxa-2 rhombomere 2-specific enhancer in Drosophila embryos revealed a distinct expression domain within the arthropod head segments, which parallels the expression domain of the Hoxa-2 homologue proboscipedia. These results suggest an evolutionary conservation between HOM-C/Hox family members, which includes a conservation of certain DNA regulatory elements and possible regulatory cascades.

The mouse Fgf8 gene encodes a family of polypeptides and is expressed in regions that direct outgrowth and patterning in the developing embryo

Evidence is accumulating that members of the FGF gene family provide signals that act locally to regulate growth and patterning in vertebrate embryos. In this report, we provide a detailed analysis of the mouse Fgf8 gene. We have mapped the Fgf8 locus to the distal region of mouse chromosome 19, and sequenced the 5′ coding region of the gene. Our data identify a new coding exon, and locate multiple splice donor and splice acceptor sites that can be used to produce at least seven transcripts encoding a family of secreted FGF8 proteins with different N termini. From these results, it appears that Fgf8 is structurally the most complex member of the FGF family described to date. In the embryo, many of the regions in which Fgf8 RNA is localized are known to direct outgrowth and patterning, including the apical ectodermal ridge of the limb bud, the primitive streak and tail bud, the surface ectoderm overlying the facial primorida and the midbrain-hindbrain junction, suggesting that FGF8 may be a component of the regulatory signals that emanate from these regions.

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 TGFalpha) and the so-called steroid/thyroid/retinoic acid superfamily. Considerable advances have been made with respect to our understanding of 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.(ABSTRACT TRUNCATED AT 400 WORDS)

Tooth crown morphogenesis and cytodifferentiations: candid questions and critical comments

Teeth are probably meristic units and crown morphogenesis leads to tooth specific distribution of functional cells. Since heterodonty is derived from homodonty, one way to understand tooth morphogenesis would be to unravel the involved phenomena in homodont species and then to characterize the "put up job" of evolution leading to species specific dentitions with particular functional abilities. Interaction of paleontologists and developmental biologists should be initiated. My naive "developmental" point of view will illustrate only one facet of mouse tooth morphogenesis and cytodifferentiation. The main concern will be to try to discriminate between known facts and speculations, between hypotheses and anticipated or deduced certitudes, to call attention to conflicting data, to suggest some further investigations and to advocate the point of view that molecular interpretations should be founded on indisputable morphological data.

Genetic interaction between hoxb-5 and hoxb-6 is revealed by nonallelic noncomplementation

hoxb-5 and hoxb-6 are adjacent genes in the mouse HoxB locus and are members of the homeotic transcription factor complex that governs establishment of the mammalian body plan. To determine the roles of these genes during development, we generated mice with a targeted disruption in each gene. Three phenotypes affecting brachiocervicothoracic structures were found in the mutant mice. First, hoxb-5- homozygotes have a rostral shift of the shoulder girdle, analogous to what is seen in the human Sprengel anomaly. This suggests a role for hoxb-5 in specifying the position of limbs along the anteroposterior axis of the vertebrate body. Second, hoxb-6- homozygotes frequently have a missing first rib and a bifid second rib. The third phenotype, an anteriorizing homeotic transformation of the cervicothoracic vertebrae from C6 through T1, is common to both hoxb-5- and hoxb-6- homozygotes. Quite unexpectedly, hoxb-5, hoxb-6 transheterozygotes (hoxb-5-hoxb-6+/hoxb-5+ hoxb-6-) also show the third phenotype. By this classical genetic complementation test, these two mutations appear as alleles of the same gene. This phenomenon is termed nonallelic noncomplementation and suggests that these two genes function together to specify this region of the mammalian vertebral column.

Insertion of a targeting construct in a Hoxd-10 allele can influence the control of Hoxd-9 expression

A neomycin resistance (neo) gene driven by the phosphoglycerokinase (PGK) promoter was inserted into the Hoxd-10 homeobox by homologous recombination in embryonic stem (ES) cells. Chimeric mice derived from ES cell-injected blastocysts died shortly after birth. Craniofacial and axial abnormalities were found in the skeleton of these chimeras, resembling some of the previously described Hox gene gain-of-function phenotypes. The spatial expression patterns of various Hoxd gene transcripts were analysed in chimeric mutant embryos by in situ hybridization. Two main observations were made: (1) a wide ectopic expression domain of the Hoxd-9 gene was found in the spinal cord of these embryos, and (2) the neo gene exhibited a specific Hox-like expression domain which extended far more rostrally than that of the Hoxd-10 gene, showing that, in the context of this mutation, the PGK promoter could be regulated as a Hox promoter. These results provide the first evidence that a targeted insertion into a Hox gene coding sequence, in the context of its own cluster, could result in misexpression of a neighbour gene of the complex.

Differential and overlapping expression domains of Dlx-2 and Dlx-3 suggest distinct roles for Distal-less homeobox genes in craniofacial development

During the development of the vertebrate head, cranial neural crest cells migrate into the branchial arches to form many of the structures of the facial skeleton. These cells follow defined developmental pathways and their fates are determined early. We have isolated and characterized the murine Distal-less homeobox gene Dlx-3 and have performed a comparative analysis of Dlx-3 and Dlx-2 expression during craniofacial development. In contrast to Dlx-2 and other vertebrate Distal-less genes, Dlx-3 is not expressed in the central nervous system and is expressed in a highly restricted region of the branchial arches. Dlx-2 and -3 display temporal and spatial differences in expression in the arches and their derivatives. In later development, these two genes are expressed in both complementary and partially overlapping domains in regions whose development is dependent on epithelial-mesenchymal interactions, such as the developing middle and inner ear, teeth and whisker follicles. The differential expression of Dlx genes in the branchial region suggests that they play key roles in craniofacial patterning and morphogenesis.

Colinearity and functional hierarchy among genes of the homeotic complexes

Homeotic genes identify structures along the anterior to posterior axis during the development of most animals. These genes are clustered into complexes, and their positions within the cluster correlates with their time of expression and the positions of the anterioposterior boundaries of their expression domains. Functional analyses have revealed that this specific genetic order also coincides with a functional hierarchy among members of these complexes, so that the products of more posterior genes in the cluster tend to be prevalent over those of more anterior genes.

Genetic and epigenetic control in neural crest development

The neural crest is a fascinating structure of the vertebrate embryo; its ontogeny includes a transient period during which its component cells undergo an epithelio-mesenchymal transition and become migratory. This phase was shown recently to be controlled by the 'Slug' gene which belongs to the 'Snail' family of Drosophila transcription factors. After homing to specific sites in the embryo, the crest-derived cells produce a large variety of phenotypes. Recent advances have shown that during migration most crest cells exhibit various degrees of pluripotentiality, some being already committed to a single and definite fate. Moreover, several lines of evidence point to the existence of totipotent stem cells in the neural crest, the progeny of which become progressively diversified through a combination of intrinsic and extrinsic influences. The latter have been documented by the disruption of several neurotrophin genes, which results in severe deficiencies of selected subsets of neural crest derivatives. The neural crest has also been shown to play an important role in the development of the vertebrate head and hypobranchial region. The genetic control of this process depends on the activity of developmental genes, among which the vertebrate Hox genes are essential, particularly at the rhombencephalic level.

Function of the retinoic acid receptors (RARs) during development (I). Craniofacial and skeletal abnormalities in RAR double mutants

Numerous congenital malformations have been observed in fetuses of vitamin A-deficient (VAD) dams [Wilson, J. G., Roth, C. B., Warkany, J., (1953), Am. J. Anat. 92, 189–217]. Previous studies of retinoic acid receptor (RAR) mutant mice have not revealed any of these malformations [Li, E., Sucov, H. M., Lee, K.-F., Evans, R. M., Jaenisch, R. (1993) Proc. Natl. Acad. Sci. USA 90, 1590–1594; Lohnes, D., Kastner, P., Dierich, A., Mark, M., LeMeur, M., Chambon, P. (1993) Cell 73, 643–658; Lufkin, T., Lohnes, D., Mark, M., Dierich, A., Gorry, P., Gaub, M. P., Lemeur, M., Chambon, P. (1993) Proc. Natl. Acad. Sci. USA 90, 7225–7229; Mendelsohn, C., Mark, M., Dolle, P., Dierich, A., Gaub, M.P., Krust, A., Lampron, C., Chambon, P. (1994a) Dev. Biol. in press], suggesting either that there is a considerable functional redundancy among members of the RAR family during ontogenesis or that the RARs are not essential transducers of the retinoid signal in vivo. In order to discriminate between these possibilities, we have generated a series of RAR compound null mutants. These RAR double mutants invariably died either in utero or shortly after birth and presented a number of congenital abnormalities, which are reported in this and in the accompanying study. We describe here multiple eye abnormalities which are found in various RAR double mutant fetuses and are similar to those previously seen in VAD fetuses. Interestingly, we found further abnormalities not previously reported in VAD fetuses.(ABSTRACT TRUNCATED AT 250 WORDS)

Axial homeosis and appendicular skeleton defects in mice with a targeted disruption of hoxd-11

Using gene targeting, we have created mice with a disruption in the homeobox-containing gene hoxd-11. Homozygous mutants are viable and the only outwardly apparent abnormality is male infertility. Skeletons of mutant mice show a homeotic transformation that repatterns the sacrum such that each vertebra adopts the structure of the next most anterior vertebra. Defects are also seen in the bones of the limb, including regional malformations at the distal end of the forelimb affecting the length and structure of phalanges and metacarpals, inappropriate fusions between wrist bones, and defects at the most distal end in the long bones of the radius and ulna. The phenotypes show both incomplete penetrance and variable expressivity. In contrast to the defects observed in the vertebral column, the phenotypes in the appendicular skeleton do not resemble homeotic transformations, but rather regional malformations in the shapes, length and segmentation of bones. Our results are discussed in the context of two other recent gene targeting studies involving the paralogous gene hoxa-11 and another member of the Hox D locus, hoxd-13. The position of these limb deformities reflects the temporal and structural colinearity of the Hox genes, such that inactivation of 3′ genes has a more proximal phenotypic boundary (affecting both the zeugopod and autopod of the limb) than that of the more 5′ genes (affecting only the autopod). Taken together, these observations suggest an important role for Hox genes in controlling localized growth of those cells that contribute to forming the appendicular skeleton.

Retinoic acid and homeobox gene regulation

Hox genes have been shown to be important regulators of pattern formation in vertebrates. Retinoic acid has been shown to affect the expression of Hox genes in vitro and in vivo, and some of its effects on development correspond to changes in Hox gene expression. The idea that retinoic acid is not simply a powerful pharmocological agent, but rather that it plays an important role in creating the normal expression patterns of Hox genes, is provided by the recent identification of retinoic acid responsive enhancers near Hox genes.

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.

More to learn from gene knockouts

Gene targeting by homologous recombination in mouse embryonic stem cells is a powerful technique to determine the physiological function of any gene product in embryonic and postnatal development and in molecular pathogenesis. Although the technique is very demanding and still in its developing stage several knockout mice carrying disrupted genes, which were once thought important for the development or molecular pathogenesis of certain tissues, have given unexpected results. A gene/function redundancy or superfluous and on-functional theory has been advanced by many investigators to explain the unexpected results. These surprising results may teach us a new lesson and lead to a revision of the strongly held view that highly conserved and abundantly expressed genes have a prominent role and function in cell physiology and development. Additional, they may also support the notion that molecular cross-talk among the genes may play an important role in determining the minimal phenotype.

Targeted disruptions of the murine Hoxa-4 and Hoxa-6 genes result in homeotic transformations of components of the vertebral column

It is becoming clear that Hox genes, which encode transcription factors of the Antennapedia homeodomain family, are key players in establishing the body plan of mammalian embryos. They have already been implicated in the formation of the central nervous system, tissues derived from neural crest, the vertebral column and the limbs. In order to examine the roles of hoxa-4 and hoxa-6 during development, mice with targeted disruptions in these genes were generated. Each shows homeotic transformation of cervical vertebrae, at positions that approximate the anterior borders of expression of these genes in the prevertebrae. Defects were not observed in other tissues that normally express these genes. Hoxa-4-/hoxa-4- mice show, with 100% penetrance, anterior transformations of the dorsal aspects of the third cervical vertebra by acquiring features normally associated with the second cervical vertebra. Mice homozygous for the hoxa-6 mutation show, with incomplete penetrance, even on opposite sides of the same animal, posterior transformations of the seventh cervical vertebra to the first thoracic vertebra. In addition, both hoxa-4-/hoxa-4- and hoxa-6-/hoxa-6- mice show variability in expressivity. These data indicate that alternative genetic pathways can partially, and at times completely, substitute for the function of these two genes. Other members of these two paralogous Hox families are good candidates for providing the substitutions. As paralogous genes lie on different chromosomes, it is possible to examine the degree of redundancy among these genes by intercrossing mice with the appropriate individual disruptions. The analysis of double, triple and even quadruple mutants should determine the ways in which these Hox genes interact in order to specify the multitude of tissues in a restricted region of the developing mouse embryo.

Genetic control of development in Xenopus laevis

In this paper we address the question of how genes can control development by using Xenopus as a model system, since it combines the classical advantages of amphibian embryology with advanced molecular techniques. Several developmental regulator genes have been shown to encode for transcription factors which trigger the activation of downstream genes, thus resulting in a cascade of regulatory events. In the first two examples, we deal with regulatory events that underlie early body patterning in vertebrates, and with the role of homeobox transcription factors in deciphering positional information along the body axis. In the third example, we address the question of the role of post-transcriptional regulation in development by studying the possible regulatory role of a cytoplasmic zinc finger protein, presumably acting through RNA-protein interactions. The general idea is that understanding how genes can control development will hopefully lead to understanding the construction of a shape, and eventually of an organism.

Msx1 deficient mice exhibit cleft palate and abnormalities of craniofacial and tooth development

The Msx1 homeobox gene is expressed at diverse sites of epithelial-mesenchymal interaction during vertebrate embryogenesis, and has been implicated in signalling processes between tissue layers. To determine the phenotypic consequences of its deficiency, we prepared mice lacking Msx1 function. All Msx1- homozygotes manifest a cleft secondary palate, a deficiency of alveolar mandible and maxilla and a failure of tooth development. These mice also exhibit abnormalities of the nasal, frontal and parietal bones, and of the malleus in the middle ear. Msx1 thus has a critical role in mediating epithelial-mesenchymal interactions during craniofacial bone and tooth development. The Msx1-/Msx1- phenotype is similar to human cleft palate, and provides a genetic model for cleft palate and oligodontia in which the defective gene is known.