Epithelial influences on skeletogenesis in the mandible of the embryonic chick

Molecular and cellular mechanisms underlying the evolution of form and function in the amniote jaw

The amniote jaw complex is a remarkable amalgamation of derivatives from distinct embryonic cell lineages. During development, the cells in these lineages experience concerted movements, migrations, and signaling interactions that take them from their initial origins to their final destinations and imbue their derivatives with aspects of form including their axial orientation, anatomical identity, size, and shape. Perturbations along the way can produce defects and disease, but also generate the variation necessary for jaw evolution and adaptation. We focus on molecular and cellular mechanisms that regulate form in the amniote jaw complex, and that enable structural and functional integration. Special emphasis is placed on the role of cranial neural crest mesenchyme (NCM) during the species-specific patterning of bone, cartilage, tendon, muscle, and other jaw tissues. We also address the effects of biomechanical forces during jaw development and discuss ways in which certain molecular and cellular responses add adaptive and evolutionary plasticity to jaw morphology. Overall, we highlight how variation in molecular and cellular programs can promote the phenomenal diversity and functional morphology achieved during amniote jaw evolution or lead to the range of jaw defects and disease that affect the human condition.

SP8 regulates signaling centers during craniofacial development

Much of the bone, cartilage and smooth muscle of the vertebrate face is derived from neural crest (NC) cells. During craniofacial development, the anterior neural ridge (ANR) and olfactory pit (OP) signaling centers are responsible for driving the outgrowth, survival, and differentiation of NC populated facial prominences, primarily via FGF. While much is known about the functional importance of signaling centers, relatively little is understood of how these signaling centers are made and maintained. In this report we describe a dramatic craniofacial malformation in mice mutant for the zinc finger transcription factor gene Sp8. At E14.5 they show facial prominences that are reduced in size and underdeveloped, giving an almost faceless phenotype. At later times they show severe midline defects, excencephaly, hyperterlorism, cleft palate, and a striking loss of many NC and paraxial mesoderm derived cranial bones. Sp8 expression was primarily restricted to the ANR and OP regions during craniofacial development. Analysis of an extensive series of conditional Sp8 mutants confirmed the critical role of Sp8 in signaling centers, and not directly in the NC and paraxial mesoderm cells. The NC cells of the Sp8 mutants showed increased levels of apoptosis and decreased cell proliferation, thereby explaining the reduced sizes of the facial prominences. Perturbed gene expression in the Sp8 mutants was examined by laser capture microdissection coupled with microarrays, as well as in situ hybridization and immunostaining. The most dramatic differences included striking reductions in Fgf8 and Fgf17 expression in the ANR and OP signaling centers. We were also able to achieve genetic and pharmaceutical partial rescue of the Sp8 mutant phenotype by reducing Sonic Hedgehog (SHH) signaling. These results show that Sp8 primarily functions to promote Fgf expression in the ANR and OP signaling centers that drive the survival, proliferation, and differentiation of the NC and paraxial mesoderm that make the face.

Modes of developmental outgrowth and shaping of a craniofacial bone in zebrafish

The morphologies of individual bones are crucial for their functions within the skeleton, and vary markedly during evolution. Recent studies have begun to reveal the detailed molecular genetic pathways that underlie skeletal morphogenesis. On the other hand, understanding of the process of morphogenesis itself has not kept pace with the molecular work. We examined, through an extended period of development in zebrafish, how a prominent craniofacial bone, the opercle (Op), attains its adult morphology. Using high-resolution confocal imaging of the vitally stained Op in live larvae, we show that the bone initially appears as a simple linear spicule, or spur, with a characteristic position and orientation, and lined by osteoblasts that we visualize by transgenic labeling. The Op then undergoes a stereotyped sequence of shape transitions, most notably during the larval period occurring through three weeks postfertilization. New shapes arise, and the bone grows in size, as a consequence of anisotropic addition of new mineralized bone matrix along specific regions of the pre-existing bone surfaces. We find that two modes of matrix addition, spurs and veils, are primarily associated with change in shape, whereas a third mode, incremental banding, largely accounts for growth in size. Furthermore, morphometric analyses show that shape development and growth follow different trajectories, suggesting separate control of bone shape and size. New osteoblast arrangements are associated with new patterns of matrix outgrowth, and we propose that fine developmental regulation of osteoblast position is a critical determinant of the spatiotemporal pattern of morphogenesis.

New directions in craniofacial morphogenesis

The vertebrate head is an extremely complicated structure: development of the head requires tissue-tissue interactions between derivates of all the germ layers and coordinated morphogenetic movements in three dimensions. In this review, we highlight a number of recent embryological studies, using chicken, frog, zebrafish and mouse, which have identified crucial signaling centers in the embryonic face. These studies demonstrate how small variations in growth factor signaling can lead to a diversity of phenotypic outcomes. We also discuss novel genetic studies, in human, mouse and zebrafish, which describe cell biological mechanisms fundamental to the growth and morphogenesis of the craniofacial skeleton. Together, these findings underscore the complex interactions leading to species-specific morphology. These and future studies will improve our understanding of the genetic and environmental influences underlying human craniofacial anomalies.

Expression of WNT signalling pathway genes during chicken craniofacial development

A comprehensive expression analysis of WNT signalling pathway genes during several stages of chicken facial development was performed. Thirty genes were surveyed including: WNT1, 2B, 3A, 4, 5A, 5B, 6, 7A, 7B, 8B, 8C, 9A, 9B, 11, 11B, 16, CTNNB1, LEF1, FRZB1, DKK1, DKK2, FZD1-8, FZD10. The strictly canonical WNTs (2B, 7A, 9B, and 16) in addition to WNT4 WNT6 (both canonical and non-canonical) are epithelially expressed, whereas WNT5A, 5B, 11 are limited to the mesenchyme. WNT16 is limited to the invaginating nasal pit, respiratory epithelium, and lip fusion zone. Antagonists DKK1 and FRZB1 are expressed in the fusing primary palate but then are decreased at stage 28 when fusion is beginning. This suggests that canonical WNT signalling may be active during lip fusion. Mediators of canonical signalling, CTNNB1, LEF1, and the majority of the FZD genes are expressed ubiquitously. These data show that activation of the canonical WNT pathway is feasible in all regions of the face; however, the localization of ligands and antagonists confers specificity.

A SHH-responsive signaling center in the forebrain regulates craniofacial morphogenesis via the facial ectoderm

Interactions among the forebrain, neural crest and facial ectoderm regulate development of the upper jaw. To examine these interactions, we activated the Sonic hedgehog (SHH) pathway in the brain. Beginning 72 hours after activation of the SHH pathway, growth within the avian frontonasal process (FNP) was exaggerated in lateral regions and impaired in medial regions. This growth pattern is similar to that in mice and superimposed a mammalian-like morphology on the upper jaw. Jaw growth is controlled by signals from the frontonasal ectodermal zone (FEZ), and the divergent morphologies that characterize birds and mammals are accompanied by changes in the FEZ. In chicks there is a single FEZ spanning the FNP, but in mice both median nasal processes have a FEZ. In treated chicks, the FEZ was split into right and left domains that resembled the pattern present in mice. Additionally, we observed that, in the brain, fibroblast growth factor 8 (Fgf8) was downregulated, and signals in or near the nasal pit were altered. Raldh2 expression was expanded, whereas Fgf8, Wnt4, Wnt6 and Zfhx1b were downregulated. However, Wnt9b, and activation of the canonical WNT pathway, were unaltered in treated embryos. At later time points the upper beak was shortened owing to hypoplasia of the skeleton, and this phenotype was reproduced when we blocked the FGF pathway. Thus, the brain establishes multiple signaling centers within the developing upper jaw. Changes in organization of the brain that occur during evolution or as a result of disease can alter these centers and thereby generate morphological variation.

Mesenchyme-dependent BMP signaling directs the timing of mandibular osteogenesis

To identify molecular and cellular mechanisms that determine when bone forms, and to elucidate the role played by osteogenic mesenchyme, we employed an avian chimeric system that draws upon the divergent embryonic maturation rates of quail and duck. Pre-migratory neural crest mesenchyme destined to form bone in the mandible was transplanted from quail to duck. In resulting chimeras, quail donor mesenchyme established significantly faster molecular and histological programs for osteogenesis within the relatively slower-progressing duck host environment. To understand this phenotype, we assayed for changes in the timing of epithelial-mesenchymal interactions required for bone formation and found that such interactions were accelerated in chimeras. In situ hybridization analyses uncovered donor-dependent changes in the spatiotemporal expression of genes, including the osteo-inductive growth factor Bmp4. Mesenchymal expression of Bmp4 correlated with an ability of quail donor cells to form bone precociously without duck host epithelium, and also relied upon epithelial interactions until mesenchyme could form bone independently. Treating control mandibles with exogenous BMP4 recapitulated the capacity of chimeras to express molecular mediators of osteogenesis prematurely and led to the early differentiation of bone. Inhibiting BMP signaling delayed bone formation in a stage-dependent manner that was accelerated in chimeras. Thus, mandibular mesenchyme dictates when bone forms by temporally regulating its interactions with epithelium and its own expression of Bmp4. Our findings offer a developmental mechanism to explain how neural crest-derived mesenchyme and BMP signaling underlie the evolution of species-specific skeletal morphology.

Moz-dependent Hox expression controls segment-specific fate maps of skeletal precursors in the face

Development of the facial skeleton depends on interactions between intrinsic factors in the skeletal precursors and extrinsic signals in the facial environment. Hox genes have been proposed to act cell-intrinsically in skeletogenic cranial neural crest cells (CNC) for skeletal pattern. However,Hox genes are also expressed in other facial tissues, such as the ectoderm and endoderm, suggesting that Hox genes could also regulate extrinsic signalling from non-CNC tissues. Here we study moz mutant zebrafish in which hoxa2b and hoxb2a expression is lost and the support skeleton of the second pharyngeal segment is transformed into a duplicate of the first-segment-derived jaw skeleton. By performing tissue mosaic experiments between moz- and wild-type embryos, we show that Moz and Hox genes function in CNC, but not in the ectoderm or endoderm,to specify the support skeleton. How then does Hox expression within CNC specify a support skeleton at the cellular level? Our fate map analysis of skeletal precursors reveals that Moz specifies a second-segment fate map in part by regulating the interaction of CNC with the first endodermal pouch(p1). Removal of p1, either by laser ablation or in the itga5b926 mutant, reveals that p1 epithelium is required for development of the wild-type support but not the moz-duplicate jaw-like skeleton. We present a model in which Moz-dependent Hox expression in CNC shapes the normal support skeleton by instructing second-segment CNC to undergo skeletogenesis in response to local extrinsic signals.

Different roles of Runx2 during early neural crest-derived bone and tooth development

We compared gene expression profiles between Runx2 null mutant mice and their wildtype littermates. Most Runx2-dependent genes in bones were different from those in teeth, implying that the target genes of Runx2 are tissue-dependent. In vitro experiments determined that Runx2 is a part of the FGF and BMP signaling pathways in tooth and bone development, respectively.

INTRODUCTION

Runx2 (Cbfa1) is expressed in the neural crest-derived mesenchyme of developing bone and tooth. Runx2 homozygous null mice lack bone through a failure in osteoblast differentiation and have arrested tooth development at the late bud stage. The aim of this study was to discover and compare the identities and the roles of Runx2 target genes in bone and tooth development.

MATERIALS AND METHODS

Wildtype and Runx2-/- tissue was collected from mouse embryos, and gene expression was compared by Affymetrix microarray analysis and radioactive in situ hybridization of embryonic tissue sections (E12-E14). Induction of target genes by growth factors in bone and tooth tissue was studied using in vitro experiments, including a novel method involving hanging-drop cultures and RT-PCR.

RESULTS

Thirteen bone and four tooth genes were identified that are Runx2-dependent. The identities of these genes do not significantly overlap between bone and tooth, indicating tissue specificity of several genes regulated by Runx2. Genes downregulated in bone development in Runx2 null mutants were Bambi, Bmp4, Bono1, Dkk1, Fgf receptor1, Gli1, Lef1, Patched, Prostaglandin F receptor1, Tcf1, Tgfbeta1, Wnt10a, and Wnt10b. Several of these genes were induced by BMPs in bone tissue in a Runx2-independent manner. Genes downregulated in tooth development were Dkk1, Dusp6, Enpp1, and Igfbp3. These genes were all induced by fibroblast growth factors (FGFs) in dental tissue. FGF-induction of Dkk1 was completely dependent on Runx2 function.

CONCLUSIONS

The contrasting identities and distinctive mechanisms that stimulate the expression of Runx2-dependent genes in bone and tooth development imply that the developmental roles of Runx2 in these separate tissues are different. In tooth development, Dkk1 may be a direct transcriptional target of Runx2. Bone genes were stimulated by BMP4 before the formation of the ossification center, suggesting that BMPs may mediate the early epithelial-mesenchymal interactions involved in bone formation.

Comparative ontogeny and phylogeny of the upper jaw skeleton in amniotes

The morphology, position, and presence of the upper jaw bones vary greatly across amniote taxa. In this review, we compare the development and anatomy of upper jaw bones from the three living amniote groups: reptiles, birds, and mammals. The study of reptiles is particularly important as comparatively little is known about the embryogenesis of the jaw in this group. Our review covers the ontogeny and phylogeny of membranous bones in the face. The aim is to identify conserved embryonic processes that may exist among the three major amniote groups. Finally, we discuss how temporal and spatial regulation of preosseous condensations and ossification centers can lead to variation in the morphology of amniote upper jaw bones.

Dlx2 over-expression regulates cell adhesion and mesenchymal condensation in ectomesenchyme

The Dlx family of homeodomain transcription factors have diverse roles in development including craniofacial morphogenesis and consists of 6 members with overlapping expression patterns. Dlx2 is expressed within the developing branchial arches in both the epithelium and mesenchyme and targeted deletion in mice has revealed roles in patterning and development of the craniofacial skeleton. Defects in Dlx2 null mice include skeletal anomalies of proximal branchial arch 1 derivatives while distal elements are largely spared indicating redundancy within the Dlx family. We have investigated the function of Dlx2 using in ovo electroporation and cell culture. Ectopic expression of Dlx2 within the neural tube beginning prior to emigration of neural crest cells at E1.25 drastically inhibits the migration of transfected cells and induces aggregation of transfected neuroepithelial cells within the neural tube at 24 h post-electroporation. By 48 h post-electroporation, the majority of transfected cells formed multicellular aggregates that were found adjacent to the basal side of the neural tube and very few Dlx2 expressing cells migrated to the level of the branchial arches. Similar results were obtained for Dlx5, suggesting these effects may be common to Dlx genes. Electroporation of the Dlx2 expression construct into branchial arch mesenchyme induced N-cadherin and NCAM, a dramatic increase in cell-cell adhesion relative to controls, and resulted in an increase in mesenchymal condensation. These results suggest a role for Dlx genes in regulating ectomesenchymal cell adhesion and supports the possibility that the skeletal dysmorphology seen in Dlx null mice may derive from abnormalities at the condensation stage.

How the turtle forms its shell: a paracrine hypothesis of carapace formation

We propose a two-step model for the evolutionary origin of the turtle shell. We show here that the carapacial ridge (CR) is critical for the entry of the ribs into the dorsal dermis. Moreover, we demonstrate that the maintenance of the CR and its ability to attract the migrating rib precursor cells depend upon fibroblast growth factor (FGF) signaling. Inhibitors of FGF allow the CR to degenerate, with the consequent migration of ribs along the ventral body wall. Beads containing FGF10 can rearrange rib migration in the chick, suggesting that the CR FGF10 plays an important role in attracting the rib rudiments. The co-ordinated growth of the carapacial plate and the ribs may be a positive feedback loop (similar to that of the limbs) caused by the induction of Fgf8 in the distal tips of the ribs by the FGF10-secreting mesenchyme of the CR. Once in the dermis, the ribs undergo endochrondral ossification. We provide evidence that the ribs act as signaling centers for the dermal ossification and that this ossification is due to bone morphogenetic proteins secreted by the rib. Thus, once the ribs are within the dermis, the ossification of the dermis is not difficult to achieve. This relatively rapid means of carapace formation would allow for the appearance of turtles in the fossil record without obvious intermediates.

moz regulates Hox expression and pharyngeal segmental identity in zebrafish

In vertebrate embryos, streams of cranial neural crest (CNC) cells migrate to form segmental pharyngeal arches and differentiate into segment-specific parts of the facial skeleton. To identify genes involved in specifying segmental identity in the vertebrate head, we screened for mutations affecting cartilage patterning in the zebrafish larval pharynx. We present the positional cloning and initial phenotypic characterization of a homeotic locus discovered in this screen. We show that a zebrafish ortholog of the human oncogenic histone acetyltransferase MOZ (monocytic leukemia zinc finger) is required for specifying segmental identity in the second through fourth pharyngeal arches. In moz mutant zebrafish, the second pharyngeal arch is dramatically transformed into a mirror-image duplicated jaw. This phenotype resembles a similar but stronger transformation than that seen in hox2 morpholino oligo (hox2-MO) injected animals. In addition, mild anterior homeotic transformations are seen in the third and fourth pharyngeal arches of moz mutants. moz is required for maintenance of most hox1-4 expression domains and this requirement probably at least partially accounts for the moz mutant homeotic phenotypes. Homeosis and defective Hox gene expression in moz mutants is rescued by inhibiting histone deacetylase activity with Trichostatin A. Although we find early patterning of the moz mutant hindbrain to be normal, we find a late defect in facial motoneuron migration in moz mutants. Pharyngeal musculature is transformed late, but not early, in moz mutants. We detect relatively minor defects in arch epithelia of moz mutants. Vital labeling of arch development reveals no detectable changes in CNC generation in moz mutants, but later prechondrogenic condensations are mispositioned and misshapen. Mirror-image hox2-dependent gene expression changes in postmigratory CNC prefigure the homeotic phenotype in moz mutants. Early second arch ventral expression of goosecoid (gsc) in moz mutants and in animals injected with hox2-MOs shifts from lateral to medial, mirroring the first arch pattern. bapx1, which is normally expressed in first arch postmigratory CNC prefiguring the jaw joint, is ectopically expressed in second arch CNC of moz mutants and hox2-MO injected animals. Reduction of bapx1 function in wild types causes loss of the jaw joint. Reduction of bapx1 function in moz mutants causes loss of both first and second arch joints, providing functional genetic evidence that bapx1 contributes to the moz-deficient homeotic pattern. Together, our results reveal an essential embryonic role and a crucial histone acetyltransferase activity for Moz in regulating Hox expression and segmental identity, and provide two early targets, bapx1 and gsc, of moz and hox2 signaling in the second pharyngeal arch.

Bone remodeling during prenatal morphogenesis of the human mental foramen

From a morphogenetic point of view, the mental foramen of the mandible is a highly suitable model to study the interactions of different tissues such as nerves, vessels, mesenchymal cells, cartilage, and bone. In previous work, we provided a three-dimensional description of the mental foramen at different developmental stages, and now we complement those studies with a three-dimensional visualization of different bone remodeling activities around the mental foramen. Histological serial sections of human embryos and fetuses, ranging in size from 25 to 117 mm crown-rump-length (CRL), were used to characterize the bone remodeling activity (apposition, inactivity, and resorption). We quantified and reconstructed this activity in three dimensions, and included information on the spatial relationship of the nerves, vessels, and dental primordia. In general, the mandible showed strong apposition at its outer surfaces. The brim of the mental foramen, however, displayed changing remodeling activity at different stages. In the depth of the bony gutter, which provides space for the nerve and the blood vessels, we found bone resorption beneath the inferior alveolar vein. Bone was also resorbed in proximity to the dental primordia. In future studies, we will relate gene expression data to these morphological findings in order to identify molecular mechanisms that regulate this complex system.

Conserved molecular program regulating cranial and appendicular skeletogenesis

The majority of in vivo studies on bone and cartilage differentiation are carried out using the appendicular skeleton as a model system, with the implicit assumption that skeletal formation is equivalent throughout the body. This assumption persists, despite differences in the cellular origins of the skeletogenic precursors. To test the hypothesis that a fundamental set of genes directs skeletal cell differentiation throughout the body, we analyzed cartilage and bone of the chick limb and head during mesenchymal condensation, and when the skeletal tissues had matured. First, we analyzed the expression patterns of transcription factors in early skeletogenic condensations, which revealed similarities among skeletal cell specification in the limb and head. For example, skeletogenic condensations that undergo endochondral ossification had equivalent expression patterns of skeletogenic transcription factors in both limb and head. In the head, many elements also differentiate through intramembranous ossification, or through persistent cartilage formation. Our analyses of these skeletogenic condensations revealed that a unique expression pattern of transcription factors distinguishes among three skeletal tissue fates. The vasculature was excluded from all three skeletogenic condensations, demonstrating that this is not a characteristic unique to endochondral ossification. Second, we compared three different types of more mature cartilage and bone tissue in both the limb and the head, by analyzing a variety of skeletal collagens and signaling molecules. Histological and molecular markers of cartilage and bone generally were conserved between the limb and head skeletons, although we uncovered subtle differences in signaling pathways that might influence cranial and appendicular skeletogenesis.

Alk8 is required for neural crest cell formation and development of pharyngeal arch cartilages

The type I TGFbeta family member receptor alk8 acts in bone morphogenetic protein (BMP) signaling pathways to establish dorsoventral patterning in the early zebrafish embryo. Here, we present evidence that alk8 is required for neural crest cell (NCC) formation and that alk8 signaling gradients direct the proper patterning of premigratory NCCs. We extend our previous functional studies of alk8 to demonstrate that ectopic expression of constitutively active and dominant negative Alk8, consistently results in more medially or laterally positioned premigratory NCCs, respectively. We also demonstrate that patterning defects in premigratory NCCs, induced by alk8 misexpression, correlate with subsequent defects in NCC-derived pharyngeal arch cartilages. Furthermore, an anteroposterior effect is revealed, where overexpression of Alk8 more severely affects anterior arch cartilages and decreased Alk8 activity more severely affects posterior arch cartilage formation. Ectopic expression studies of alk8 are supported by analyses of zygotic and maternal-zygotic laf/alk8 mutants and of several BMP pathway mutants. Pharyngeal mesodermal and endodermal defects in laf/alk8 mutants suggest additional roles for alk8 in patterning of these tissues. Our results provide insight into alk8-mediated BMP signaling gradients and the establishment of premigratory NCC mediolateral positioning, and extend the model for BMP patterning of the neural crest to include that of NCC-derived pharyngeal arch cartilages.

A zone of frontonasal ectoderm regulates patterning and growth in the face

A fundamental set of patterning genes may define the global organization of the craniofacial region. One of our goals has been to identify these basic patterning genes and understand how they regulate outgrowth of the frontonasal process, which gives rise to the mid and upper face. We identified a molecular boundary in the frontonasal process ectoderm, defined by the juxtaposed domains of Fibroblast growth factor 8 and Sonic hedgehog, which presaged the initial site of frontonasal process outgrowth. Fate maps confirmed that this boundary region later demarcated the dorsoventral axis of the upper beak. Ectopic transplantation of the ectodermal boundary region activated a cascade of molecular events that reprogrammed the developmental fate of neural crest-derived mesenchyme, which resulted in duplications of upper and lower beak structures. We discuss these data in the context of boundary/morphogen models of patterning, and in view of the recent controversy regarding neural crest pre-patterning versus neural crest plasticity.

Endothelin 1-mediated regulation of pharyngeal bone development in zebrafish

Endothelin 1 (Edn1), a secreted peptide expressed ventrally in the primordia of the zebrafish pharyngeal arches, is required for correct patterning of pharyngeal cartilage development. We have studied mutants and morpholino-injected larvae to examine the role of the Edn1 signal in patterning anterior pharyngeal arch bone development during the first week after fertilization. We observe a remarkable variety of phenotypic changes in dermal bones of the anterior arches after Edn1 reduction, including loss, size reduction and expansion, fusion and shape change. Notably, the changes that occur appear to relate to the level of residual Edn1. Mandibular arch dermal bone fusions occur with severe Edn1 loss. In the dorsal hyoid arch, the dermal opercle bone is usually absent when Edn1 is severely reduced and is usually enlarged when Edn1 is only mildly reduced, suggesting that the same signal can act both positively and negatively in controlling development of a single bone. Position also appears to influence the changes: a branchiostegal ray, a dermal hyoid bone normally ventral to the opercle, can be missing in the same arch where the opercle is enlarged. We propose that Edn1 acts as a morphogen; different levels pattern specific positions, shapes and sizes of bones along the dorso-ventral axis. Changes involving Edn1 may have occurred during actinopterygian evolution to produce the efficient gill-pumping opercular apparatus of teleosts.

The zebrafishnecklessmutation reveals a requirement forraldh2in mesodermal signals that pattern the hindbrain

We describe a new zebrafish mutation, neckless, and present evidence that it inactivates retinaldehyde dehydrogenase type 2, an enzyme involved in retinoic acid biosynthesis. neckless embryos are characterised by a truncation of the anteroposterior axis anterior to the somites, defects in midline mesendodermal tissues and absence of pectoral fins. At a similar anteroposterior level within the nervous system, expression of the retinoic acid receptor α and hoxb4 genes is delayed and significantly reduced. Consistent with a primary defect in retinoic acid signalling, some of these defects in neckless mutants can be rescued by application of exogenous retinoic acid. We use mosaic analysis to show that the reduction in hoxb4 expression in the nervous system is a non-cell autonomous effect, reflecting a requirement for retinoic acid signalling from adjacent paraxial mesoderm. Together, our results demonstrate a conserved role for retinaldehyde dehydrogenase type 2 in patterning the posterior cranial mesoderm of the vertebrate embryo and provide definitive evidence for an involvement of endogenous retinoic acid in signalling between the paraxial mesoderm and neural tube.

Analysis of cranial neural crest migratory pathways in axolotl using cell markers and transplantation

We have examined the ability of normal and heterotopically transplanted neural crest cells to migrate along cranial neural crest pathways in the axolotl using focal DiI injections and in situ hybridization with the neural crest marker, AP-2. DiI labeling demonstrates that cranial neural crest cells migrate as distinct streams along prescribed pathways to populate the maxillary and mandibular processes of the first branchial arch, the hyoid arch and gill arches 1–4, following migratory pathways similar to those observed in other vertebrates. Another neural crest marker, the transcription factor AP-2, is expressed by premigratory neural crest cells within the neural folds and migrating neural crest cells en route to and within the branchial arches. Rotations of the cranial neural folds suggest that premigratory neural crest cells are not committed to a specific branchial arch fate, but can compensate when displaced short distances from their targets by migrating to a new target arch. In contrast, when cells are displaced far from their original location, they appear unable to respond appropriately to their new milieu such that they fail to migrate or appear to migrate randomly. When trunk neural folds are grafted heterotopically into the head, trunk neural crest cells migrate in a highly disorganized fashion and fail to follow normal cranial neural crest pathways. Importantly, we find incorporation of some trunk cells into branchial arch cartilage despite the random nature of their migration. This is the first demonstration that trunk neural crest cells can form cartilage when transplanted to the head. Our results indicate that, although cranial and trunk neural crest cells have inherent differences in ability to recognize migratory pathways, trunk neural crest can differentiate into cranial cartilage when given proper instructive cues.

Signaling pathways crucial for craniofacial development revealed by endothelin-A receptor-deficient mice

Most of the bone and cartilage in the craniofacial region is derived from cephalic neural crest cells, which undergo three primary developmental events: migration from the rhombomeric neuroectoderm to the pharyngeal arches, proliferation as the ectomesenchyme within the arches, and differentiation into terminal structures. Interactions between the ectomesenchymal cells and surrounding cells are required in these processes, in which defects can lead to craniofacial malformation. We have previously shown that the G-protein-coupled endothelin-A receptor (ET(A)) is expressed in the neural crest-derived ectomesenchyme, whereas the cognate ligand for ET(A), endothelin-1 (ET-1), is expressed in arch epithelium and the paraxial mesoderm-derived arch core; absence of either ET(A) or ET-1 results in numerous craniofacial defects. In this study we have attempted to define the point at which cephalic neural crest development is disrupted in ET(A)-deficient embryos. We find that, while neural crest cell migration in the head of ET(A)(-/-) embryos appears normal, expression of a number of transcription factors in the arch ectomesenchymal cells is either absent or significantly reduced. These ET(A)-dependent factors include the transcription factors goosecoid, Dlx-2, Dlx-3, dHAND, eHAND, and Barx1, but not MHox, Hoxa-2, CRABP1, or Ufd1. In addition, the size of the arches in E10.5 to E11.5 ET(A)(-/-) embryos is smaller and an increase in ectomesenchymal apoptosis is observed. Thus, ET(A) signaling in ectomesenchymal cells appears to coordinate specific aspects of arch development by inducing expression of transcription factors in the postmigratory ectomesenchyme. Absence of these signals results in retarded arch growth, defects in proper differentiation, and, in some mesenchymal cells, apoptosis. In particular, this developmental pathway appears distinct from the pathway that includes UFD1L, implicated as a causative gene in CATCH 22 patients, and suggests parallel complementary pathways mediating craniofacial development.

The role of sonic hedgehog in normal and abnormal craniofacial morphogenesis

There is growing evidence that implicates a role for Sonic hedgehog (SHH) in morphogenesis of the craniofacial complex. Mutations in human and murine SHH cause midline patterning defects that are manifested in the head as holoprosencephaly and cyclopia. In addition, teratogens such as jervine, which inhibit the response of tissues to SHH, also produce cyclopia. Thus, the loss of SHH signaling during early stages of neural plate patterning has a profound influence of craniofacial morphogenesis. However, the severity of these defects precludes analyses of SHH function during later stages of craniofacial development. We have used an embryonic chick system to study the role of SHH during these later stages of craniofacial development. Using a combination of surgical and molecular experiments, we show here that SHH is essential for morphogenesis of the frontonasal and maxillary processes (FNP and MXPs), which give rise to the mid- and upper face. Transient loss of SHH signaling in the embryonic face inhibits growth of the primordia and results in defects analogous to hypotelorism and cleft lip/palate, characteristics of the mild forms of holoprosencephaly. In contrast, excess SHH leads to a mediolateral widening of the FNP and a widening between the eyes, a condition known as hypertelorism. In severe cases, this widening is accompanied by facial duplications. Collectively, these experiments demonstrate that SHH has multiple and profound effects on the entire spectrum of craniofacial development, and perturbations in SHH signaling are likely to underlie a number of human craniofacial anomalies.

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.

Embryonic origin and fate of chondroid tissue and secondary cartilages in the avian skull

BACKGROUND

Chondroid tissue is an intermediate calcified tissue, mainly involved in desmocranial morphogenesis. Often associated with secondary cartilages, it remained of unprecise embryonic origin.

METHODS

The latter was studied by performing isotopic isochronic grafts of quail encephalon onto 30 chick embryos. The so-obtained chimeras were sacrificed at the 9th, 12th, and 14th day of incubation. The contribution of graft- and host-derived cells to the histogenesis of chondroid tissue, bone, and secondary cartilages was analyzed on both microradiographs of thick undecalcified sections and on classical histological sections after several DNA or ECM specific staining procedures.

RESULTS

Chondroid tissue is deposited in the primitive anlage of all membranous bones of the avian skull. Also present on their sutural edges, it uniformly arises from the neural crest. In the face, bone and secondary cartilages share this mesectodermal origin. However, secondary cartilages located along the basal chondrocranium and bone formed on the chondroid primordium of the cranial vault, originate from the cephalic mesoderm.

CONCLUSIONS

These facts provide evidence that chondroid tissue arises from a specific differentiation of neural crest derived cells and that this original skeletogenic program differs from that of secondary chondrogenesis. Moreover, they obviously indicate that in membraneous bone ontogenesis, chondroid tissue replaces functions devoted to mesodermal primary cartilages of the cranial base, and so corroborates at the tissue level, the dual embryonic and phyletic origin of the skull.

Spatial and temporal distribution of Indian hedgehog mRNA in the embryonic mouse mandible

Hedgehog genes are involved in pattern formation during embryonic development. A recent report showed that Sonic hedgehog is expressed in the mouse mandible in the presumptive incisor region. In the present study, Indian hedgehog (Ihh) transcripts were present from gestational day 9 to 14 in the mouse mandible (reverse transcription/polymerase chain reaction analysis). Ihh mRNA was present in the dental lamina in both incisor and molar regions and in the developing whiskers (in-situ hybridization). Ihh may be involved in the site-specific proliferation of mandibular epithelium during the formation of the dental lamina. This is consistent with the observation that endogenous synthesis of retinoic acid is necessary for the initiation of odontogenesis and that retinoic acid induces hedgehog expression.

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.

Spatial and temporal distribution of sonic hedgehog mRNA in the embryonic mouse mandible by reverse transcription/polymerase chain reaction and in situ hybridization analysis

Hedgehog genes have recently been implicated in the control of pattern formation in many developing organ system. Vertebrate homologues of the Drosophila hedgehog have been identified in mouse and rate embryos. The temporal regulation of sonic hedgehog (mouse homologue) has previously been studied by Northern analysis of whole embryos with varying results. Sonic hedgehog transcript expression in the mouse mandibular process was now characterized using polymerase chain reaction (PCR) an in situ hybridization techniques. PCR analysis revealed transcripts at gestational days 10 and 11, before the formation of the dental lamina, but not at days 12-14, after tooth buds have formed. Transcripts were localized to, primarily, the epithelium in the presumptive incisor region of the mandibular midline at gestational day 10. No mRNA was detected by in situ hybridization techniques in the presumptive molar regions of odontogenic epithelium. Sonic hedgehog expression may be involved in the regulation of pattern formation through establishment of an incisor-molar axis of polarity.

Experimental analysis of Msx-1 and Msx-2 gene expression during chick mandibular morphogenesis

Homeobox-containing genes are thought to be involved in regulating pattern formation in a variety of tissues during embryogenesis. We have examined the expression of the homeobox-related genes Msx-1 and Msx-2 during the development of the chick mandibular arch. Northern blot hybridization indicates that transcripts for both Msx-1 (1.6 Kb) and Msx-2 (3 Kb) are present in the mandibular arch as early as stage 18. The levels of both transcripts in the whole mandible decrease as cartilage is formed in vivo and in vitro. Using in situ hybridization, transcripts of Msx-1 were localized in high amounts to the mesenchyme of the mesial tips of the arches. Msx-2 transcripts were localized in high amounts to medial regions of the arches. Little or no hybridization of either probe was detected in the chondrogenic and myogenic regions of the arches. Transcripts of both genes were also excluded from calcified bone and cartilage. Our results further demonstrate that the mesial tip mesenchyme expressing Msx-1 includes areas of highly proliferative cells and has in vitro chondrogenic potential. The region of mesenchymal cells expressing the Msx-2 gene overlap with areas of developmentally programmed cell death which also contain very few proliferative cells and lack chondrogenic potential in vitro. These results are consistent with the possibility that Msx-1 may be involved in the outgrowth of the mandibular arch and Msx-2 may be involved in both developmentally programmed cell death and delineating the non-chondrogenic region of the medial part of the mandibular arch.

Spatial distribution of epidermal growth-factor transcripts and effects of exogenous epidermal growth factor on the pattern of the mouse dental lamina

The initiation of odontogenesis is characterized by the site-specific proliferation of mandibular epithelium in the formation of the dental lamina. The epidermal growth factor (EGF) gene is expressed in the developing mandible immediately before the appearance of the dental lamina. This expression is necessary for the formation of the dental lamina and subsequent development of teeth. Previous work has demonstrated that retinoids and EGF may interact in the establishment of the pattern of the dentition. In the present study explanted mandibles that were treated with exogenous EGF (40 ng/ml of medium) contained supernumerary buds of mandibular epithelium in the diastema region. These pattern changes were the same as in previous retinoid-treated explants. These results, in addition to the previously reported effects of retinoids on the expression of the EGF gene, support the hypothesis that retinoids and EGF interact in controlling, at least in part, the pattern of the dentition by affecting the pattern of the dental lamina. The spatial distribution of EGF transcripts was also characterized. The location of EGF transcripts in the mesenchyme adjacent to the mandibular epithelium suggests a paracrine mechanism in the stimulation of epithelial proliferation in the formation of the dental lamina.

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)

Enhancement of avian mandibular chondrogenesis in vitro in the absence of epithelium

The roles of mandibular epithelium in chondrogenesis and growth of mandibular mesenchyme were examined in organ cultures. Epithelium and mesenchyme were separated from the mandibular arches of chick embryos at stages before and after the onset of chondrogenesis in vivo (stages 18-28). Isochronic and heterochronic tissue recombinations were prepared. Removal of the mandibular epithelium resulted in reduced growth of the explants and enhanced chondrogenesis, resulting in increased levels of mRNAs for type II collagen and aggrecan. The presence of mandibular epithelium promoted cell division in loosely arranged undifferentiated tissue from the mandibular mesenchyme and resulted in increased levels of type I collagen mRNA. Enhanced chondrogenesis was also observed in the mesenchyme isolated with basement membrane and isolated mesenchyme grown within Matrigel. These findings suggest that mandibular epithelium has mitogenic and chondrogenic-inhibitory effects on the underlying mesenchyme that are stage independent. Furthermore, the chondrogenic-inhibitory effect of mandibular epithelium on the underlying mesenchymal cells is not mediated by basement membrane.

Effects of retinol on the temporal expression of transforming growth factor-alpha mRNA in the embryonic mouse mandible

Development of the mouse embryonic mandible from days 9 to 14 involves tissue interactions in the formation of bone, cartilage, salivary glands and teeth. Growth factors may play an important role in these interactions. Epidermal growth factor (EGF) mRNA expression has been characterized and its presence has been shown to be necessary for odontogenesis. In addition, retinol alters the pattern of dental lamina formation; this effect is correlated with an alteration of the expression of the mRNA for this mitogen (EGF). Transforming growth factor-alpha (TGF alpha) mRNA expression has now been characterized by polymerase chain reaction for this entire period of development (days 9-14). Although the mRNA is present at the same time as EGF (days 9 and 10 only), retinol does not alter the expression of this mitogen as it does EGF. This suggests that retinoids may act to control the proliferative pattern of the dental lamina through EGF expression and not TGF alpha expression, although mRNAs for both mitogens are present at the same time.

Developmental differences in the ossification process of the human corpus and ramus mandibulae

A correlation was sought between the organization of the dental crest and the ossification of the corpus mandibulae in 14 human embryos and 13 human fetuses. The different types of ossification between the corpus and the ramus mandibulae suggest that the cartilago mandibularis (meckeliensis) guides the formation of the mandibula, while the dental crest acts as a coorganizer. In the area of the foramen mentale, the lamina dentalis begins to invaginate (to give rise to the dental crest), and at this level intramembranous ossification of the corpus mandibulae commences. These findings, together with the presence of the cartilago mandibularis before the appearance of the dental crest, and the fact that the former is seen along the entire length of the mandibula (from the symphysis mandibulae to the capsula otica), support the hypothesis that the dental crest, rather than the cartilago mandibularis, acts as the coorganizer in the corpus mandibulae.

The role of retinoids in normal and abnormal embryonic craniofacial morphogenesis

The objective of this article is to evaluate the role of retinoids in the developing head and face. This article covers two lines of evidence that strongly support a role for retinoids in craniofacial development. First, the specific effects of exogenous retinoids on the head and face are covered and mechanisms for the specificity discussed. Second, the function of endogenous retinoids in facial development is discussed in relation to the distribution of retinoid-binding substances in the face. Finally, the interaction of retinoids with other genes known to be expressed in the face as well as other factors required for facial growth is discussed.

Mesoderm and jaw development in vertebrates: the role of growth factors

The head and neck arise during development as the result of a complex series of cellular and molecular interactions that begin in the fertilized egg. In this article, the role of an important class of molecules, growth factors, is examined in two main steps of the developmental sequence: the initial induction of mesoderm and the later induction of jaw cartilage and bone. The article focuses particularly on the roles of members of the transforming growth factor-beta (TGF-beta), fibroblast growth factor (FGF), and epithelial growth factor (EGF) families in these processes and current models of growth factor involvement. Possible experiments for the future are discussed.

Stage-related chondrogenic potential of avian mandibular ectomesenchymal cells

We have examined the in vitro stage-related chondrogenic potential of avian mandibular ectomesenchymal cells using micromass cultures. Our results indicate that mandibular ectomesenchymal cells as early as stage 16, soon after the formation of the mandibular arches and well before the initiation of in vivo chondrogenesis, have chondrogenic potential which is expressed in micromass culture. There is an increase in the total area of the cultures occupied by cartilage when cells from increasing stages of development are used. The nodular pattern of chondrogenesis in these cultures indicates that mandibular ectomesenchymal cells are a heterogenous population from the time of mandibular arch formation. In addition, we studied the temporal expression of the genes for extracellular matrix proteins during in vitro chondrogenesis and correlated the morphological changes with the pattern of gene expression. Low levels of type II collagen mRNA are present in the cultures prior to detection of any stainable cartilage matrix and increase 5 fold just before the onset of chondrogenesis in vitro. On the other hand mRNA for cartilage proteoglycan core protein was not detected until the second day of culture when stainable cartilage matrix was present and progressively increased thereafter. Messenger RNA for type I collagen was present at the time of initiation of cultures and continuously increased during the culture period. Our experiments also indicated that embryonic epithelia can inhibit the in vitro chondrogenesis of mandibular ectomesenchymal cells and that the inhibitory effect of embryonic epithelia is independent of its age and site of origin.

Temporal and spatial expression of genes for cartilage extracellular matrix proteins during avian mandibular arch development

We have examined the temporal expression of genes for extracellular matrix proteins (type I collagen, type II collagen, and the cartilage specific proteoglycan core protein) during the development of the avian mandibular arch. We detected low levels of type II collagen mRNA in the mandibular arch as early as stage 15. Type II collagen mRNA remained low but increased slightly as development progressed from stage 15 to stage 25. More dramatic increases occurred after stage 25 coincident with overt chondrogenesis. In contrast, mRNA for the core protein of cartilage specific proteoglycan was not detected prior to the onset of chondrogenesis, appeared at stage 25, and increased thereafter. Type I collagen mRNA was also present as early as stage 15 and dramatically increased after stage 28/29, coincident with initiation of osteogenesis. Using in situ hybridization, we found that type II collagen mRNA became detectable in the center of the mandible around stage 24/25 coincident with the initiation of chondrogenesis. At later stages (26-32) type II collagen mRNA was localized in the cartilaginous rudiment. The pattern of hybridization observed with the proteoglycan core protein probe at later stages of development was essentially identical to that observed with the type II collagen probe. In contrast, the probe for the alpha 1 (I) collagen mRNA was localized over the perichondrium, over differentiated bone, and in areas within the mandibular arch where bone formation had been initiated.

The effects of surgical section of the embryonic chick mandibular arch

The embryonic chick mandibular arch was surgically sectioned in ovo on day 7 of incubation and the subsequent wound healing of the arch, together with the response of Meckel's cartilage to fracture, was examined. The repair process observed (in contrast to that in adults) was characterised by minimal haematoma formation or cell death and the absence of formation of either cellular blastema or fracture callus. re-epithelialisation was complete within 48 h with no scar tissue formed. Continuity of Meckel's cartilage, together with restoration of its histological appearance and that of the surrounding soft tissues, was re-established within 24 h in 88% of cases. In the case of the cartilage this was due to fusion of the matrix followed apparently by chondrocytic and perichondrial proliferation. This differs from the repair of embryonic long bone cartilages. In 12% of cases, however, mal-union or non-union of the cartilage resulted in mandibular arch deviation. This observation suggests that mandibular arch growth and morphogenesis may parallel the development of Meckel's cartilage. Where cartilaginous non-union occurred, some irregularities in the pattern of the developing mandibular bones were evident, and it is argued that deformity in the cartilage may ultimately affect the length and shape of the adult mandible.

Development and evolutionary origins of vertebrate skeletogenic and odontogenic tissues

This review deals with the following seven aspects of vertebrate skeletogenic and odontogenic tissues. 1. The evolutionary sequence in which the tissues appeared amongst the lower craniate taxa. 2. The topographic association between skeletal (cartilage, bone) and dental (dentine, cement, enamel) tissues in the oldest vertebrates of each major taxon. 3. The separate developmental origin of the exo- and endoskeletons. 4. The neural-crest origin of cranial skeletogenic and odontogenic tissues in extant vertebrates. 5. The neural-crest origin of trunk dermal skeletogenic and odontogenic tissues in extant vertebrates. 6. The developmental processes that control differentiation of skeletogenic and odontogenic tissues in extant vertebrates. 7. Maintenance of developmental interactions regulating skeletogenic/odontogenic differentiation across vertebrate taxa. We derive twelve postulates, eight relating to the earliest vertebrate skeletogenic and odontogenic tissues and four relating to the development of these tissues in extant vertebrates and extrapolate the developmental data back to the evolutionary origin of vertebrate skeletogenic and odontogenic tissues. The conclusions that we draw from this analysis are as follows. 8. The dermal exoskeleton of thelodonts, heterostracans and osteostracans consisted of dentine, attachment tissue (cement or bone), and bone. 9. Cartilage (unmineralized) can be inferred to have been present in heterostracans and osteostracans, and globular mineralized cartilage was present in Eriptychius, an early Middle Ordovician vertebrate unassigned to any established group, but assumed to be a stem agnathan. 10. Enamel and possibly also enameloid was present in some early agnathans of uncertain affinities. The majority of dentine tubercles were bare. 11. The contemporaneous appearance of cellular and acellular bone in heterostracans and osteostracans during the Ordovician provides no clue as to whether one is more primitive than the other. 12. We interpret aspidin as being developmentally related to the odontogenic attachment tissues, either closer to dentine or a form of cement, rather than as derived from bone. 13. Dentine is present in the stratigraphically oldest (Cambrian) assumed vertebrate fossils, at present some only included as Problematica, and is cladistically primitive, relative to bone. 14. The first vertebrate exoskeletal skeletogenic ability was expressed as denticles of dentine. 15. Dentine, the bone of attachment associated with dentine, the basal bone to which dermal denticles are fused and cartilage of the Ordovician agnathan dermal exoskeleton were all derived from the neural crest and not from mesoderm. Therefore the earliest vertebrate skeletogenic/odontogenic tissues were of neural-crest origin.(ABSTRACT TRUNCATED AT 400 WORDS)

Autoradiographic localization of osteogenin binding sites in cartilage and bone during rat embryonic development

Osteogenin, a novel bone differentiation factor isolated from bone, has been recently purified and the amino acid sequence determined. Osteogenin in conjunction with a collagenous bone matrix substratum induces cartilage and bone formation in vivo. In order to understand the developmental role of osteogenin during cartilage and bone morphogenesis we examined the binding and distribution of iodinated osteogenin in developing rat embryos. Whole embryo tissue sections were made from 11, 12, 13, 15, 18, and 20 day fetuses. The specific binding of osteogenin at different stages of rat embryonic development was determined by autoradiography. Maximal binding was observed in mesodermal tissues such as cartilage, bone, perichondrium, and periosteum. During Days 11-15, peak binding was localized to perichondrium during limb and vertebral morphogenesis. By Day 18 periosteum exhibited the highest concentration of autoradiographic grains. Osteogenin was also localized in developing membranous bones of the calvarium and other craniofacial bones. Considerably less binding was observed, in decreasing order, in muscle, liver, spleen, skin, brain, heart, kidney, and intestine. The observed maximal binding during skeletal morphogenesis implies a developmental role for osteogenin.

Cartilage, bone and tooth induction during early embryonic mouse mandibular morphogenesis using serumless, chemically-defined medium

Studies were designed to test the hypothesis that plasma- and serum-deprived embryonic cells and tissues in vitro are capable of producing growth regulating factors which augment cartilage, bone and tooth induction during mouse mandibular process development. Embryonic mouse first branchial arch-derived mandibular processes (E11-E12, Theiler stages 18-19) or cap stage molar tooth (M1) organs (E15-E16, Theiler stage 23) expressed morphogenesis, histogenesis and cytodifferentiation (e.g., Meckel's cartilage and mandibular bone) when cultured as explants in permissive serumless and chemically-defined BGJB medium for periods up to 31 days in vitro. Organ cultures of early mandibular process explants in serumless conditions showed DNA synthesis comparable to the time- and position-restricted patterns characteristic for control in vivo development. As a paradigm for embryonic cell expression of putative growth factors, sense and antisense oligodeoxynucleotide probes corresponding to amino acids 1070-1081 for preproEGF, and antibodies directed against amino acids 348-691 of preproEGF, were used to identify and localize mRNA transcripts and translation products. Our preliminary evidence suggests that odontogenic epithelial and ectomesenchyme cells produce EGF-like products during instructive phases of tooth development. We suggest that plasma- and serum-deprived cells and tissues in vitro produce autocrine and/or paracrine growth factors which mediate embryonic mandibular morphogenesis, histogenesis and cytodifferentiation.

Influence of epithelial-mesenchymal interaction on the viability of facial mesenchyme in vitro

Separation and recombination experiments, employing a variety of tissue configurations in organ culture, were performed to determine the extent to which the epithelium of the maxillary process influences the viability of the underlying mesenchyme during organogenesis. The results of these studies indicated that the viability of mesenchyme of the maxillary process of early stage embryos was severely impaired when separated from the overlying epithelium. The influence of epithelium on the viability of mesenchyme was stage dependent; that is, the requirement for the presence of epithelium for the maintenance of the viability of mesenchyme became progressively less pronounced at older developmental stages. The response of mesenchyme to the presence of recombined epithelium resulted in the appearance of a delimited zone of influence extending, within specific boundaries, from the epithelial-mesenchymal interface. Preliminary data from homotypic (maxillary epithelium-maxillary mesenchyme), heterotypic (limb apical ectodermal ridge-maxillary mesenchyme) and heterochronic (stage 28 epithelium-stage 22 mesenchyme) recombination experiments indicated that viability of mesenchyme could be achieved only through direct epithelial-mesenchymal contact which allowed restoration of normal morphological relationships at the interface of the two tissues.