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

The development of the neural crest in the human

The first systematic account of the neural crest in the human has been prepared after an investigation of 185 serially sectioned staged embryos, aided by graphic reconstructions. As many as fourteen named topographical subdivisions of the crest were identified and eight of them give origin to ganglia (Table 2). Significant findings in the human include the following. (1) An indication of mesencephalic neural crest is discernible already at stage 9, and trigeminal, facial, and postotic components can be detected at stage 10. (2) Crest was not observed at the level of diencephalon 2. Although pre-otic crest from the neural folds is at first continuous (stage 10), crest-free zones are soon observable (stage 11) in Rh.1, 3, and 5. (3) Emigration of cranial neural crest from the neural folds at the neurosomatic junction begins before closure of the rostral neuropore, and later crest cells do not accumulate above the neural tube. (4) The trigeminal, facial, glossopharyngeal and vagal ganglia, which develop from crest that emigrates before the neural folds have fused, continue to receive contributions from the roof plate of the neural tube after fusion of the folds. (5) The nasal crest and the terminalis-vomeronasal complex are the last components of the cranial crest to appear (at stage 13) and they persist longer. (6) The optic, mesencephalic, isthmic, accessory, and hypoglossal crest do not form ganglia. Cervical ganglion 1 is separated early from the neural crest and is not a Froriep ganglion. (7) The cranial ganglia derived from neural crest show a specific relationship to individual neuromeres, and rhombomeres are better landmarks than the otic primordium, which descends during stages 9-14. (8) Epipharyngeal placodes of the pharyngeal arches contribute to cranial ganglia, although that of arch 1 is not typical. (9) The neural crest from rhombomeres 6 and 7 that migrates to pharyngeal arch 3 and from there rostrad to the truncus arteriosus at stage 12 is identified here, for the first time in the human, as the cardiac crest. (10) The hypoglossal crest provides cells that accompany those of myotomes 1-4 and form the hypoglossal cell cord at stages 13 and 14. (11) The occipital crest, which is related to somites 1-4 in the human, differs from the spinal mainly in that it does not develop ganglia. (12) The occipital and spinal portions of the crest migrate dorsoventrad and appear to traverse the sclerotomes before the differentiation into loose and dense zones in the latter. (13) Embryonic examples of synophthalmia and anencephaly are cited to emphasize the role of the neural crest in the development of cranial ganglia and the skull.

Neural crest cell deficiency of c-myc causes skull and hearing defects

The proto-oncogene c-myc has a central role in multiple processes important for embryonic development, including cell proliferation, growth, apoptosis, and differentiation. We have investigated the role of c-myc in neural crest by using Wnt1-Cre-mediated deletion of a conditional mutation of the c-myc gene. c-myc deficiency in neural crest resulted in viable adult mice that have defects in coat color, skull frontal bone, and middle ear ossicle development. Physiological hearing studies demonstrated a significant hearing deficit in the mutant mice. In this report, we focus on the craniofacial and hearing defects. To further examine neural crest cells affected by c-myc deficiency, we fate mapped Wnt1-Cre expressing neural crest cells using the ROSA26 Cre reporter transgene. The phenotype obtained demonstrates the critical role that c-myc has in neural crest during craniofacial development as well as in providing a model for examining human congenital skull defects and deafness.

Neural Crest Cells and the Community of Plan for Craniofacial Development

After their initial discovery in the mid 1800s, neural crest cells transitioned from the category of renegade intra-embryonic wanderers to achieve rebel status, provoked especially by the outrageous claim that they participate in skeletogenesis, an embryonic event theretofore reserved exclusively for mesoderm. Much of the 20th century found neural crest cells increasingly viewed as a unique population set apart from other embryonic populations and more often treated as orphans rather than fully embraced by mainstream developmental biology. Now frequently touted as a fourth germ layer, the neural crest has become a fundamental character for distinguishing craniates from other metazoans, and has radically redefined perceptions about the organization and evolution of the vertebrate jaws and head. In this chapter we provide an historical overview of four main research areas in which the neural crest have incited fervent discord among workers past and present. Specifically, we describe how discussions surrounding the neural crest threatened the germ layer theory, upended traditional schemes of vertebrate head organization, challenged assumptions about morphological conservation and homology, and redefined concepts on mechanisms of craniofacial patterning. In each case we frame these debates in the context of recent data on the developmental fate and roles of the neural crest.

Neural Crest Stem Cells

Stem cells are defined by their ability to both self-renew and give rise to multiple lineages in vivo and/or in vitro. As discussed in other chapters in this volume, the embryonic neural crest is a multipotent tissue that gives rise to a plethora of differentiated cell types in the adult organism and is unique to vertebrate embryos. From the point of view of stem cell biology, the neural crest is an ideal source for multipotent adult stem cells. Significant advances have been made in the past few years isolating neural crest stem cell lines that can be maintained in vitro and can give rise to many neural crest derivatives either in vitro or when placed back into the context of an embryo. The initial work identifying these stem cells was carried out with premigratory neural crest from the embryonic neural tube. Later, neural crest stem cells were isolated from postmigratory neural crest, presumably more restricted in developmental potential. More recently it has been demonstrated that neural crest stem cell progenitors persist in the adult in at least two differentiated tissues, the enteric nervous system of the gut and the whisker follicles of the facial skin. In all cases, the properties of the stem cells derived reflect their tissue of origin and the potential of the progenitors becomes more restricted with age. In this chapter we will review this work and speculate on future possibilities with respect to combining our knowledge of neural crest gene function in the embryo and the manipulation of adult neural crest stem cells in vitro and eventually in vivo.

A hierarchical NGF signaling cascade controls Ret-dependent and Ret-independent events during development of nonpeptidergic DRG neurons

NGF controls survival, differentiation, and target innervation of both peptidergic and nonpeptidergic DRG sensory neurons. The common receptor for GDNF family ligands, Ret, is highly expressed in nonpeptidergic neurons, but its function during development of these neurons is unclear. Here, we show that expression of Ret and its coreceptors GFRalpha1 and GFRalpha2 is dependent on NGF. GFR/Ret signaling, in turn, autoregulates expression of both GFRalpha1 and GFRalpha2 and promotes expression of TrpA1, MrgA1, MrgA3, and MrgB4, acquisition of normal neuronal size, axonal innervation of the epidermis, and postnatal extinction of the NGF receptor TrkA. Moreover, NGF controls expression of several other genes characteristic of nonpeptidergic neurons, such as TrpC3, TrpM8, MrgD, and the transcription factor Runx1, via a Ret-independent signaling pathway. These findings support a model in which NGF controls maturation of nonpeptidergic DRG neurons through a combination of GFR/Ret-dependent and -independent signaling pathways.

Cell autonomous requirement for TGF-beta signaling during odontoblast differentiation and dentin matrix formation

TGF-beta subtypes are expressed in tissues derived from cranial neural crest cells during early mouse craniofacial development. TGF-beta signaling is critical for mediating epithelial-mesenchymal interactions, including those vital for tooth morphogenesis. However, it remains unclear how TGF-beta signaling contributes to the terminal differentiation of odontoblast and dentin formation during tooth morphogenesis. Towards this end, we generated mice with conditional inactivation of the Tgfbr2 gene in cranial neural crest derived cells. Odontoblast differentiation was substantially delayed in the Tgfbr2(fl/fl);Wnt1-Cre mutant mice at E18.5. Following kidney capsule transplantation, Tgfbr2 mutant tooth germs expressed a reduced level of Col1a1 and Dspp and exhibited defects including decreased dentin thickness and absent dentinal tubules. In addition, the expression of the intermediate filament nestin was decreased in the Tgfbr2 mutant samples. Significantly, exogenous TGF-beta2 induced nestin and Dspp expression in dental pulp cells in the developing tooth organ. Our data suggest that TGF-beta signaling controls odontoblast maturation and dentin formation during tooth morphogenesis.

Expression of the epithelial marker E-cadherin by thyroid C cells and their precursors during murine development

Studies of chick-quail chimeras have reported that avian ultimobranchial C cells originate from the neural crest. It has consequently been assumed, without much supporting evidence, that mammalian thyroid C cells also originate from the neural crest. To test this notion, we employed both Connexin43-lacZ and Wnt1-Cre/R26R transgenic mice, because their neural crest cells can be marked. We also examined the immunohistochemical expression of a number of markers that identify migratory or postmigratory neural crest cells, namely, TuJ1, neurofilament 160, nestin, P75NTR, and Sox10. Moreover, we examined the expression of E-cadherin, an epithelial cell marker. At embryonic day (E)10.5, the neural crest cells densely populated the pharyngeal arches but were not distributed in the pharyngeal pouches, including the fourth pouch. At E11.5, the ultimobranchial rudiment formed from the fourth pouch and was located close to the fourth arch artery. At E13.0, this organ came into contact with the thyroid lobe, and at E13.5, it fused with this lobe. However, the ultimobranchial body was not colonized by neural crest-derived cells at any of these developmental stages. Instead, all ultimobranchial cells, as well as the epithelium of the fourth pharyngeal pouch, were intensely immunoreactive for E-cadherin. Furthermore, confocal microscopy of newborn mouse thyroid glands revealed colocalization of calcitonin and E-cadherin in the C cells. The cells, however, were not marked in the Wnt-Cre/R26R mice. These results indicated that murine thyroid C cells are derived from the endodermal epithelial cells of the fourth pharyngeal pouch and do not originate from neural crest cells.

Localization, quantification, and characterization of tuftelin in soft tissues

Tuftelin was initially found in the developing and mature extracellular enamel. Here we describe our novel discovery of tuftelin cellular distribution (protein and mRNA) in six soft tissues. The expression levels of tuftelin mRNA were significantly higher in mouse kidney and testis, in which oxygen levels are hovering closely to hypoxia under normal conditions.

Prostate Cancer Associated withp53andRbDeficiency Arises from the Stem/Progenitor Cell–Enriched Proximal Region of Prostatic Ducts

Recently, we have shown that prostate epithelium-specific deficiency for p53 and Rb tumor suppressors leads to metastatic cancer, exhibiting features of both luminal and neuroendocrine differentiation. Using stage-by-stage evaluation of carcinogenesis in this model, we report that all malignant neoplasms arise from the proximal region of the prostatic ducts, the compartment highly enriched for prostatic stem/progenitor cells. In close similarity to reported properties of prostatic stem cells, the cells of the earliest neoplastic lesions express stem cell marker stem cell antigen 1 and are not sensitive to androgen withdrawal. Like a subset of normal cells located in the proximal region of prostatic ducts, the early neoplastic cells coexpress luminal epithelium markers cytokeratin 8, androgen receptor, and neuroendocrine markers synaptophysin and chromogranin A. Inactivation of p53 and Rb also takes place in the lineage-committed transit-amplifying and/or differentiated cells of the distal region of the prostatic ducts. However, the resulting prostatic intraepithelial neoplasms never progress to carcinoma by the time of mouse death. Interestingly, in an ectopic transplantation assay, early mutant cells derived from either region of the prostatic ducts are capable of forming neoplasms within 3 months. These findings indicate that p53 and Rb are critically important for the regulation of the prostatic stem cell compartment, the transformation in which may lead to particularly aggressive cancers in the context of microenvironment.

Embryonic development of Python sebae – II: Craniofacial microscopic anatomy, cell proliferation and apoptosis

This study explores the microscopic craniofacial morphogenesis of the oviparous African rock python (Python sebae) spanning the first two-thirds of the post-oviposition period. At the time of laying, the python embryo consists of largely undifferentiated mesenchyme and epithelium with the exception of the cranial base and trabeculae cranii, which are undergoing chondrogenesis. The facial prominences are well defined and are at a late stage, close to the time when lip fusion begins. Later (11-12d), specializations in the epithelia begin to differentiate (vomeronasal and olfactory epithelia, teeth). Dental development in snakes is different from that of mammals in several aspects including an extended dental lamina with the capacity to form 4 sets of generational teeth. In addition, the ophidian olfactory system is very different from the mammalian. There is a large vomeronasal organ, a nasal cavity proper and an extraconchal space. All of these areas are lined with a greatly expanded olfactory epithelium. Intramembranous bone differentiation is taking place at stage 3 with some bones already ossifying whereas most are only represented as mesenchymal condensations. In addition to routine histological staining, PCNA immunohistochemistry reveals relatively higher levels of proliferation in the extending dental laminae, in osseous mesenchymal condensations and in the olfactory epithelia. Areas undergoing apoptosis were noted in the enamel organs of the teeth and osseous mesenchymal condensations. We propose that localized apoptosis helps to divide a single condensation into multiple ossification centres and this is a mechanism whereby novel morphology can be selected in response to evolutionary pressures. Several additional differences in head morphology between snakes and other amniotes were noted including a palatal groove separating the inner and outer row of teeth in the upper jaw, a tracheal opening within the tongue and a pharyngeal adhesion that closes off the pharynx from the oral cavity between stages 1 and 4. Our studies on these and other differences in the python will provide valuable insights into in developmental, molecular and evolutionary mechanisms of patterning.

Influence of extended operation time and of occlusal force on determination of pulpal healing pattern in replanted mouse molars

The mechanism regulating the divergent healing processes following tooth replantation is unclear. This study clarifies the relationship between the healing pattern, the time taken for tooth replantation, and the influence of occlusal force. We investigated the pulpal healing process after tooth replantation by immunohistochemistry for 5-bromo-2'-deoxyuridine and nestin and by histochemistry for tartrate-resistant acid phosphatase. The upper right first molar of 3-week-old mice was extracted and repositioned in the original socket immediately or 30 min to 6 h after the operation. We divided the animals into a non-occluded group in which the lower right first molar was extracted and an occluded group without extraction of the counterpart tooth. In control teeth (upper left first molar), the periphery of the coronal dental pulp showed intense nestin-positive reaction. Tooth replantation weakened the nestin-positive reaction in the pulp tissue. On postoperative days 5-7, tubular dentin formation commenced next to preexisting dentin in which nestin-positive odontoblast-like cells were arranged in successful cases. In other cases, bone-like tissue formation occurred in the pulp chamber until day 14. The ratio of tertiary dentin formation was significantly higher in the non-occluded group. The intentionally prolonged time for the completion of tooth replantation induced bone-like tissue formation, expanded inflammatory reaction, or fibrous tissue formation in pulp tissue. Thus, the lack of a proper oxygenated medium is probably decisive for the survival of odontoblast-lineage cells, and occlusal force during and/or after operation worsens the fate of these cells.

Neural crest progenitors and stem cells

In the vertebrate embryo, multiple cell types originate from a common structure, the neural crest (NC), which forms at the dorsal tips of the neural epithelium. The NC gives rise to migratory cells that colonise a wide range of embryonic tissues and later differentiate into neurones and glial cells of the peripheral nervous system (PNS), pigment cells (melanocytes) in the skin and endocrine cells in the adrenal and thyroid glands. In the head and the neck, the NC also yields mesenchymal cells that form craniofacial cartilages, bones, dermis, adipose tissue, and vascular smooth muscle cells. The NC is therefore a model system to study cell diversification during embryogenesis and phenotype maintenance in the adult. By analysing the developmental potentials of quail NC cells in clonal cultures, we have shown that the migratory NC is a collection of heterogeneous progenitors, including various types of intermediate precursors and highly multipotent cells, some of which being endowed of self-renewal capacity. We also have identified common progenitors for mesenchymal derivatives and neural/melanocytic cells in the cephalic NC. These results are consistent with a hierarchical model of lineage segregation wherein environmental cytokines control the fate of progenitors and stem cells. One of these cytokines, the endothelin3 peptide, promotes the survival, proliferation, and self-renewal capacity of common progenitors for glial cells and melanocytes. At post-migratory stages, when they have already differentiated, NC-derived cells exhibit phenotypic plasticity. Epidermal pigment cells and Schwann cells from peripheral nerves in single-cell culture are able to reverse into multipotent NC-like progenitors endowed with self-renewal. Therefore, stem cell properties are expressed by a variety of NC progenitors and can be re-acquired by differentiated cells of NC origin, suggesting potential function for repair.

EphB/ephrin-B interaction mediates adult stem cell attachment, spreading, and migration: implications for dental tissue repair

Human adult dental pulp stem cells (DPSCs) reside predominantly within the perivascular niche of dental pulp and are thought to originate from migrating neural crest cells during development. The Eph family of receptor tyrosine kinases and their ligands, the ephrin molecules, play an essential role in the migration of neural crest cells during development and stem cell niche maintenance. The present study examined the expression and function of the B-subclass Eph/ephrin molecules on DPSCs. Multiple receptors were primarily identified on DPSCs within the perivascular niche, whereas ephrin-B1 and ephrin-B3 were expressed by the surrounding pulp tissue. EphB/ephrin-B bidirectional signaling inhibited cell attachment and spreading, predominately via the mitogen-activated protein kinase (MAPK) pathway for forward signaling and phosphorylation of Src family tyrosine kinases via reverse ephrin-B signaling. DPSC migration was restricted through unidirectional ephrin-B1-activated EphB forward signaling, primarily signaling through the MAPK pathway. Furthermore, we observed that ephrin-B1 was downregulated in diseased adult teeth compared with paired uninjured controls. Collectively, these studies suggest that EphB/ephrin-B molecules play a role in restricting DPSC attachment and migration to maintain DPSCs within their stem cell niche under steady-state conditions. These results may have implications for dental pulp development and regeneration.

Developmental biology and genetics of dental malformations

The synthesis of tooth development biology with human studies focusing on inherited conditions that specifically interfere with tooth development is improving our understanding of normal and pathological tooth formation. The type of inherited dental malformations observed in a given kindred relate to when, during odontogenesis, the defective gene is critically expressed. Information about the protein encoded by the defective gene and the resulting dental phenotype helps us understand the major processes underway at different stages during tooth development. Genes affecting early tooth development (PAX9, MSX1, and AXIN2) are associated with familial tooth agenesis or oligodontia. Genes expressed by odontoblasts (COL1A1, COL1A2, and DSPP), and ameloblasts (AMELX, ENAM, MMP20, and KLK4) during the crown formation stage, are associated with dentinogenesis imperfecta, dentin dysplasia, and amelogenesis imperfecta. Late genes expressed during root formation (ALPL and DLX3) are associated with cementum agenesis (hypophosphatasia) and taurodontism. Understanding the relationships between normal tooth development and the dental pathologies associated with inherited diseases improves our ability to diagnose and treat patients suffering the manifestations of inherited dental disorders.

Stem cells and the dental pulp: potential roles in dentine regeneration and repair

The dentine-pulp complex displays exquisite regenerative potential in response to injury. The postnatal dental pulp contains a variety of potential progenitor/stem cells, which may participate in dental regeneration. A population of multipotent mesenchymal progenitor cells known as dental pulp stem cells with high proliferative potential for self-renewal has been described and may be important to the regenerative capacity of the tissue. The nature of the progenitor/stem cell populations in the pulp is of importance in understanding their potentialities and development of isolation or recruitment strategies, and allowing exploitation of their use in regeneration and tissue engineering. Various strategies will be required to ensure not only effective isolation of these cells, but also controlled signalling of their differentiation and regulation of secretory behaviour. Characterization of these cells and determination of their potentialities in terms of specificity of regenerative response will form the foundation for development of new clinical treatment modalities, whether involving directed recruitment of the cells and seeding of stem cells at sites of injury for regeneration or use of the stem cells with appropriate scaffolds for tissue engineering solutions. Such approaches will provide an innovative and novel biologically based new generation of clinical treatments for dental disease.

Molecular neuroanatomy's "Three Gs": a primer

New genetic technologies are transforming nervous system studies in mice, impacting fields from neural development to the neurobiology of disease. Of necessity, alongside these methodological advances, new concepts are taking shape with respect to both vocabulary and form. Here we review aspects of both burgeoning areas. Presented are technologies which, by co-opting site-specific recombinase systems, enable select genes to be turned on or off in specific brain cells of otherwise undisturbed mouse embryos or adults. Manipulated genes can be endogenous loci or inserted transgenes encoding reporter, sensor, or effector molecules, making it now possible to assess not only gene function, but also cell function, origin, fate, connectivity, and behavioral output. From these methodological advances, a new form of molecular neuroscience is emerging that may be said to lean on the concepts of genetic access, genetic lineage, and genetic anatomy – the three ‘Gs’ – much like a general education rests on the basics of reading, ‘riting and ‘rithmetic.

Cranial muscle defects of Pitx2 mutants result from specification defects in the first branchial arch

Pitx2 expression is observed during all states of the myogenic progression in embryonic muscle anlagen and persists in adult muscle. Pitx2 mutant mice form all but a few muscle anlagen. Loss or degeneration in muscle anlagen could generally be attributed to the loss of a muscle attachment site induced by some other aspect of the Pitx2 phenotype. Muscles derived from the first branchial arch were absent, whereas muscles derived from the second branchial arch were merely distorted in Pitx2 mutants at midgestation. Pitx2 was expressed well before, and was required for, initiation of the myogenic progression in the first, but not second, branchial arch mesoderm. Pitx2 was also required for expression of premyoblast specification markers Tbx1, Tcf21, and Msc in the first, but not second, branchial arch. First, but not second, arch mesoderm of Pitx2 mutants failed to enlarge after embryonic day 9.5, well before the onset of the myogenic progression. Thus, Pitx2 contributes to specification of first, but not second, arch mesoderm. The jaw of Pitx2 mutants was vestigial by midgestation, but significant size reductions were observed as early as embryonic day 10.5. The diminutive first branchial arch of mutants could not be explained by loss of mesoderm alone, suggesting that Pitx2 contributes to the earliest specification of jaw itself.

Myocardin-related transcription factor B is required for normal mouse vascular development and smooth muscle gene expression

Smooth muscle gene expression is required for the proper development and function of multiple organ systems. Expression of smooth muscle genes is critical for contractile function and tissue architectural integrity. One critical transcription factor for smooth muscle gene expression is the Serum Response Factor (SRF). SRF is expressed ubiquitously, but tissue-specific transcriptional regulation is conferred by its binding to cofactors such as myocardin. Myocardin-related transcription factor B (MRTF-B) is a member of a family of genes (Myocardin(Myocd),Myocardin-related transcription factor A(MRTF-A),MRTF-B) that provides tissue-specificity and potentiate SRF-dependent transcription. Unlike myocardin, which is expressed specifically in smooth and cardiac muscle, MRTF-B is expressed in a wide variety of tissues. To examine the function of MRTF-B, we generated mice containing an insertional mutation of MRTF-B. MRTF-B homozygous mutants die in late gestation with vascular defects and liver hemorrhage. At E9.5, MRTF-B is expressed strongly in the septum transversum mesoderm critical for development of the vitelline system that produces the liver sinusoids and portal venous system. MRTF-B deficiency results in defective smooth muscle gene expression in the liver sinusoids, vitelline veins, and yolk sac, which contributes significantly to the lethal phenotype. These data support our hypothesis that MRTF-B has a unique role in regulating smooth muscle genes important for liver, yolk sac, and portal vascular development.

Neural crest and mesoderm lineage-dependent gene expression in orofacial development

The present study utilizes a combination of genetic labeling/selective isolation of pluripotent embryonic progenitor cells, and oligonucleotide-based microarray technology, to delineate and compare the "molecular fingerprint" of two mesenchymal cell populations from distinct lineages in the developing embryonic orofacial region. The first branchial arches-bi-lateral tissue primordia that flank the primitive oral cavity-are populated by pluripotent mesenchymal cells from two different lineages: neural crest (neuroectoderm)- and mesoderm-derived mesenchymal cells. These cells give rise to all of the connective tissue elements (bone, cartilage, smooth and skeletal muscle, dentin) of the orofacial region (maxillary and mandibular portion), as well as neurons and glia associated with the cranial ganglia, among other tissues. In the present study, neural crest- and mesoderm-derived mesenchymal cells were selectively isolated from the first branchial arch of gestational day 9.5 mouse embryos using laser capture microdissection (LCM). The two different embryonic cell lineages were distinguished through utilization of a novel two component transgenic mouse model (Wnt1Cre/ZEG) in which the neural crest cells and their derivatives are indelibly marked (i.e., expressing enhanced green fluorescent protein, EGFP) throughout the pre- and post-natal lifespan of the organism. EGFP-labeled neural crest-derived, and non-fluorescent mesoderm-derived mesenchymal cells from the first branchial arch were visualized in frozen tissue sections from gestational day 9.5 mouse embryos and independently isolated by LCM under epifluorescence optics. RNA was extracted from the two populations of LCM-procured cells, and amplified by double-stranded cDNA synthesis and in vitro transcription. Gene expression profiles of the two progenitor cell populations were generated via hybridization of the cell-type specific cRNA samples to oligo-based GeneChip microarrays. Comparison of gene expression profiles of neural crest- and mesoderm-derived mesenchymal cells from the first branchial arch revealed over 140 genes that exhibited statistically significant differential levels of expression. The gene products of many of these differentially expressed genes have previously been linked to the development of mesoderm- or neural crest-derived tissues in the embryo. Interestingly, however, hitherto uncharacterized coding sequences with highly significant differences in expression between the two embryonic progenitor cell types were also identified. These lineage-dependent mesenchymal cell molecular fingerprints offer the opportunity to elucidate additional mechanisms governing cellular growth, differentiation, and morphogenesis of the embryonic orofacial region. The chemokine stromal cell-derived factor 1, (SDF-1), was found to exhibit greater expression in mesoderm-derived mesenchyme in the branchial arch when compared with neurectoderm, suggesting a possible chemotactic role for SDF-1 in guiding the migratory neural crest cells to their destination. The novel combination of genetic labeling of the neural crest cell population by EGFP coupled with isolation of cells by LCM for gene expression analysis has enabled, for the first time, the generation of gene expression profiles of distinct embryonic cell lineages.

Bone marrow-derived mesenchymal stem cells for regenerative medicine in craniofacial region

The craniofacial region contains many specified tissues including bone, cartilage, muscle, blood vessels and neurons. Defect or dysfunction of the craniofacial tissue after post-cancer ablative surgery, trauma, congenital malformations and progressive deforming skeletal diseases has a huge influence on the patient's life. Therefore, functional reconstruction of damaged tissues is highly expected. Bone marrow-derived mesenchymal stem cells (BMMSCs) are one of the most well characterized postnatal stem cell populations, and considered to be utilized for cell-based clinical therapies. Here, the current understanding and the potential applications in craniofacial tissue regeneration of BMMSCs are reviewed, and the current limitations and drawbacks are also discussed.

Establishment and characterization of an immortomouse-derived odontoblast-like cell line to evaluate the effect of insulin-like growth factors on odontoblast differentiation

Insulin-like growth factors (IGF-I and IGF-II) play important roles in regulating growth and differentiation of many different organs including teeth. The presence of these factors in the developing tooth has been demonstrated. In vitro studies using tooth explants grown in the presence of IGFs suggest that they promote differentiation of ameloblast and odontoblasts cells. This is achieved by inducing or repressing gene expression associated with these cells. Since some of the genes involved in tooth differentiation are expressed by both cells, to determine the effect of IGF on odontoblast cell differentiation we first need a cell line in which a controlled environment can be created. In this study, we report the establishment and characterization of an Immortomouse-derived odontoblast-like cell line. This conditional cell line can grow indefinitely under permissive conditions in the presence of INF-gamma at 33 degrees C, differentiate into odontoblast-like cells and produce a mineralized extracellular matrix when the INF-gamma is removed and cell maintained at 39 degrees C. Addition of exogenous IGFs to the media results in an accelerated production of a mineralized matrix. This is the result of increased transcription of genes associated with bone mineralization while down regulating genes associated with dentin formation like DSPP. This data suggest that IGFs induce dental papillae mesenchyme cells to produce a bone-like mineralized extracellular matrix.

Advances in defining regulators of cementum development and periodontal regeneration

Substantial advancements have been made in defining the cells and molecular signals that guide tooth crown morphogenesis and development. As a result, very encouraging progress has been made in regenerating crown tissues by using dental stem cells and recombining epithelial and mesenchymal tissues of specific developmental ages. To date, attempts to regenerate a complete tooth, including the critical periodontal tissues of the tooth root, have not been successful. This may be in part due to a lesser degree of understanding of the events leading to the initiation and development of root and periodontal tissues. Controversies still exist regarding the formation of periodontal tissues, including the origins and contributions of cells, the cues that direct root development, and the potential of these factors to direct regeneration of periodontal tissues when they are lost to disease. In recent years, great strides have been made in beginning to identify and characterize factors contributing to formation of the root and surrounding tissues, that is, cementum, periodontal ligament, and alveolar bone. This review focuses on the most exciting and important developments over the last 5 years toward defining the regulators of tooth root and periodontal tissue development, with special focus on cementogenesis and the potential for applying this knowledge toward developing regenerative therapies. Cells, genes, and proteins regulating root development are reviewed in a question-answer format in order to highlight areas of progress as well as areas of remaining uncertainty that warrant further study.

Antifungal triazole derivative triadimefon induces ectopic maxillary cartilage by altering the morphogenesis of the first branchial arch

BACKGROUND

The triazole derivative, triadimefon (FON), induces branchial arch abnormalities in post-implantation rat embryos cultured in vitro, and cranio-facial malformations in mouse fetuses. Ectopic maxillary cartilage has been also described as a typical FON-related malformation. This work studies the morphogenesis of the ectopic cartilage in rat embryos and fetuses exposed in vivo to FON during the early postimplantation period.

METHODS

Pregnant rats were treated with 0, 250, and 500 mg/kg FON on Day 9.5 of pregnancy (D9.5) and sacrificed at term (D20), during the early fetal period (D17) or at different embryogenetic periods (D10, D11, D12). The skeleton was examined after stain of bone and cartilage or of cartilage alone respectively at term or at D17. The neural crest cell (NCC) migration and compaction was investigated at D10 and D11 and the cranial nerve organization described at D12.

RESULTS

Triadimefon is teratogenic in rats under the chosen experimental conditions. The malformations were at the level of the cranio-facial and axial skeleton at term and of the hindbrain nerves in embryos. A NCC abnormal migration and compaction was observed at the level of the first branchial arch: in FON-exposed embryos NCC were detected at the level of both maxillary and mandibular processes, whereas control embryos showed the immunostained tissue only at the level of the mandibular bud.

CONCLUSIONS

The pathogenic pathway, proposed to explain the ectopic cartilage, is the displacement of part of the NCC-derived tissues at the maxillary region of the first branchial arch.

Analysis of mouse development with conditional mutagenesis

Explorations into the molecular embryology of the mouse have played a vital role in our understanding of the basic mechanisms of gene regulation that govern development and disease. In the last 15 years, these mechanisms have been analyzed with vastly greater precision and clarity with the advent of systems that allow the conditional control of gene expression. Typically, this control is achieved by silencing or activating the gene of interest with site-specific DNA recombination or transcriptional transactivation. In this review, I discuss the application of these technologies to mouse development, focusing on recent innovations and experimental designs that specifically aid the study of the mouse embryo.

Ephrin-B2 forward signaling regulates somite patterning and neural crest cell development

Genetic studies in the mouse have implicated ephrin-B2 (encoded by the gene Efnb2) in blood vessel formation, cardiac development and remodeling of the lymphatic vasculature. Here we report that loss of ephrin-B2 leads to defects in populations of cranial and trunk neural crest cells (NCC) and to defective somite development. In addition, we show that Efnb1/Efnb2 double heterozygous embryos exhibit phenotypes in a number of NCC derivatives. Expression of one copy of a mutant version of Efnb2 that lacks tyrosine phosphorylation sites was sufficient to rescue the embryonic phenotypes associated with loss of Efnb2. Our results uncover an important role for ephrin-B2 in NCC and somites during embryogenesis and suggest that ephrin-B2 exerts many of its embryonic function via activation of forward signaling.

Application of lentivirus-mediated RNAi in studying gene function in mammalian tooth development

RNA interference (RNAi) has recently become a powerful tool to silence gene expression in mammalian cells, but its application in assessing gene function in mammalian developing organs remains highly limited. Here we describe several unique developmental properties of the mouse molar germ. Embryonic molar mesenchyme, but not the incisor mesenchyme, once dissociated into single cell suspension and re-aggregated, retains its odontogenic potential, the capability of a tissue to instruct an adjacent tissue to initiate tooth formation. Dissociated molar mesenchymal cells, even after being plated in cell culture, retain odontogenic competence, the capability of a tissue to respond to odontogenic signals and to support tooth formation. Most interestingly, while dissociated epithelial and mesenchymal cells of molar tooth germ are mixed and re-aggregated, the epithelial cells are able to sort out from the mesenchymal cells and organize into a well-defined dental epithelial structure, leading to the formation of a well-differentiated tooth organ after sub-renal culture. These unique molar developmental properties allow us to develop a strategy using a lentivirus-mediated RNAi approach to silence gene expression in dental mesenchymal cells and assess gene function in tooth development. We show that knockdown of Msx1 or Dlx2 expression in the dental mesenchyme faithfully recapitulates the tooth phenotype of their targeted mutant mice. Silencing of Barx1 expression in the dental mesenchyme causes an arrest of tooth development at the bud stage, demonstrating a crucial role for Barx1 in tooth formation. Our studies have established a reliable and rapid assay that would permit large-scale analysis of gene function in mammalian tooth development.

The role of TGF-beta signaling in regulating chondrogenesis and osteogenesis during mandibular development

During craniofacial development, Meckel's cartilage and the mandible bone derive from the first branchial arch, and their development depends upon the contribution of cranial neural crest (CNC) cells. We previously demonstrated that conditional inactivation of Tgfbr2 in the neural crest of mice (Tgfbr2(fl/fl);Wnt1-Cre) results in severe defects in mandibular development, although the specific cellular and molecular mechanisms by which TGF-beta signaling regulates the fate of CNC cells during mandibular development remain unknown. We show here that loss of Tgfbr2 does not affect the migration of CNC cells during mandibular development. TGF-beta signaling is specifically required for cell proliferation in Meckel's cartilage and the mandibular anlagen and for the formation of the coronoid, condyle and angular processes. TGF-beta-mediated connective tissue growth factor (CTGF) signaling is critical for CNC cell proliferation. Exogenous CTGF rescues the cell proliferation defect in Meckel's cartilage of Tgfbr2(fl/fl);Wnt1-Cre mutants, demonstrating the biological significance of this signaling cascade in chondrogenesis during mandibular development. Furthermore, TGF-beta signaling controls Msx1 expression to regulate mandibular osteogenesis as Msx1 expression is significantly reduced in Tgfbr2(fl/fl);Wnt1-Cre mutants. Collectively, our data suggest that there are differential signal cascades in response to TGF-beta to control chondrogenesis and osteogenesis during mandibular development.

Epithelial-mesenchymal transformation during craniofacial development

Epithelial to mesenchymal phenotype transition is a common phenomenon during embryonic development, wound healing, and tumor metastasis. This transition involves cellular changes in cytoskeleton architecture and protein expression. Specifically, this highly regulated biological event plays several important roles during craniofacial development. This review focuses on the regulation of epithelial-mesenchymal transformation (EMT) during neural crest cell migration, and fusion of the secondary palate and the upper lip.

Craniofacial tissue engineering by stem cells

Craniofacial tissue engineering promises the regeneration or de novo formation of dental, oral, and craniofacial structures lost to congenital anomalies, trauma, and diseases. Virtually all craniofacial structures are derivatives of mesenchymal cells. Mesenchymal stem cells are the offspring of mesenchymal cells following asymmetrical division, and reside in various craniofacial structures in the adult. Cells with characteristics of adult stem cells have been isolated from the dental pulp, the deciduous tooth, and the periodontium. Several craniofacial structures--such as the mandibular condyle, calvarial bone, cranial suture, and subcutaneous adipose tissue--have been engineered from mesenchymal stem cells, growth factor, and/or gene therapy approaches. As a departure from the reliance of current clinical practice on durable materials such as amalgam, composites, and metallic alloys, biological therapies utilize mesenchymal stem cells, delivered or internally recruited, to generate craniofacial structures in temporary scaffolding biomaterials. Craniofacial tissue engineering is likely to be realized in the foreseeable future, and represents an opportunity that dentistry cannot afford to miss.

Characterization of established cementoblast-like cell lines from human cementum-lining cells in vitro and in vivo

To study cellular characteristics of human cementoblasts using a cellular model is important for understanding the mechanisms of homeostasis and regeneration of periodontal tissues. However, at present no immortalized human cementoblast cell line has been established due to limitation of the life span. In the present study, therefore, we attempted to establish human cementoblast-like cell lines by transfection with telomerase catalytic subunit hTERT gene. Two stable clones (HCEM-1 and -2) with high telomerase activity were obtained and they grew over 200 population doublings without significant growth retardation. The expression of mRNA for differentiation markers, type I collagen, alkaline phosphatase (ALP), runt-related transcription factor 2, osteocalcin, bone sialoprotein and cementum-derived protein was revealed in these clones by RT-PCR. Moreover, these cells showed high ALP activity and calcified nodule formation in vitro. Interestingly, HCEM-2 showed cementum like formation on the surface of hydroxyapatites granules by subcutaneous transplantation into immunodeficient mice with hydroxyapatite granules. Thus, we established human cementoblast-like cell lines. We suggest that HCEM cell lines can be useful cell models for investigating the characteristics of human cementoblasts.

Impaired neural development caused by inducible expression of Axin in transgenic mice

Ablations of the Axin family genes demonstrated that they modulate Wnt signaling in key processes of mammalian development. The ubiquitously expressed Axin1 plays an important role in formation of the embryonic neural axis, while Axin2 is essential for craniofacial skeletogenesis. Although Axin2 is also highly expressed during early neural development, including the neural tube and neural crest, it is not essential for these processes, apparently due to functional redundancy with Axin1. To further investigate the role of Wnt signaling during early neural development, and its potential regulation by Axins, we developed a mouse model for conditional gene activation in the Axin2-expressing domains. We show that gene expression can be successfully targeted to the Axin2-expressing cells in a spatially and temporally specific fashion. High levels of Axin in this domain induce a region-specific effect on the patterning of neural tube. In the mutant embryos, only the development of midbrain is severely impaired even though the transgene is expressed throughout the neural tube. Axin apparently regulates beta-catenin in coordinating cell cycle progression, cell adhesion and survival of neuroepithelial precursors during development of ventricles. Our data support the conclusion that the development of embryonic neural axis is highly sensitive to the level of Wnt signaling.

Establishment of immortalized dental follicle cells for generating periodontal ligament in vivo

The dental follicle is a mesenchymal tissue that surrounds the developing tooth germ. During tooth root formation, periodontal components, viz., cementum, periodontal ligament (PDL), and alveolar bone, are created by dental follicle progenitors. Here, we report the presence of PDL progenitors in mouse dental follicle (MDF) cells. MDF cells were obtained from mouse incisor tooth germs and immortalized by the expression of a mutant human papilloma virus type 16 E6 gene lacking the PDZ-domain-binding motif. MDF cells expressing the mutant E6 gene (MDF( E6-EGFP ) cells) had an extended life span, beyond 150 population doublings (PD). In contrast, normal MDF cells failed to proliferate beyond 10 PD. MDF( E6-EGFP ) cells expressed tendon/ligament phenotype-related genes such as Scleraxis (Scx), growth and differentiation factor-5, EphA4, Six-1, and type I collagen. In addition, the expression of periostin was observed. To elucidate the differentiation capacity of MDF( E6-EGFP ) cells in vivo, the cells were transplanted into severe combined immunodeficiency mice. At 4 weeks, MDF( E6-EGFP ) cell transplants had the capacity to generate a PDL-like tissue that expressed periostin, Scx, and type XII collagen and the fibrillar assembly of type I collagen. Our findings suggest that MDF( E6-EGFP ) cells can act as PDL progenitors, and that these cells may be a useful research tool for studying PDL formation and for developing regeneration therapies.

Use of a ROSA26:GFP transgenic line for long-term Xenopus fate-mapping studies

Widespread and persistent marker expression is a prerequisite for many transgenic applications, including chimeric transplantation studies. Although existing transgenic tools for the clawed frog, Xenopus laevis, offer a number of promoters that drive widespread expression during embryonic stages, obtaining transgene expression through metamorphosis and into differentiated adult tissues has been difficult to achieve with this species. Here we report the application of the murine ROSA26 promoter in Xenopus. GFP is expressed in every transgenic tissue and cell type examined at post-metamorphic stages. Furthermore, transgenic ROSA26:GFP frogs develop normally, with no apparent differences in growth or morphology relative to wild-type frogs. ROSA26 transgenes may be used as a reliable marker for embryonic fate-mapping of adult structures in Xenopus laevis. Utility of this transgenic line is illustrated by its use in a chimeric grafting study that demonstrates the derivation of the adult bony jaw from embryonic cranial neural crest.

Endogenous bone morphogenetic protein antagonists regulate mammalian neural crest generation and survival

We demonstrate here that Chordin and Noggin function as bone morphogenetic protein (BMP) antagonists in vivo to promote mammalian neural crest development. Using Chrd and Nog single and compound mutants, we find that Noggin has a major role in promoting neural crest formation, in which Chordin is partially redundant. BMP signaling is increased in dorsal tissues lacking Noggin and is further increased when Chordin is also absent. The early neural crest domain is expanded with decreased BMP antagonism in vivo. Noggin and Chordin also regulate subsequent neural crest cell emigration from the neural tube. However, reduced levels of these BMP antagonists ultimately result in perturbation of neural crest cell derived peripheral nervous system and craniofacial skeletal elements. Such defects reflect, at least in part, a function to limit apoptosis in neural crest cells. Noggin and Chordin, therefore, function together to regulate both the generation and survival of neural crest cells in mammalian development.

Potential of dental mesenchymal cells in developing teeth

The tooth, composed of dentin and enamel, develops through epithelium-mesenchyme interactions. Neural crest (NC) cells contribute to the dental mesenchyme in the developing tooth and differentiate into dentin-secreting odontoblasts. NC cells are known to differentiate into chondrocytes and osteoblasts in the craniofacial region. However, it is not clear whether the dental mesenchymal cells in the developing tooth possess the potential to differentiate into a lineage(s) other than the odontoblast lineage. In this study, we prepared mesenchymal cells from E13.5 tooth germ cells and assessed their potential for differentiation in culture. They differentiated into odontoblasts, chondrocyte-like cells, and osteoblast-like cells. Their derivation was confirmed by tracing NC-derived cells as LacZ(+) cells using P0-Cre/Rosa26R mice. Using the flow cytometry-fluorescent di-beta-D-galactosidase system, which makes it possible to detect LacZ(+) cells as living cells, cell surface molecules of dental mesenchymal cells were characterized. Large number of LacZ(+) NC-derived cells expressed platelet-derived growth factor receptor alpha and integrins. Taken together, these results suggest that NC-derived cells with the potential to differentiate into chondrocyte-like and osteoblast-like cells are present in the developing tooth, and these cells may contribute to tooth organogenesis.

Tissue Engineering of the TMJ disc: a review

The potential impact of a tissue-engineered temporomandibular joint (TMJ) disc is immense. Currently, patients suffering from a severely dysfunctional TMJ have few options. Facing the general lack of safe, effective TMJ disc implants, many patients undergo discectomy, a procedure that removes the injured TMJ disc in hopes of reducing debilitating symptoms associated with severe TMJ disorders. This procedure may not be ideal as the TMJ is left without an important functional component. Tissue engineering is a promising approach for the creation of viable, effective implants. The first attempt to investigate TMJ disc cells on a biomaterial was conducted in 1991. The first TMJ tissue-engineered constructs to be tested biochemically and biomechanically were formed in 1994; however, in examining this study in retrospect, it is clear how little TMJ knowledge was available at that time. Within the last 10 to 15 years, multiple studies have investigated critical TMJ disc characteristics, and while this characterization is not complete, these data have created a solid foundation for tissue-engineering research. Thus, the last 5 years have yielded core studies investigating the principal elements of tissue engineering: scaffold, cell source, and biological/biomechanical stimuli. Although TMJ disc tissue engineering is still in its formative years, its future is quite promising. Key studies are now being conducted that will assist in the establishment of a solid TMJ disc tissue-engineering approach. As the challenges of tissue engineering are faced and met, the ultimate goal of creating a functional biological implant nears.

Abnormal migration and distribution of neural crest cells in Pax6 heterozygous mutant eye, a model for human eye diseases

PAX6/Pax6 gene encodes a transcription factor that is crucially required for eye development. Pax6 heterozygous mutant mouse (Pax6(Sey/+)) shows various ocular defects, especially in the anterior segment. It has been well known that the induction of the lens and development of the cornea and retina are dependent on PAX6/Pax6 in a cell-autonomous fashion, although the influence of PAX6/Pax6 on the other tissues derived from the ocular mesenchyme is largely unknown. Using transgenic mouse lines in which neural crest cells are genetically marked by LacZ or EGFP, we revealed the extensive contribution of neural crest derived cells (NCDCs) to the ocular tissues. Furthermore, various eye defects in Pax6(Sey/+) mouse were accompanied by abnormal distribution of NCDCs from early developmental stages to the adult. In Pax6(Sey/+) mouse mice, neural crest cells abnormally migrated into the developing eye in a cell nonautonomous manner at early embryonic stages. These results indicate that normal distribution and integration of NCDCs in ocular tissues depend on a proper dosage of Pax6, and that Pax6(Sey/+) eye anomalies are caused by cell autonomous and nonautonomous defects due to Pax6 haploinsufficiency.

Capacity of dental pulp differentiation after tooth transplantation

Under pathological conditions, dental pulp elaborates both bone and dentin matrix in which the contribution of periodontal tissue cannot be excluded. This study has aimed to clarify the capability of dental pulp to deposit bone matrix in an auto-graft experiment by using (1) immunohistochemistry for 5-bromo-2'-deoxyuridine (BrdU) and nestin and (2) histochemistry for tartrate-resistant acid phosphatase (TRAP). Following the extraction of the molars of 3-week-old mice, the roots and pulp floor were resected and immediately transplanted into the sublingual region. On Days 5-7, tubular dentin formation commenced next to the pre-existing dentin at the pulp horn in which nestin-positive odontoblast-like cells were arranged. Up until Day 14, bone-like tissue formation occurred in the pulp chamber in which intense TRAP-positive cells appeared. These results suggest that odontoblast- and osteoblast-lineage cells reside in the dental pulp. Overall, specific dental pulp regeneration should provide fundamental knowledge for the realization of human tooth regeneration in the near future.

Generation and maintenance of Dmbx1 gene-targeted mutant alleles

Dmbx1 encodes a paired-like homeodomain protein that is expressed in neural tissues at mouse embryonic and postnatal stages. We previously generated two Dmbx1 mutant alleles, Dmbx1 (-) and Dmbx1 ( z ), by homologous recombination in mouse embryonic stem (ES) cells. In this article we report the generation of three novel Dmbx1 mutant alleles, Dmbx1 (tauZ ), Dmbx1 (tauG ), and Dmbx1 ( Cre ), that carry the intronic insertion of tau (tau)-lacZ, tau-eGFP, and Cre reporter genes, respectively. Dmbx1 (tauZ ) and Dmbx1 (tauG ) recapitulated the Dmbx1 expression, and the reporter gene expression was detected in the diencephalon and mesencephalon during embryogenesis. The crossing of Dmbx1 ( Cre ) mice with Rosa26 reporter mice identified the Cre-mediated DNA excision in the postnatal midbrain, cerebellum, medulla oblongata, and spinal cord. To maintain the Dmbx1 mutant alleles without genotyping, we crossed Dmbx1 mutant mice with Inv4(1) ( Brd ) mice that possess the inversion between D4Mit117 and D4Mit281 on Chromosome 4, where Dmbx1 is located. The intercrossing of the non-agouti (a/a) albino (Tyr ( c-Brd )/Tyr ( c-Brd )) Dmbx1 mutant mice carrying Inv4(1) ( Brd ) tagged with K14-Agouti and Tyrosinase coat-color markers resulted in the generation of dark brown Dmbx1 wild-type [Inv4(1) ( Brd )/Inv4(1) ( Brd )], light brown Dmbx1 heterozygous [Dmbx1 ( tm )/Inv4(1) ( Brd )], and albino Dmbx1 homozygous (Dmbx1 ( tm )/Dmbx1 ( tm )) mutant mice. To our knowledge, this is the first demonstration of the proof-of-principle of the maintenance of viable gene-targeted alleles using coat-color-tagged nonlethal balancer chromosomes.

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.

Nell-1 induces acrania-like cranioskeletal deformities during mouse embryonic development

We previously reported NELL-1 as a novel molecule overexpressed during premature cranial suture closure in patients with craniosynostosis (CS). Nell-1 overexpression also results in premature suture closure/craniosynostosis in newborn transgenic mice. On a cellular level, increased levels of Nell-1 induce osteoblast differentiation and apoptosis. In this report, mice over-expressing Nell-1 were examined during embryonic development as well as shortly after birth for further analysis of craniofacial defects including neural tube defects (NTDs). The results demonstrated that overexpression of Nell-1 could induce acrania at relatively late gestation stage (E15.5) in mouse embryos, through massive apoptosis in calvarial osteoblasts and neural cells. The induced apoptosis was associated with an increase in Fas and Fas-L production. In addition, transgenic E15.5 and newborn transgenic mice with the CS phenotype displayed distortion of the chondrocranium associated with premature hypertrophy and increased apoptosis of chondrocytes. These findings were also verified in vitro with primary chondrocytes transduced with AdNell-1. In conclusion, Nell-1 overexpression can induce craniofacial anomalies associated with neural tube defects during embryonic development and may involve mechanisms of massive apoptosis associated with the Fas/Fas-L signaling pathway. NELL-1: used when describing the human gene; NELL-1: used when describing the human protein; Nell-1: used when describing the rodent gene; Nell-1: used when describing the rodent protein.

Optimized beta-galactosidase staining method for simultaneous detection of endogenous gene expression in early mouse embryos

beta-Galactosidase (beta-gal) is one of the popular reporters for detecting the expression of endogenous or exogenous genes. Here we report 6-chloro-3-indoxyl-beta-D-galactopyranoside (S-gal) is more sensitive for beta-gal activity than 5-bromo-4-chloro-3-indolyl-beta-D-galactoside (X-gal), particularly during the early developmental stages of mouse embryos. Further, we successfully combined beta-gal staining with S-gal and in situ hybridization using DIG-labeled probes in both whole and sections of early stage embryos due to the sensitivity and color compatibility of S-gal.

Hepatic stellate cells do not derive from the neural crest

BACKGROUND/AIMS

Hepatic stellate cells (HSC) have been hypothesised to derive from the neural crest, based on their expression of multiple neural/neuroendocrine features and their contacts with autonomic nerve endings.

METHODS

We studied the emergence of HSC in the liver during embryonic development in a transgenic mouse line expressing yellow fluorescent protein (YFP) in all neural crest cells and their derivatives. Cellular YFP expression in these mice was compared with desmin expression between embryonic day (E) 11.5 and adulthood.

RESULTS

YFP was abundantly expressed in neural crest cells delaminating from the neural tube and in all known neural crest-derived structures and cell populations. In particular, YFP expressing cells perfectly mimicked the spatial and temporal pattern of enteric nervous system development from neural crest cells migrating from the postotic region. Cells within the adrenal medulla were also YFP positive. Analysis of the liver showed that desmin-expressing, stellate-shaped, perisinusoidally located HSC were evident from E11.5 onwards. However, no detectable YFP expression was seen in the developing liver or in HSC, from E11.5 until adulthood.

CONCLUSIONS

These findings suggest HSC do not descend from the neural crest, and therefore may derive from the septum transversum mesenchyme, from endoderm or from the mesothelial liver capsule.

Skeletal site-specific characterization of orofacial and iliac crest human bone marrow stromal cells in same individuals

Autologous grafts from axial and appendicular bones commonly used to repair orofacial bone defects often result in unfavorable outcome. This clinical observation, along with the fact that many bone abnormalities are limited to craniofacial bones, suggests that there are significant differences in bone metabolism in orofacial, axial and appendicular bones. It is plausible that these differences are dictated by site-specificity of embryological progenitor cells and osteogenic properties of resident multipotent human bone marrow stromal cells (hBMSCs). This study investigated skeletal site-specific phenotypic and functional differences between orofacial (maxilla and mandible) and axial (iliac crest) hBMSCs in vitro and in vivo. Primary cultures of maxilla, mandible and iliac crest hBMSCs were established with and without osteogenic inducers. Site-specific characterization included colony forming efficiency, cell proliferation, life span before senescence, relative presence of surface markers, adipogenesis, osteogenesis and transplantation in immunocompromised mice to compare bone regenerative capacity. Compared with iliac crest cells, orofacial hBMSCs (OF-MSCs) proliferated more rapidly with delayed senescence, expressed higher levels of alkaline phosphatase and demonstrated more calcium accumulation in vitro. Cells isolated from the three skeletal sites were variably positive for STRO 1, a marker of hBMSCs. OF-MSCs formed more bone in vivo, while iliac crest hBMSCs formed more compacted bone that included hematopoietic tissue and were more responsive in vitro and in vivo to osteogenic and adipogenic inductions. These data demonstrate that hBMSCs from the same individuals differ in vitro and in vivo in a skeletal site-specific fashion and identified orofacial marrow stromal cells as unique cell populations. Further understanding of site-specific properties of hBMSCs and their impact on site-specific bone diseases and regeneration are needed.

Cell autonomous requirement for Tgfbr2 in the disappearance of medial edge epithelium during palatal fusion

Palatal fusion is a complex, multi-step developmental process; the consequence of failure in this process is cleft palate, one of the most common birth defects in humans. Previous studies have shown that regression of the medial edge epithelium (MEE) upon palatal fusion is required for this process, and TGF-beta signaling plays an important role in regulating palatal fusion. However, the fate of the MEE and the mechanisms underlying its disappearance are still unclear. By using the Cre/lox system, we are able to label the MEE genetically and to ablate Tgfbr2 specifically in the palatal epithelial cells. Our results indicate that epithelial-mesenchymal transformation does not occur in the regression of MEE cells. Ablation of Tgfbr2 in the palatal epithelial cells causes soft palate cleft, submucosal cleft and failure of the primary palate to fuse with the secondary palate. Whereas wild-type MEE cells disappear, the mutant MEE cells continue to proliferate and form cysts and epithelial bridges in the midline of the palate. Our study provides for the first time an animal model for soft palate cleft and submucous cleft. At the molecular level, Tgfb3 and Irf6 have similar expression patterns in the MEE. Mutations in IRF6 disrupt orofacial development and cause cleft palate in humans. We show here that Irf6 expression is downregulated in the MEE of the Tgfbr2 mutant. As a recent study shows that heterozygous mutations in TGFBR1 or TGFBR2 cause multiple human congenital malformations, including soft palate cleft, we propose that TGF-beta mediated Irf6 expression plays an important, cell-autonomous role in regulating the fate of MEE cells during palatogenesis in both mice and humans.

Evolution of novelty in the cichlid dentition

The shape of teeth occupies a central position in various biological disciplines, from paleo-ecology to molecular biology to cosmetic and reconstructive dentistry. Despite a long tradition of study in mammals, important questions remain regarding the genetic and developmental basis of differences in tooth shape. Here, we use natural mutants of cichlid fish from East Africa, which exhibit tremendous dental diversity, to help fill the gaps in our understanding of vertebrate odontogenesis. We employ an expanded genetic linkage map to demonstrate that cusp number segregates as a gene of major effect, which explains approximately 40% of the phenotypic variance, on cichlid chromosome 5. Furthermore, we examine patterns of Bmp4 expression in early odontogenesis to address and refine predictions of models linking tooth shape and tooth number. Mutations in the Bmp4 cistron do not control tooth shape in this mapping cross. Our data suggest that the evolution of novelty in the cichlid dentition is galvanized by a small number of genetic changes, echoing similar conclusions from recent studies of other vertebrate adaptive morphologies.

An in vitro model for characterizing the post-migratory cranial neural crest cells of the first branchial arch

The cranial neural crest (CNC) is a transient cell population that originates at the crest of the neural fold and gives rise to multiple cell types during craniofacial development. Traditionally, researchers have used tissue explants, such as the neural tube, to obtain primary neural crest cells for their studies. However, this approach has inevitably resulted in simultaneous isolation of neural and non-neural crest cells as both of these cells migrate away from tissue explants. Using the Wnt1-Cre/R26R mouse model, we have obtained a pure population of neural crest cells and established a primary CNC cell culture system in which the cell culture medium best supports the proliferation of E10.5 first branchial arch CNC cells and maintains these cells in their undifferentiated state. Differentiation of CNC cells can be initiated by switching to a differentiation medium. In this model, cultured CNC cells can give rise to neurons, glial cells, osteoblasts, and other cell types, faithfully mimicking the differentiation process of the post-migratory CNC cells in vivo. Taken together, our study shows that the Wnt1-Cre/R26R mouse first branchial arch provides an excellent model for obtaining post-migratory neural crest cells free of any mesodermal contaminants. The cultured neural crest cells are under sustained proliferative, undifferentiated, or lineage-enhanced conditions, hence, serving as a tool for the investigation of the regulatory mechanism of CNC cell fate determination in normal and abnormal craniofacial development.