Synergy betweenHoxa1andHoxb1: the relationship between arch patterning and the generation of cranial neural crest

Craniofacial malformations: intrinsic vs extrinsic neural crest cell defects in Treacher Collins and 22q11 deletion syndromes

The craniofacial complex is anatomically the most sophisticated part of the body. It houses all the major sensory organ systems and its origins are synonymous with vertebrate evolution. Of fundamental importance to craniofacial development is a specialized population of stem and progenitor cells, known as the neural crest, which generate the majority of the bone, cartilage, connective and peripheral nerve tissue in the head. Approximately one third of all congenital abnormalities exhibit craniofacial malformations and consequently, most craniofacial anomalies are considered to arise through primary defects in neural crest cell development. Recent advances however, have challenged this classical dogma, underscoring the influence of tissues with which the neural crest cells interact as the primary origin of patterning defects in craniofacial morphogenesis. In this review we discuss these neural crest cell interactions with mesoderm, endoderm and ectoderm in the head in the context of a better understanding of craniofacial malformations such as in Treacher Collins and 22q11 deletion syndromes.

The role of the endoderm in the development and evolution of the pharyngeal arches

The oro-pharyngeal apparatus has its origin in a series of bulges found on the lateral surface of the embryonic head, the pharyngeal arches. Significantly, the development of these structures is extremely complex, involving interactions between a number of disparate embryonic cell types: ectoderm, endoderm, mesoderm and neural crest, each of which generates particular components of the arches, and whose development must be co-ordinated to generate the functional adult oro-pharyngeal apparatus. In the past most studies have emphasized the role played by the neural crest, which generates the skeletal elements of the arches, in directing pharyngeal arch development. However, it is now apparent that the pharyngeal endoderm plays an important role in directing arch development. Here we discuss the role of the pharyngeal endoderm in organizing the development of the pharyngeal arches, and the mechanisms that act to pattern the endoderm itself and those which direct its morphogenesis. Finally, we discuss the importance of modification to the pharyngeal endoderm during vertebrate evolution. In particular, we focus on the emergence of the parathyroid gland, which we have recently shown to be the result of the internalization of the gills.

Recent advances in craniofacial morphogenesis

Craniofacial malformations are involved in three fourths of all congenital birth defects in humans, affecting the development of head, face, or neck. Tremendous progress in the study of craniofacial development has been made that places this field at the forefront of biomedical research. A concerted effort among evolutionary and developmental biologists, human geneticists, and tissue engineers has revealed important information on the molecular mechanisms that are crucial for the patterning and formation of craniofacial structures. Here, we highlight recent advances in our understanding of evo-devo as it relates to craniofacial morphogenesis, fate determination of cranial neural crest cells, and specific signaling pathways in regulating tissue-tissue interactions during patterning of craniofacial apparatus and the morphogenesis of tooth, mandible, and palate. Together, these findings will be beneficial for the understanding, treatment, and prevention of human congenital malformations and establish the foundation for craniofacial tissue regeneration.

Neural crest cell plasticity. size matters

Patterning and morphogenesis of neural crest-derived tissues within a developing vertebrate embryo rely on a complex balance between signals acquired by neural crest cells in the neuroepithelium during their formation and signals from the tissues that the neural crest cells contact during their migration. Axial identity of hindbrain neural crest is controlled by a combinatorial pattern of Hox gene expression. Cellular interactions that pattern neural crest involve signals from the same key molecular families that regulate other aspects of patterning and morphogenesis within a developing embryo, namely the BMP, SHH and FGF pathways. The developmental program that regulates neural crest cell fate is both plastic and fixed. As a cohort of interacting cells, neural crest cells carry information that directs the axial pattern and species-specific morphology of the head and face. As individual cells, neural crest cells are responsive to signals from each other as well as from non-neural crest tissues in the environment. General rules and fundamental mechanisms have been important for the conservation of basic patterning of neural crest, but exceptions are notable and relevant. The key to furthering our understanding of important processes such as craniofacial development will require a better characterization of the molecular determinants of the endoderm, ectoderm and mesoderm and the effects that these molecules have on neural crest cell development.

Specification of neural crest cell formation and migration in mouse embryos

Of all the model organisms used to study human development, rodents such as mice most accurately reflect human craniofacial development. Collective advances in mouse embryology and mouse genetics continue to shape our understanding of neural crest cell development and by extrapolation the etiology of human congenital head and facial birth defects. The aim of this review is to highlight the considerable progress being made in our understanding of cranial neural crest cell patterning in mouse embryos.

Neural crest contribution to mammalian tooth formation

The cranial neural crest cells, which are specialized cells of neural origin, are central to the process of mammalian tooth development. They are the only source of mesenchyme able to sustain tooth development, and give rise not only to most of the dental tissues, but also to the periodontium, the surrounding tissues that hold teeth in position. Tooth organogenesis is regulated by a series of interactions between cranial neural crest cells and the oral epithelium. In the development of a tooth, the epithelium covering the inside of the developing oral cavity provides the first instructive signals. Signaling molecules secreted by the oral epithelium 1) establish large cellular fields competent to form a specific tooth shape (mono- or multicuspid) along a proximodistal axis; 2) define an oral (capable of forming teeth) and non-oral mesenchyme along a rostrocaudal axis; and 3) position the sites of future tooth development. The critical information to model tooth shape resides later in the neural crest-derived mesenchyme. Cranial neural crest cells ultimately differentiate into highly specialized cell types to produce mature dental organs. Some cranial neural crest cells located in the dental pulp, however, maintain plasticity in their developmental potential up to postnatal life, offering new prospects for regeneration of dental tissues.

Non-neural ectoderm is really neural: evolution of developmental patterning mechanisms in the non-neural ectoderm of chordates and the problem of sensory cell homologies

In chordates, the ectoderm is divided into the neuroectoderm and the so-called non-neural ectoderm. In spite of its name, however, the non-neural ectoderm contains numerous sensory cells. Therefore, the term "non-neural" ectoderm should be replaced by "general ectoderm." At least in amphioxus and tunicates and possibly in vertebrates as well, both the neuroectoderm and the general ectoderm are patterned anterior/posteriorly by mechanisms involving retinoic acid and Hox genes. In amphioxus and tunicates the ectodermal sensory cells, which have a wide range of ciliary and microvillar configurations, are mostly primary neurons sending axons to the CNS, although a minority lack axons. In contrast, vertebrate mechanosensory cells, called hair cells, are all secondary neurons that lack axons and have a characteristic eccentric cilium adjacent to a group of microvilli of graded lengths. It has been highly controversial whether the ectodermal sensory cells in the oral siphons of adult tunicates are homologous to vertebrate hair cells. In some species of tunicates, these cells appear to be secondary neurons, and microvillar and ciliary configurations of some of these cells approach those of vertebrate hair cells. However, none of the tunicate cells has all the characteristics of a hair cell, and there is a high degree of variation among ectodermal sensory cells within and between different species. Thus, similarities between the ectodermal sensory cells of any one species of tunicate and craniate hair cells may well represent convergent evolution rather than homology.

Specification and patterning of neural crest cells during craniofacial development

Craniofacial evolution is considered fundamental to the origin of vertebrates and central to this process was the formation of a migratory, multipotent cell population known as the neural crest. The number of cell types that arise from the neural crest is truly astonishing as is the number of tissues and organs to which the neural crest contributes. In addition to forming melanocytes as well as many neurons and glia in the peripheral nervous system, neural crest cells also contribute much of the cartilage, bone and connective tissue of the face. These multipotent migrating cells are capable of self renewing decisions and based upon these criteria are often considered stem cells or stem cell-like. Rapid advances in our understanding of neural crest cell patterning continue to shape our appreciation of the evolution of neural crest cells and their impact on vertebrate craniofacial morphogenesis.

Molecular Regulation of Müllerian Development by Hox Genes

HOX genes are a family of regulatory molecules that encode conserved transcription factors controlling aspects of morphogenesis and cell differentiation during normal embryonic development. All metazoans possess a common genetic system for embryonic patterning, and this system is also used in the reproductive tract. Hox genes are also expressed in the adult uterus. Hox genes are essential both for the development of mullerian tract in the embryonic period and adult function. Sex steroids regulate Hox gene expression during embryonic and endometrial development in the menstrual cycle. EMX2 and beta(3)-integrin acting downstream of Hoxa10 gene are likely involved in both these developmental processes. This article reviews the role and molecular regulation of Hox genes in reproductive tract development.

Loss of function but no gain of function caused by amino acid substitutions in the hexapeptide of Hoxa1 in vivo

Homeodomain containing transcription factors of the Hox family play critical roles in patterning the anteroposterior embryonic body axis, as well as in controlling several steps of organogenesis. Several Hox proteins have been shown to cooperate with members of the Pbx family for the recognition and activation of identified target enhancers. Hox proteins contact Pbx via a conserved hexapeptide motif. Previous biochemical studies provided evidence that critical amino acid substitutions in the hexapeptide sequence of Hoxa1 abolish its interaction with Pbx. As a result, these substitutions also abolish Hoxa1 activity on known target enhancers in cellular models, suggesting that Hoxa1 activity relies on its capacity to interact with Pbx. Here, we show that mice with mutations in the Hoxa1 hexapeptide display hindbrain, cranial nerve, and skeletal defects highly reminiscent of those reported for the Hoxa1 loss of function. Since similar hexapeptide mutations in the mouse Hoxb8 and the Drosophila AbdA proteins result in activity modulation and gain of function, our data demonstrate that the functional importance of the hexapeptide in vivo differs according to the Hox proteins.

Gene-regulatory interactions in neural crest evolution and development

In this review, we outline the gene-regulatory interactions driving neural crest development and compare these to a hypothetical network operating in the embryonic ectoderm of the cephalochordate amphioxus. While the early stages of ectodermal patterning appear conserved between amphioxus and vertebrates, later activation of neural crest-specific factors at the neural plate border appears to be a vertebrate novelty. This difference may reflect co-option of genetic pathways which conferred novel properties upon the evolving vertebrate neural plate border, potentiating the evolution of definitive neural crest.

AP-2 and HNK-1 define distinct populations of cranial neural crest cells

OBJECTIVE

To determine if distinct populations of cranial neural crest cells (CNCC) exist by characterization of their divergent gene expression patterns.

DESIGN

Identification of unique populations of CNCC was determined by a combination of lineage and immunohistochemical analyses.

SETTING

Department of Pathology, Children's Hospital of Philadelphia and the University of Pennsylvania School of Medicine, Philadelphia, PA 19104.

RESULTS

We found antibodies of two proteins previously described as identifying all CNCC, label three populations of CNCC at specific time-points. Furthermore, the activating protein 2 (AP-2) expressing CNCC become neural or mesenchymal NCC derivatives whereas the HNK-1 labeled cells do not participate in the mesenchymal lineage.

CONCLUSION

These data provide molecular markers for unique CNCC fates and thus will be invaluable in the characterizing of craniofacial anomalies related to defects in NCCS. In addition, our data suggest AP-2 may function in determining the unique mesenchymal fate of CNCCs.

Expression of mouse fibroblast growth factor and fibroblast growth factor receptor genes during early inner ear development

The inner ear, which mediates hearing and equilibrium, develops from an ectodermal placode located adjacent to the developing hindbrain. Induction of the placode and its subsequent morphogenesis and differentiation into the inner ear epithelium and its sensory neurons, involves signalling interactions within and between otic and non-otic tissues. Several members of the fibroblast growth factor (FGF) family play important roles at various stages of otic development; however, there are additional family members that have not been evaluated. In this study, we surveyed the expression patterns of 18 mouse Fgf and 3 Fgf receptor (Fgfr) genes during early otic development. Two members of the Fgf family, Fgf4 and Fgf16, and all three tested members of the Fgfr family, Fgfr2c, Fgfr3c, and Fgfr4, were expressed in tissues relevant to inner ear development. Fgf4 transcripts were expressed in the preplacodal and placodal ectoderm, suggesting potential roles in placode induction and/or maintenance. Fgf16 was expressed in the posterior otic cup and vesicle, suggesting roles in otic cell fate decisions and/or axis formation.

The role of cell mixing in branchial arch development

Compartmental structures are the basis of a number of developing systems, including parts of the vertebrate head. One of the characteristics of a series of compartments is that mixing between cells in adjacent units is restricted. This is a consequence of differential chemoaffinity between neighbouring cells in adjacent compartments. We set out to determine whether mesenchymal cells in the branchial arches and their precursors show cell-mixing properties consistent with a compartmental organisation. In chimaeric avian embryos we found no evidence of preferential association or segregation of neural crest cells when surrounded by cells derived from a different axial level. In reassociation assays using mesenchymal cells isolated from chick branchial arches at stage 18, cells reformed into clusters without exhibiting a preferential affinity for cells derived from the same branchial arch. We find no evidence for differential chemoaffinity in vivo or in vitro between mesenchymal cells in different branchial arches. Our findings suggest that branchial arch mesenchyme is not organised into a series of compartments.

Aortic arch and pharyngeal phenotype in the absence of BMP-dependent neural crest in the mouse

Neural crest cells are essential for proper development of a variety of tissues and structures, including peripheral and autonomic nervous systems, facial skeleton, aortic arches and pharyngeal glands like the thymus and parathyroids. Previous work has shown that bone morphogenic protein (BMP) signalling is required for the production of migratory neural crest cells that contribute to the neurogenic and skeletogenic lineages. We show here that BMP-dependent neural crest cells are also required for development of the embryonic aortic arches and pharynx-derived glands. Blocking formation or migration of this crest cell population from the caudal hindbrain resulted in strong phenotypes in the cardiac outflow tract and the thymus. Thymic aplasia or hypoplasia occurs despite uncompromised gene induction in the pharyngeal endoderm. In addition, when hypoplastic thymic tissue is found, it is ectopically located, but functional in thymopoiesis. Our data indicate that thymic phenotypes produced by neural crest deficits result from aberrant formation of pharyngeal pouches and impaired migration of thymic primordia because the mesenchymal content in the branchial arches is below a threshold level.

Targeted insertion results in a rhombomere 2-specific Hoxa2 knockdown and ectopic activation of Hoxa1 expression

Recent studies indicated that retention of selectable marker cassettes in targeted Hox loci may cause unexpected phenotypes in mutant mice, due to neighborhood effects. However, the molecular mechanisms have been poorly investigated. Here, we analysed the effects of the targeted insertion of a PGK-neo cassette in the 3' untranslated region of Hoxa2. Even at this 3' position, the insertion resulted in homozygous mutants that unexpectedly did not survive beyond 3 weeks of age. Molecular analysis of the targeted allele revealed a selective "knockdown" of Hoxa2 expression in rhombomere 2 and associated patterning abnormalities. Moreover, Hoxa1 was ectopically expressed in the hindbrain and branchial arches of mutant embryos. Of interest, we demonstrated that the ectopic expression was due to the generation of neo-Hoxa1 fusion transcripts, resulting from aberrant alternative splicing. These defects could be rescued after removal of the PGK-neo cassette by Flp-mediated recombination. These results underscore the complexity of transcriptional regulation at Hox loci and provide insights into the in vivo regulation of Hoxa2 segmental expression. They also provide a molecular basis for the interpretation of unexpected Hox knockout phenotypes in which the targeted selectable marker is retained in the locus.

Krox20 and kreisler co-operate in the transcriptional control of segmental expression of Hoxb3 in the developing hindbrain

In the segmented vertebrate hindbrain, the Hoxa3 and Hoxb3 genes are expressed at high relative levels in the rhombomeres (r) 5 and 6, and 5, respectively. The single enhancer elements responsible for these activities have been identified previously and shown to constitute direct targets of the transcription factor kreisler, which is expressed in r5 and r6. Here, we have analysed the contribution of the transcription factor Krox20, present in r3 and r5. Genetic analyses demonstrated that Krox20 is required for activity of the Hoxb3 r5 enhancer, but not of the Hoxa3 r5/6 enhancer. Mutational analysis of the Hoxb3 r5 enhancer, together with ectopic expression experiments, revealed that Krox20 binds to the enhancer and synergizes with kreisler to promote Hoxb3 transcription, restricting enhancer activity to their domain of overlap, r5. These analyses also suggested contributions from an Ets-related factor and from putative factors likely to heterodimerize with kreisler. The integration of multiple independent inputs present in overlapping domains by a single enhancer is likely to constitute a general mechanism for the patterning of subterritories during vertebrate development.

Hox genes, neural crest cells and branchial arch patterning

Proper craniofacial development requires the orchestrated integration of multiple specialized tissue interactions. Recent analyses suggest that craniofacial development is not dependent upon neural crest pre-programming as previously thought but is regulated by a more complex integration of cell and tissue interactions. In the absence of neural crest cells it is still possible to obtain normal arch patterning indicating that neural crest is not responsible for patterning all of arch development. The mesoderm, endoderm and surface ectoderm tissues play a role in the patterning of the branchial arches, and there is now strong evidence that Hoxa2 acts as a selector gene for the pathways that govern second arch structures.