Secondary induction and the development of tooth nerve supply

Ablation of CNTN2+ Pyramidal Neurons During Development Results in Defects in Neocortical Size and Axonal Tract Formation

Corticothalamic axons express Contactin-2 (CNTN2/TAG-1), a neuronal recognition molecule of the immunoglobulin superfamily involved in neurogenesis, neurite outgrowth, and fasciculation. TAG-1, which is expressed transiently by cortical pyramidal neurons during embryonic development, has been shown to be fundamental for axonal recognition, cellular migration, and neuronal proliferation in the developing cortex. Although Tag-1^−/− mice do not exhibit any obvious defects in the corticofugal system, the role of TAG-1+ neurons during the development of the cortex remains elusive. We have generated a mouse model expressing EGFP under the Tag-1 promoter and encompassing the coding sequence of Diptheria Toxin subunit A (DTA) under quiescence with no effect on the expression of endogenous Tag-1. We show that while the line recapitulates the expression pattern of the molecule, it highlights an extended expression in the forebrain, including multiple axonal tracts and neuronal populations, both spatially and temporally. Crossing these mice to the Emx1-Cre strain, we ablated the vast majority of TAG-1+ cortical neurons. Among the observed defects were a significantly smaller cortex, a reduction of corticothalamic axons as well as callosal and commissural defects. Such defects are common in neurodevelopmental disorders, thus this mouse could serve as a useful model to study physiological and pathophysiological cortical development.

Deciduous autologous tooth stem cells regenerate dental pulp after implantation into injured teeth

Pulp necrosis arrests root development in injured immature permanent teeth, which may result in tooth loss. However, dental pulp regeneration and promotion of root development remains challenging. We show that implantation of autologous tooth stem cells from deciduous teeth regenerated dental pulp with an odontoblast layer, blood vessels, and nerves in two animal models. These results prompted us to enroll 40 patients with pulp necrosis after traumatic dental injuries in a randomized, controlled clinical trial. We randomly allocated 30 patients to the human deciduous pulp stem cell (hDPSC) implantation group and 10 patients to the group receiving traditional apexification treatment. Four patients were excluded from the implantation group due to loss at follow-up (three patients) and retrauma of the treated tooth (one patient). We examined 26 patients (26 teeth) after hDPSC implantation and 10 patients (10 teeth) after apexification treatment. hDPSC implantation, but not apexification treatment, led to regeneration of three-dimensional pulp tissue equipped with blood vessels and sensory nerves at 12 months after treatment. hDPSC implantation increased the length of the root (P < 0.0001) and reduced the width of the apical foramen (P < 0.0001) compared to the apexification group. In addition, hDPSC implantation led to regeneration of dental pulp tissue containing sensory nerves. To evaluate the safety of hDPSC implantation, we followed 20 patients implanted with hDPSCs for 24 months and did not observe any adverse events. Our study suggests that hDPSCs are able to regenerate whole dental pulp and may be useful for treating tooth injuries due to trauma.

Establishment of tooth blood supply and innervation is developmentally regulated and takes place through differential patterning processes

Teeth are richly supported by blood vessels and peripheral nerves. The aim of this study was to describe in detail the developmental time-course and localization of blood vessels during early tooth formation and to compare that to innervation, as well as to address the putative role of vascular endothelial growth factor (VEGF), which is an essential regulator of vasculature development, in this process. The localization of blood vessels and neurites was compared using double immunofluorescence staining on sections at consecutive stages of the embryonic (E) and postnatal (PN) mandibular first molar tooth germ (E11-PN7). Cellular mRNA expression domains of VEGF and its signaling receptor VEGFR2 were studied using sectional radioactive in situ hybridization. Expression of VEGF mRNA and the encoded protein were studied by RT-PCR and western blot analysis, respectively, in the cap and early bell stage tooth germs, respectively. VEGFR2 was immunolocalized on tooth tissue sections. Smooth muscle cells were investigated by anti-alpha smooth muscle actin (αSMA) antibodies. VEGF showed developmentally regulated epithelial and mesenchymal mRNA expression domains including the enamel knot signaling centers that correlated with the growth and navigation of the blood vessels expressing Vegfr2 and VEGFR2 to the dental papilla and enamel organ. Developing blood vessels were present in the jaw mesenchyme including the presumptive dental mesenchyme before the appearance of the epithelial dental placode and dental neurites. Similarly, formation of a blood vessel plexus around the bud stage tooth germ and ingrowth of vessels into dental papilla at E14 preceded ingrowth of neurites. Subsequently, pioneer blood vessels in the dental papilla started to receive smooth muscle coverage at the early embryonic bell stage. Establishment and patterning of the blood vessels and nerves during tooth formation are developmentally regulated, stepwise processes that likely involve differential patterning mechanisms. Development of tooth vascular supply is proposed to be regulated by local, tooth-specific regulation by epithelial-mesenchymal tissue interactions and involving tooth target expressed VEGF signaling. Further investigations on tooth vascular development by local VEGF signaling, as well as how tooth innervation and development of blood vessels are integrated with advancing tooth organ formation by local signaling mechanisms, are warranted.

Pulp Stem Cell-Mediated Functional Pulp Regeneration

The preservation of vital dental pulp with vasculature and nerve components remains one of the most significant challenges in modern dentistry. Due to the immense potential for neurovascularization, mesenchymal stem cell (MSC) transplantation has shown emerging promise in regenerative medicine and dental translational practice. Actually, pulp mesenchymal stem cells, including postnatal dental pulp stem cells (from permanent teeth) and stem cells from human exfoliated deciduous teeth, possess unique properties based on their origins from neural crest or glial cells. Furthermore, they reside in a neurovascular niche and have the potential for neurogenesis, angiogenesis, and neurovascular inductive activity. According to current pulp regeneration strategies, pulp stem cell-mediated approaches to regeneration have demonstrated convincing evidence that they can rebuild the complex histologic structure of native pulp in situ with highly organized physiologic patterns or even achieve de novo regeneration of complete dental pulp tissues. More importantly, recent clinical studies emphasized in situ neurovascularization outcomes in successful regeneration of vitalized pulp via pulp stem cell transplantation. In this review, we summarize recent breakthroughs in pulp stem cell-mediated pulp regeneration, emphasizing the crucial achievement of neurovascularization. This functional pulp regeneration represents an innovative and promising approach for future regenerative endodontics.

Integration of tooth morphogenesis and innervation by local tissue interactions, signaling networks, and semaphorin 3A

The tooth, like many other organs, develops from both epithelial and mesenchymal tissues, and has proven to be a valuable tool with which to investigate organ formation and peripheral innervation. Tooth formation is regulated by local epithelial-mesenchymal tissue interactions, and is closely integrated with stereotypic dental nerve navigation and patterning. Recent analyses of the function and regulation of semaphorin 3A (SEMA3A) have shed light on the regulatory mechanisms that coordinate organogenesis and innervation at the tissue and molecular levels. In the tooth, SEM3A acts as a developmentally regulated secretory chemo-repellent, that controls tooth innervation during embryonic and postnatal development. The tooth germ governs its own innervation by a combination of local tissue interactions and SEMA3A expression. SEMA3A signaling, in turn, is controlled by a number of conserved signaling effectors, including TGF-β superfamily members, FGF, and WNT; all function in embryo and organ development, and are essential for tooth histo-morphogenesis. Thus, SEMA3A driven axon guidance is integrated into key odontogenic signaling networks, establishing this protein as a critical molecular tether between 2 distinct developmental processes (morphogenesis and sensory innervation), both of which are required to obtain a functional tooth.

Tooth Organ Bioengineering: Cell Sources and Innovative Approaches

Various treatment approaches for restoring missing teeth are being utilized nowadays by using artificial dental crowns/bridges or the use of dental implants. All aforementioned restorative modalities are considered to be the conventional way of treating such cases. Although these artificial therapies are commonly used for tooth loss rehabilitation, they are still less conservative, show less biocompatibility and fail to restore the natural biological and physiological function. Adding to that, they are considered to be costly due to the risk of failure and they also require regular maintenance. Regenerative dentistry is currently considered a novel therapeutic concept with high potential for a complete recovery of the natural function and esthetics of teeth. Biological-cell based dental therapies would involve replacement of teeth by using stem cells that will ultimately grow a bioengineered tooth, thereby restoring both the biological and physiological functions of the natural tooth, and are considered to be the ultimate goal in regenerative dentistry. In this review, various stem cell-based therapeutic approaches for tooth organ bioengineering will be discussed.

The innervation of the zebrafish pharyngeal jaws and teeth

Zebrafish (Danio rerio) teeth are increasingly used as a model to study odontogenesis in non-mammalians. Using serial semi-thin section histology and immunohistochemistry, the nerves innervating the pharyngeal jaws and teeth have been identified. The last pair of branchial arches, which are non-gill bearing but which carry the teeth, are innervated by an internal branch of a post-trematic ramus of the vagal nerve. Another, external, branch is probably responsible for the motor innervation of the branchiomeric musculature. Nerve fibres appear in the pulp cavity of the teeth only late during cytodifferentiation, and are therefore likely not involved in early steps of tooth formation. The precise role of the nervous system during continuous tooth replacement remains to be determined. Nonetheless, this study provides the necessary morphological background information to address this question.

Glial origin of mesenchymal stem cells in a tooth model system

Mesenchymal stem cells occupy niches in stromal tissues where they provide sources of cells for specialized mesenchymal derivatives during growth and repair. The origins of mesenchymal stem cells have been the subject of considerable discussion, and current consensus holds that perivascular cells form mesenchymal stem cells in most tissues. The continuously growing mouse incisor tooth offers an excellent model to address the origin of mesenchymal stem cells. These stem cells dwell in a niche at the tooth apex where they produce a variety of differentiated derivatives. Cells constituting the tooth are mostly derived from two embryonic sources: neural crest ectomesenchyme and ectodermal epithelium. It has been thought for decades that the dental mesenchymal stem cells giving rise to pulp cells and odontoblasts derive from neural crest cells after their migration in the early head and formation of ectomesenchymal tissue. Here we show that a significant population of mesenchymal stem cells during development, self-renewal and repair of a tooth are derived from peripheral nerve-associated glia. Glial cells generate multipotent mesenchymal stem cells that produce pulp cells and odontoblasts. By combining a clonal colour-coding technique with tracing of peripheral glia, we provide new insights into the dynamics of tooth organogenesis and growth.

Coordination of tooth morphogenesis and neuronal development through tissue interactions: lessons from mouse models

In addition to being an advantageous model to investigate general molecular mechanisms of organ formation, the tooth is a distinct target organ for peripheral nerve innervation. These nerves are required for the function and protection of the teeth and, as shown in fish, also for their regeneration. This review focuses on recent findings of the local tissue interactions and molecular signaling mechanisms that regulate the early nerve arrival and patterning of mouse mandibular molar tooth sensory innervation. Dental sensory nerve growth and patterning is a stepwise process that is intimately linked to advancing tooth morphogenesis. In particular, nerve growth factor and semaphorin 3A serve as essential functions during and are iteratively used at different stages of tooth innervation. The tooth germ controls development of its own nerve supply, and similar to the development of the tooth organ proper, tissue interactions between dental epithelial and mesenchymal tissues control the establishment of tooth innervation. Tgf-β, Wnt, and Fgf signaling, which regulate tooth formation, are implicated to mediate these interactions. Therefore, tissue interactions mediated by conserved signal families may constitute key mechanism for the integration of tooth organogenesis and development of its peripheral nerve supply.

Nicotinic receptor Alpha7 expression during tooth morphogenesis reveals functional pleiotropy

The expression of nicotinic acetylcholine receptor (nAChR) subtype, alpha7, was investigated in the developing teeth of mice that were modified through homologous recombination to express a bi-cistronic IRES-driven tau-enhanced green fluorescent protein (GFP); alpha7GFP) or IRES-Cre (alpha7Cre). The expression of alpha7GFP was detected first in cells of the condensing mesenchyme at embryonic (E) day E13.5 where it intensifies through E14.5. This expression ends abruptly at E15.5, but was again observed in ameloblasts of incisors at E16.5 or molar ameloblasts by E17.5–E18.5. This expression remains detectable until molar enamel deposition is completed or throughout life as in the constantly erupting mouse incisors. The expression of alpha7GFP also identifies all stages of innervation of the tooth organ. Ablation of the alpha7-cell lineage using a conditional alpha7Cre×ROSA26-LoxP(diphtheria toxin A) strategy substantially reduced the mesenchyme and this corresponded with excessive epithelium overgrowth consistent with an instructive role by these cells during ectoderm patterning. However, alpha7knock-out (KO) mice exhibited normal tooth size and shape indicating that under normal conditions alpha7 expression is dispensable to this process. The function of ameloblasts in alpha7KO mice is altered relative to controls. High resolution micro-computed tomography analysis of adult mandibular incisors revealed enamel volume of the alpha7KO was significantly reduced and the organization of enamel rods was altered relative to controls. These results demonstrate distinct and varied spatiotemporal expression of alpha7 during tooth development, and they suggest that dysfunction of this receptor would have diverse impacts upon the adult organ.

Developmentally regulated expression of Sema3A chemorepellant in the developing mouse incisor

OBJECTIVE

Semaphorin 3A (Sema3A) is an essential chemorepellant controlling peripheral axon pathfinding and patterning, but also serves non-neuronal cellular functions. Incisors of rodent are distinctive from molars as they erupt continuously, have only one root and enamel is present only on the labial side. The aim of this study is to address putative regulatory roles of Sema3A chemorepellant in the development of incisor innervation and formation.

MATERIALS AND METHODS

This study analyzed expression of Sema3A mRNAs during embryonic and early post-natal stages of mouse mandibular incisor using sectional radioactive in situ hybridization.

RESULTS

Although Sema3A mRNAs were observed in condensed dental mesenchyme during the early bud stage, they were absent in dental papilla or pulp at later stages. Sema3A mRNAs were observed in the dental epithelium including the cervical loops and a prominent expression was also seen in alveolar bone. Interestingly, transcripts were absent from the mesenchymal dental follicle target area (future periodontal ligament) throughout the studied stages.

CONCLUSION

The expression patterns of Sema3A indicate that it may control the timing and patterning of the incisor innervation. In particular, Sema3A appears to regulate innervation of the periodontal ligament, while nerve penetration into the incisor dental pulp appears not to be dependent on Sema3A. Moreover, Sema3A may regulate the functions of cervical loops and the development of alveolar bone. Future study with Sema3A deficient mice will help to elucidate the putative neuronal and non-neuronal functions of Sema3A in incisor tooth development.

Expression patterns of Sema3F, PlexinA4, -A3, Neuropilin1 and -2 in the postnatal mouse molar suggest roles in tooth innervation and organogenesis

OBJECTIVE

Semaphorins form a family of axon wiring molecules but still little is known about their role in tooth formation. A class 3 semaphorin, Semaphorin3F (Sema3F), besides acting as a chemorepellant for different types of axons, controls a variety of non-neuronal developmental processes.

MATERIALS AND METHODS

Cellular mRNA expression patterns of Sema3F as well as neuropilin 1 (Npn1), neuropilin 2 (Npn2), plexinA3 and plexinA4 receptors were analyzed by sectional in situ hybridization in the mouse molar tooth during postnatal days 0-7. The expression of the receptors was studied in PN5 trigeminal ganglia.

RESULTS

Sema3F, Npn1, -2 and plexinA4 exhibited distinct, spatiotemporally changing expression patterns, whereas plexinA3 was not observed in the tooth germs. Besides being expressed in the base of the dental mesenchyme Sema3F, like plexinA4, Npn1 and -2, was present in the ameloblast cell lineage. Npn1 and Npn2 were additionally seen in the pulp horns and endothelial cells and like PlexinA4 in the developing alveolar bone. Npn1, plexinA3 and -A4 were observed in trigeminal ganglion neurons.

CONCLUSIONS

Sema3F may act as a tooth target-derived axonal chemorepellant controlling establishment of the tooth nerve supply. Furthermore, Sema3F, like Npn1, -2 and plexinA4 may serve non-neuronal functions by controlling the development of the ameloblast cell lineage. Moreover, Npn1 and Npn2 may regulate dental vasculogenesis and, together with PlexinA4, alveolar bone formation. Further analyses such as investigation of transgenic mouse models will be required to elucidate in vivo signaling functions of Sema3F and the receptors in odontogenesis.

Developmentally regulated expression of intracellular Fgf11-13, hormone-like Fgf15 and canonical Fgf16, -17 and -20 mRNAs in the developing mouse molar tooth

OBJECTIVE

To investigate and compare the cellular expression of non-secreted Fgf11-14 and secreted Fgf15-18 and -20 mRNAs during tooth formation.

MATERIALS AND METHODS

mRNA expression was analyzed from the morphological initiation of the mouse mandibular first molar development to the onset of crown calcification using sectional in situ hybridization.

RESULTS

This study found distinct, differentially regulated expression patterns for the Fgf11-13, -15-17 and -20, in particular in the epithelial-mesenchymal interface, whereas Fgf14 and 18 mRNAs were not detected. Fgf11, -15, -16, -17 and -20 were seen in the epithelium, whereas Fgf12 and -13 signals were restricted to the mesenchymal tissue component of the tooth. Fgf11 was observed in the putative epithelial signaling areas, the tertiary enamel knots and enamel free areas of the calcifying crown. Fgf15, Fgf17 and -20 were transiently colocalized in the thickened dental epithelium at E11.5. Later Fgf15 and -20 were exclusively expressed in the epithelial enamel knot signaling centers. In contrast, Fgf13 was present in the dental mesenchyme including odontoblasts cell lineage, whereas Fgf12 appeared transiently in the preodontoblasts.

CONCLUSIONS

The expression of the Fgf11-13, -15, -17 and -20 in the epithelial signaling centers and/or epithelial-mesenchymal interfaces at key stages of the tooth formation suggest important functions in odontogenesis. Future analyses of the transgenic mice will help elucidate in vivo functions of the studied Fgfs during odontogenesis and whether any of the functions of the tooth expressed epithelial and mesenchymal Fgfs of different sub-families are redundant.

Dact1-3 mRNAs exhibit distinct expression domains during tooth development

Wnt signaling is essential for tooth formation and Dact proteins modulate Wnt signaling by binding to the intracellular protein Dishevelled (Dvl). Comparison of the three known mouse Dact genes, Dact1-3, from the morphological initiation of mandibular first molar development through the onset of root formation using section in situ hybridization showed distinct, complementary and overlapping expression patterns for these genes. Whereas Dact2 expression was restricted to the dental epithelium, including the enamel knot signaling centers and pre-ameloblasts, Dact1 and Dact3 showed developmentally regulated expression in the dental mesenchyme. Both Dact1 and Dact3 mRNAs were first detected in the presumptive dental mesenchyme. After being downregulated from the condensing dental mesenchyme of the bud stage tooth germ, Dact1 was upregulated in the dental follicle mesenchyme at the cap stage and subsequently also in the dental papilla at the bell stage, where the expression persisted to the postnatal stages. In contrast, Dact3 transcripts persisted throughout the dental mesenchyme, including the preodontoblasts, during embryogenesis before transcripts were largely downregulated from the tooth germ postnatally. Collectively, these results suggest that Dact1 and -3 may contribute to early tooth formation by modulation of Wnt signaling pathways in the mesenchyme, including preodontoblasts, whereas Dact2 may play important signal-modulating roles in the adjacent epithelial cells including the enamel knot signaling centers and pre-ameloblasts. Future loss-of-function studies will help elucidate whether any of these functions are redundant, particularly for Dact1 and Dact3.