Fate of the mammalian cranial neural crest during tooth and mandibular morphogenesis

Use of type I collagen green fluorescent protein transgenes to identify subpopulations of cells at different stages of the osteoblast lineage

Green fluorescent protein (GFP)-expressing transgenic mice were produced containing a 3.6-kilobase (kb; pOBCol3.6GFPtpz) and a 2.3-kb (pOBCol2.3GFPemd) rat type I collagen (Col1a1) promoter fragment. The 3.6-kb promoter directed strong expression of GFP messenger RNA (mRNA) to bone and isolated tail tendon and lower expression in nonosseous tissues. The 2.3-kb promoter expressed the GFP mRNA in the bone and tail tendon with no detectable mRNA elsewhere. The pattern of fluorescence was evaluated in differentiating calvarial cell (mouse calvarial osteoblast cell [mCOB]) and in marrow stromal cell (MSC) cultures derived from the transgenic mice. The pOBCol3.6GFPtpz-positive cells first appeared in spindle-shaped cells before nodule formation and continued to show a strong signal in cells associated with bone nodules. pOBCol2.3GFPemd fluorescence first appeared in nodules undergoing mineralization. Histological analysis showed weaker pOBCol3.6GFPtpz-positive fibroblastic cells in the periosteal layer and strongly positive osteoblastic cells lining endosteal and trabecular surfaces. In contrast, a pOBCol2.3GFPemd signal was limited to osteoblasts and osteocytes without detectable signal in periosteal fibroblasts. These findings suggest that Col1a1GFP transgenes are marking different subpopulations of cells during differentiation of skeletal osteoprogenitors. With the use of other promoters and color isomers of GFP, it should be possible to develop experimental protocols that can reflect the heterogeneity of cell differentiation in intact bone. In primary culture, this approach will afford isolation of subpopulations of these cells for molecular and cellular analysis.

Tissue origins and interactions in the mammalian skull vault

During mammalian evolution, expansion of the cerebral hemispheres was accompanied by expansion of the frontal and parietal bones of the skull vault and deployment of the coronal (fronto-parietal) and sagittal (parietal-parietal) sutures as major growth centres. Using a transgenic mouse with a permanent neural crest cell lineage marker, Wnt1-Cre/R26R, we show that both sutures are formed at a neural crest-mesoderm interface: the frontal bones are neural crest-derived and the parietal bones mesodermal, with a tongue of neural crest between the two parietal bones. By detailed analysis of neural crest migration pathways using X-gal staining, and mesodermal tracing by DiI labelling, we show that the neural crest-mesodermal tissue juxtaposition that later forms the coronal suture is established at E9.5 as the caudal boundary of the frontonasal mesenchyme. As the cerebral hemispheres expand, they extend caudally, passing beneath the neural crest-mesodermal interface within the dermis, carrying with them a layer of neural crest cells that forms their meningeal covering. Exposure of embryos to retinoic acid at E10.0 reduces this meningeal neural crest and inhibits parietal ossification, suggesting that intramembranous ossification of this mesodermal bone requires interaction with neural crest-derived meninges, whereas ossification of the neural crest-derived frontal bone is autonomous. These observations provide new perspectives on skull evolution and on human genetic abnormalities of skull growth and ossification.

The BMP antagonists Chordin and Noggin have essential but redundant roles in mouse mandibular outgrowth

Here we investigate the roles of the Bone Morphogenetic Protein (BMP) antagonists Chordin and Noggin in development of the mandible, which is derived from the first branchial arch (BA1). Both genes are expressed in the pharynx during early mandibular outgrowth and later in the mandibular process. Mice mutant for either Nog or Chd have only mild mandibular defects; however, pups of the genotype Chd(-/-);Nog(+/-) exhibit a range of mandibular truncation phenotypes, from normal to agnathia. A few embryos homozygous null for both genes survive to late gestation; many are agnathic, though a few have significant mandibular outgrowth. In mandibular explants, ectopic BMP4 rapidly induces expression of both Chd and Nog, consistent with results obtained in vivo with mutant embryos. Previous work has shown that FGF8 is a survival factor for cells populating the mandibular bud. We find that excess BMP4 represses Fgf8 transcription in mandibular explants. Embryos lacking these BMP antagonists often show a strong reduction in Fgf8 expression in the pharyngeal ectoderm, and increased cell death in the mandibular bud. We suggest that the variable mandibular hypoplasia in double mutants involves increased BMP activity downregulating Fgf8 expression in the pharynx, decreasing cell survival during mandibular outgrowth.

Neutrophil collagenase (MMP-8) is expressed during early development in neural crest cells as well as in adult melanoma cells

Matrix metalloproteinase-8 (MMP-8) is a neutral metalloproteinase of the fibrillar collagenase family that also includes MMP-1 and MMP-13. In contrast to the other collagenases, MMP-8 has a very limited tissue distribution, thought to be restricted to neutrophils and chondrocytes. In a previous study, we observed MMP-8 expression in human melanoma cells. This observation led us to assess in more detail the expression of MMP-8 in normal and malignant melanocytic cells. We found that MMP-8 was expressed by 11 out of 12 human melanoma cell lines tested and all 10 primary melanomas we examined, but was not expressed by four primary neonatal melanocyte strains. Since melanocytes arise from highly motile neural crest cells, we examined the hypothesis that MMP-8 might be expressed by neural crest cells. RT-PCR analysis of post-implantation mouse embryos indicated the presence of MMP-8 transcripts at E9.5. In situ hybridization and immunohistochemistry of mouse embryos between E9.5-E14.5 demonstrated MMP-8 expression in the surface ectoderm, neural crest cells and chondrocytes. MMP-8 was also detected in neural crest cell migration located in the circumference of the neural tube, branchial arches and the notochord. In addition, MMP-8 expression was observed between the somites, in circumscriptive areas of the developing brain, heart, and eye, and in the interdigital zones of the limbs. In summary, we found MMP-8 to be the first fibrillar collagenase expressed during development. In contrast to its restricted tissue expression post-partum, MMP-8 was present in multiple embryonic tissues, including neural crest cells. The production of MMP-8 by migrating neural crest cells may contribute to their ability to degrade fibrillar collagen matrices while in transit.

Antagonistic effects of Smad2 versus Smad7 are sensitive to their expression level during tooth development

Members of the transforming growth factor-beta (TGF-beta) superfamily regulate cell proliferation, differentiation, and apoptosis, controlling the development and maintenance of most tissues. TGF-beta signal is transmitted through the phosphorylation of Smad proteins by TGF-beta receptor serine/threonine kinase. During early tooth development, TGF-beta inhibits proliferation of enamel organ epithelial cells but the underlying molecular mechanisms are largely unknown. Here we tested the hypothesis that antagonistic effects between Smad2 and Smad7 regulate TGF-beta signaling during tooth development. Attenuation of Smad2 gene expression resulted in significant advancement of embryonic tooth development with increased proliferation of enamel organ epithelial cells, while attenuation of Smad7 resulted in significant inhibition of embryonic tooth development with increased apoptotic activity within enamel organ epithelium. These findings suggest that different Smads may have differential activities in regulating TGF-beta-mediated cell proliferation and death. Furthermore, functional haploinsufficiency of Smad2, but not Smad3, altered TGF-beta-mediated tooth development. The results indicate that Smads are critical factors in orchestrating TGF-beta-mediated gene regulation during embryonic tooth development. The effectiveness of TGF-beta signaling is highly sensitive to the level of Smad gene expression.

The genetic basis of modularity in the development and evolution of the vertebrate dentition

The construction of organisms from units that develop under semi-autonomous genetic control (modules) has been proposed to be an important component of their ability to undergo adaptive phenotypic evolution. The organization of the vertebrate dentition as a system of repeated parts provides an opportunity to study the extent to which phenotypic modules, identified by their evolutionary independence from other such units, are related to modularity in the genetic control of development. The evolutionary history of vertebrates provides numerous examples of both correlated and independent evolution of groups of teeth. The dentition itself appears to be a module of the dermal exoskeleton, from which it has long been under independent genetic control. Region-specific tooth loss has been a common trend in vertebrate evolution. Novel deployment of teeth and reacquisition of lost teeth have also occurred, although less frequently. Tooth shape differences within the dentition may be discontinuous (referred to as heterodonty) or graded. The occurrence of homeotic changes in tooth shape provides evidence for the decoupling of tooth shape and location in the course of evolution. Potential mechanisms for region-specific evolutionary tooth loss are suggested by a number of mouse gene knockouts and human genetic dental anomalies, as well as a comparison between fully-developed and rudimentary teeth in the dentition of rodents. These mechanisms include loss of a tooth-type-specific initiation signal, alterations of the relative strength of inductive and inhibitory signals acting at the time of tooth initiation and the overall reduction in levels of proteins required for the development of all teeth. Ectopic expression of tooth initiation signals provides a potential mechanism for the novel deployment or reacquisition of teeth; a single instance is known of a gene whose ectopic expression in transgenic mice can lead to ectopic teeth. Differences in shape between incisor and molar teeth in the mouse have been proposed to be controlled by the region-specific expression of signalling molecules in the oral epithelium. These molecules induce the expression of transcription factors in the underlying jaw mesenchyme that may act as selectors of tooth type. It is speculated that shifts in the expression domains of the epithelial signalling molecules might be responsible for homeotic changes in tooth shape. The observation that these molecules are regionally restricted in the chicken, whose ancestors were not heterodont, suggests that mammalian heterodonty may have evolved through the use of patterning mechanisms already acting on skeletal elements of the jaws. In general, genetic and morphological approaches identify similar types of modules in the dentition, but the data are not yet sufficient to identify exact correspondences. It is speculated that modularity may be achieved by gene expression differences between teeth or by differences in the time of their development, causing mutations to have cumulative effects on later-developing teeth. The mammalian dentition, for which virtually all of the available developmental genetic data have been collected, represents a small subset of the dental diversity present in vertebrates as a whole. In particular, teleost fishes may have a much more extensive dentition. Extension of research on the genetic control of tooth development to this and other vertebrate groups has great potential to further the understanding of modularity in the dentition.

Conditional control of gene expression in the mouse

One of the most powerful tools that the molecular biology revolution has given us is the ability to turn genes on and off at our discretion. In the mouse, this has been accomplished by using binary systems in which gene expression is dependent on the interaction of two components, resulting in either transcriptional transactivation or DNA recombination. During recent years, these systems have been used to analyse complex and multi-staged biological processes, such as embryogenesis and cancer, with unprecedented precision. Here, I review these systems and discuss certain studies that exemplify the advantages and limitations of each system.

[Hemifacial microsomia. Embryological and clinical approach]

Hemifacial microsomia is an otomandibular dysplasia which includes congenital malformations affecting the jaw and ear apparatus. The knowledge of normal embryonic development is a prerequisite for optimal clinical management of those malformations. The development of craniofacial structures is a multi-step process, which involves many developmental events ranging from the migration of neural crest cells from the neural folds of the young neurula embryo to molecular signaling interactions that coordinate outgrowth and patterning of the facial primordia. Our current knowledge of craniofacial development is limited, but the use of animal developmental models will contribute significantly to our understanding of human otomandibular dysplasias. In this review we discuss both the classical and current aspects of otomandibular development. A clinical approach to hemifacial microsomia is proposed. Current pathogenetic hypotheses of hemifacial microsomia and also mandibulofacial dysostosis are reviewed.

Genetics of craniofacial development and malformation

The head is anatomically the most sophisticated part of the body and its evolution was fundamental to the origin of vertebrates; understanding its development is a formidable problem in biology. A synthesis of embryology, evolution and mouse genetics is shaping our understanding of head development and in this review we discuss its application to studies of human craniofacial malformations. Many of these disorders have their origins in specific embryological processes, including abnormalities of brain patterning, of the migration and fusion of tissues in the face, and of bone differentiation in the skull vault.

Inactivation of the (β)-catenin gene by Wnt1-Cre-mediated deletion results in dramatic brain malformation and failure of craniofacial development

('bgr;)-Catenin is a central component of both the cadherin-catenin cell adhesion complex and the Wnt signaling pathway. We have investigated the role of (β)-catenin during brain morphogenesis, by specifically inactivating the (β)-catenin gene in the region of Wnt1 expression. To achieve this, mice with a conditional ('floxed') allele of (β)-catenin with required exons flanked by loxP recombination sequences were intercrossed with transgenic mice that expressed Cre recombinase under control of Wnt1 regulatory sequences. (β)-catenin gene deletion resulted in dramatic brain malformation and failure of craniofacial development. Absence of part of the midbrain and all of the cerebellum is reminiscent of the conventional Wnt1 knockout (Wnt1(−)(/)(−)), suggesting that Wnt1 acts through (β)-catenin in controlling midbrain-hindbrain development. The craniofacial phenotype, not observed in embryos that lack Wnt1, indicates a role for (β)-catenin in the fate of neural crest cells. Analysis of neural tube explants shows that (β)-catenin is efficiently deleted in migrating neural crest cell precursors. This, together with an increased apoptosis in cells migrating to the cranial ganglia and in areas of prechondrogenic condensations, suggests that removal of (β)-catenin affects neural crest cell survival and/or differentiation. Our results demonstrate the pivotal role of (β)-catenin in morphogenetic processes during brain and craniofacial development.

Formation of the middle ear: recent progress on the developmental and molecular mechanisms

The middle ear allows animals to hear while moving in an aerial medium. It is composed of a cavity harbouring a chain of three ossicles that transmit vibrations produced by airborne sound in the tympanic membrane into the inner ear, where they are converted into neural impulses. The middle ear develops in the branchial arches, and this requires sequential interactions between the epithelia and the underlying mesenchyme. Gene-inactivation experiments have identified genes required for the formation of different middle ear components. Some encode for signalling molecules, including Endothelin1 and Fgf8, probable mediators of epithelial-mesenchymal interactions. Other genes, including Eya1, Prx1, Hoxa1, Hoxa2, Dlx1, Dlx2, Dlx5, and Gsc, are most likely involved in patterning and morphogenetic processes in the neural crest-derived mesenchyme. Mechanisms controlling formation of a functional tympanic membrane are also discussed. Basically, the tympanic ring, which serves as support for the tympanic membrane, directs invagination of the first pharyngeal cleft ectoderm to form the external acoustic meatus (EAM), which provides the outer layer of the membrane. Gsc and Prx1 are essential for tympanic ring development. While invaginating, the EAM controls skeletogenesis in the underlying mesenchyme to form the manubrium of the malleus, the link between the membrane and the middle ear ossicles.

Teeth outside the mouth in teleost fishes: how to benefit from a developmental accident

Evolution proceeds by the selection of characters that enhance survival rates so that the long-term outcome for a species is better adaptation to its environment. These new characters are "accidentally" acquired, mostly through mutations leading to modifications of developmental events. However, changes that lead to the ectopic expression of an organ are rare and, whereas their subsequent selection for a new role is even more rare, such a scenario has apparently occurred for denticles in some teleost fish. Small, conical denticles are present, mainly on the dermal bones of the head, in a few, unrelated lineages of living teleosts. Here, I show that the morphology and structure of the denticles in Atherion elymus, an atheriniform, is similar to those of teeth inside the oral cavity. These denticles are not derived evolutionarily from odontodes of early vertebrates, nor do they represent a re-expression as such (i.e., a long-lasting ability to make odontodes outside the oral cavity). Teeth and odontodes are homologous organs but the origin of the denticles is to be found in teeth, not in odontodes. The denticles are simply teeth that form outside the mouth, probably derived from a sub-population of odontogenically pre-specified neural crest cells. These "accidental" extra-oral teeth have arisen independently in these lineages and were selectively advantageous in a hydrodynamic context.

Stem cell potential: can anything make anything?

Recent results suggest that stem cells from one tissue can give rise to cells from developmentally unrelated tissues. These results strongly support the idea that certain progenitors retain much broader developmental potentials than expected, and other progenitors may be able to acquire broader potentials in culture.

Derivation of the mammalian skull vault

This review describes the evolutionary history of the mammalian skull vault as a basis for understanding its complex structure. Current information on the developmental tissue origins of the skull vault bones (mesoderm and neural crest) is assessed for mammals and other tetrapods. This information is discussed in the context of evolutionary changes in the proportions of the skull vault bones at the sarcopterygian-tetrapod transition. The dual tissue origin of the skull vault is considered in relation to the molecular mechanisms underlying osteogenic cell proliferation and differentiation in the sutural growth centres and in the proportionate contributions of different sutures to skull growth.

Construction for the modern head: current concepts in craniofacial development

The vertebrate head is a highly complex composite structure whose morphological characteristics are controlled at the level of the gene. There is now increasing evidence for the role of gene families that encode transcription factors in determining the embryonic plan of the developing craniofacial complex. These genes act as regulators of gene transcription being intimately involved with the control of complex interactions between multiple downstream genes. Combinatorial expression of the Hox genes (a family of highly conserved master regulatory genes related to the homeotic genes of the fruitfly Drosophila) have been shown to play a definitive role in patterning distinct regions of the craniofacial complex. In the vertebrate, Hox genes pattern the hindbrain and branchial regions of the developing head up to and including structures derived from the second branchial arch. The first branchial arch and more rostral regions of the head are patterned by groups of homeobox genes more diverged from the original Hox clusters. Transgenic mice, with targeted disruptions in many of these genes, are now providing insights into the molecular mechanisms that lie behind a number of craniofacial defects seen in man.

Fate of the mammalian cardiac neural crest

A subpopulation of neural crest termed the cardiac neural crest is required in avian embryos to initiate reorganization of the outflow tract of the developing cardiovascular system. In mammalian embryos, it has not been previously experimentally possible to study the long-term fate of this population, although there is strong inference that a similar population exists and is perturbed in a number of genetic and teratogenic contexts. We have employed a two-component genetic system based on Cre/lox recombination to label indelibly the entire mouse neural crest population at the time of its formation, and to detect it at any time thereafter. Labeled cells are detected throughout gestation and in postnatal stages in major tissues that are known or predicted to be derived from neural crest. Labeling is highly specific and highly efficient. In the region of the heart, neural-crest-derived cells surround the pharyngeal arch arteries from the time of their formation and undergo an altered distribution coincident with the reorganization of these vessels. Labeled cells populate the aorticopulmonary septum and conotruncal cushions prior to and during overt septation of the outflow tract, and surround the thymus and thyroid as these organs form. Neural-crest-derived mesenchymal cells are abundantly distributed in midgestation (E9.5-12.5), and adult derivatives of the third, fourth and sixth pharyngeal arch arteries retain a substantial contribution of labeled cells. However, the population of neural-crest-derived cells that infiltrates the conotruncus and which surrounds the noncardiac pharyngeal organs is either overgrown or selectively eliminated as development proceeds, resulting for these tissues in a modest to marginal contribution in late fetal and postnatal life.