Robert Frank: A Pioneer and Visionary Scientist in Oral Medicine

The Faculty of Dental Surgery of the University of Strasbourg would not be the great institute it is today if it were not for an admirable, or rather extraordinary, man: Robert Frank (May 21, 1924-August 7, 2020). He was the first dean of the Faculty of Dental Surgery at the University of Strasbourg, France. He was a dynamic and notorious leader. He brought forward amazing progress-in administration, dental research, and public health at both national and international levels. He was recognized for his top-quality innovative research and elected the 60th president of the International Association for Dental Research (1983-1984). Upon retirement, he continued his commitment to advancing humanity through painting his vision of the microscopic world and developing a "nanoart" vision.

Heimler Syndrome

Heimler syndrome is a rare syndrome associating sensorineural hearing loss with retinal dystrophy and amelogenesis imperfecta due to PEX1 or PEX6 biallelic pathogenic variations. This syndrome is one of the less severe forms of peroxisome biogenesis disorders. In this chapter, we will review clinical, biological, and genetic knowledges about the Heimler syndrome.

Gene Expression during Palate Fusion in vivo and in vitro

Failure of secondary palate fusion during embryogenesis is a cause of cleft palate. Disappearance of the medial epithelial seam (MES) is required to allow merging of the mesenchyme from both palatal shelves. This involves complex changes of the medial edge epithelial (MEE) cells and surrounding structures that are controlled by several genes whose spatio-temporal expression is tightly regulated. We have carried out morphological analyses and used a semi-quantitative RT-PCR technique to evaluate whether morphological changes and modulation in the expression of putative key genes, such as twist, snail, and E-cadherin, during the fusion process in palate organ culture parallel those observed in vivo, and show that this is indeed the case. We also show, using the organotypic model of palate fusion, that the down-regulation of the transcription factor snail that occurs with the progression of palate development is not dependent on fusion of the palatal shelves. Abbreviations: dsg1, desmoglein1; EMT, epithelial-mesenchymal transition; MEE, medial edge epithelium; MES, medial epithelial seam; RT-PCR, reverse-transcriptase polymerase chain-reaction.

Amelogenesis Imperfecta: 1 Family, 2 Phenotypes, and 2 Mutated Genes

Amelogenesis imperfecta (AI) is a clinically and genetically heterogeneous group of diseases characterized by enamel defects. The authors have identified a large consanguineous Moroccan family segregating different clinical subtypes of hypoplastic and hypomineralized AI in different individuals within the family. Using targeted next-generation sequencing, the authors identified a novel heterozygous nonsense mutation in COL17A1 (c.1873C>T, p.R625*) segregating with hypoplastic AI and a novel homozygous 8-bp deletion in C4orf26 (c.39_46del, p.Cys14Glyfs*18) segregating with hypomineralized-hypoplastic AI in this family. This study highlights the phenotypic and genotypic heterogeneity of AI that can exist even within a single consanguineous family. Furthermore, the identification of novel mutations in COL17A1 and C4orf26 and their correlation with distinct AI phenotypes can contribute to a better understanding of the pathophysiology of AI and the contribution of these genes to amelogenesis.

Homozygous and compound heterozygous MMP20 mutations in amelogenesis imperfecta

In this article, we focus on hypomaturation autosomal-recessive-type amelogenesis imperfecta (type IIA2) and describe 2 new causal Matrix metalloproteinase 20 (MMP20) mutations validated in two unrelated families: a missense mutation p.T130I at the expected homozygous state, and a compound heterozygous mutation having the same mutation combined with a nucleotide deletion, leading to a premature stop codon (p.N120fz*2). We characterized the enamel structure of the latter case using scanning electron microscopy analysis and microanalysis (Energy-dispersive X-ray Spectroscopy, EDX) and confirmed the hypomaturation-type amelogenesis imperfecta as identified in the clinical diagnosis. The mineralized content was slightly decreased, with magnesium substituting for calcium in the crystal structure. The anomalies affected enamel with minimal inter-rod enamel present and apatite crystals perpendicular to the enamel prisms, suggesting a possible new role for MMP20 in enamel formation.

From the transcription of genes involved in ectodermal dysplasias to the understanding of associated dental anomalies

Orodental anomalies are one aspect of rare diseases and are increasingly identified as diagnostic and predictive traits. To understand the rationale behind gene expression during tooth or other ectodermal derivative development and the disruption of odontogenesis or hair and salivary gland formation in human syndromes we analyzed the expression patterns of a set of genes (Irf6, Nfkbia, Ercc3, Evc2, Map2k1) involved in human ectodermal dysplasias in mouse by in situ hybridization. The expression patterns of Nfkbia, Ercc3 and Evc2 during odontogenesis had never been reported previously. All genes were indeed transcribed in different tissues/organs of ectodermal origin. However, for Nfkbia, Ercc3, Evc2, and Map2k1, signals were also present in the ectomesenchymal components of the tooth germs. These expression patterns were consistent in timing and localization with the known dental anomalies (tooth agenesis, microdontia, conical shape, enamel hypoplasia) encountered in syndromes resulting from mutations in those genes. They could also explain the similar orodental anomalies encountered in some of the corresponding mutant mouse models. Translational approaches in development and medicine are relevant to gain understanding of the molecular events underlying clinical manifestations.

Mutation update for the CSB/ERCC6 and CSA/ERCC8 genes involved in Cockayne syndrome

Cockayne syndrome is an autosomal recessive multisystem disorder characterized principally by neurological and sensory impairment, cachectic dwarfism, and photosensitivity. This rare disease is linked to mutations in the CSB/ERCC6 and CSA/ERCC8 genes encoding proteins involved in the transcription-coupled DNA repair pathway. The clinical spectrum of Cockayne syndrome encompasses a wide range of severity from severe prenatal forms to mild and late-onset presentations. We have reviewed the 45 published mutations in CSA and CSB to date and we report 43 new mutations in these genes together with the corresponding clinical data. Among the 84 reported kindreds, 52 (62%) have mutations in the CSB gene. Many types of mutations are scattered along the whole coding sequence of both genes, but clusters of missense mutations can be recognized and highlight the role of particular motifs in the proteins. Genotype-phenotype correlation hypotheses are considered with regard to these new molecular and clinical data. Additional cases of molecular prenatal diagnosis are reported and the strategy for prenatal testing is discussed. Two web-based locus-specific databases have been created to list all identified variants and to allow the inclusion of future reports (www.umd.be/CSA/ and www.umd.be/CSB/).

Genetische Störungen der Zahnentwicklung und Dentition

Die Zähne sind Organe, die aus ektodermalen epithelialen Aussackungen im Bereich des 1. Kiemenbogens entstehen, gesteuert von epitheliomesenchymalen Interaktionen. Dabei spielen zahlreiche Signalmoleküle speziell der 4 großen Familien TGF-β, FGF, Hedgehog und WNT sowie diverse Transkriptionsfaktoren eine Rolle. Eine Beteiligung der Retinoide an der Odontogenese ist durch umfangreiche Befunde belegt, auch wenn die Inaktivierung relevanter Gene in Mausmodellen meist keine Zahnanomalien verursacht. Die Zahnentwicklung wird klassischerweise in verschiedene Stadien eingeteilt: Entstehung der Zahnleiste, der Zahnknospe, der Schmelzkappe, der Schmelzglocke, die Wurzelbildung und der Zahndurchbruch. Anomalien der Zahnentwicklung können isoliert oder gemeinsam mit anderen Symptomen im Zusammenhang mit Syndromen auftreten. Sie können genetisch bedingt sein oder unter Einwirkung teratogener Stoffe während der Bildung und Mineralisierung der Zahnkeime zustande kommen. Dentibukkale Entwicklungsanomalien treten im Kontext seltener Erkrankungen auf und finden zunehmend Beachtung, da sie bei bestimmten Erkrankungen in der Diagnostik und als prädikative Faktoren wichtige Anhaltspunkte geben können. Allerdings ist hierfür eine interdisziplinäre und internationale Kooperation notwendig, die bislang erst in Ansätzen verwirklicht wurde.

Oro-dental features as useful diagnostic tool in Rubinstein-Taybi syndrome

Rubinstein-Taybi syndrome (RTS; OMIM # 180849) is a well-known disorder characterized by mental and growth retardation, broad thumbs and great toes, and unusual facial characteristics. We studied oro-dental findings in a group of RTS patients: 12 from the UK, 2 from Greece, and 26 from France. All were examined by two investigators, using the Diagnosing Dental Defects Database record form to document these. Various oro-dental features were found: small mouth, retrognathia, micrognathia, highly arched and narrow palate, talon cusps, expressed crowding, screwdriver incisors, cross bites, and enamel hypoplasia. Eruption was usually normal. Specific attention for these anomalies should facilitate diagnosis and help adequate management.

Septo-optic dysplasia, subglottic stenosis and skeletal abnormalities: a case report

We report a girl with septo-optic dysplasia in association with subglottic stenosis, sagittal craniosynostosis, osteoporosis and dental anomalies. It is uncommon for patients with septo-optic dysplasia to have multiple, extra-cranial malformations. A number of differential diagnoses were considered in this case, including Cole-Carpenter syndrome, Pfeiffer syndrome and osteoglophonic dwarfism. However, none can account for all the abnormalities seen. We therefore believe that this is a previously unreported, but highly distinctive, phenotype.

Systematic review of studies comparing the anti-caries efficacy of children's toothpaste containing 600 ppm of fluoride or less with high fluoride toothpastes of 1,000 ppm or above

OBJECTIVE

To determine the clinical effectiveness of low fluoride (F) toothpastes, containing 600 ppm or less, with toothpaste containing 1,000 ppm or more fluoride in preventing dental caries.

DESIGN

Systematic review of randomised controlled trials comparing low fluoride toothpastes containing 600 ppm F or less with toothpastes containing 1,000 ppm or more in children or adults.

RESULTS

Seven randomised controlled trials were included. These were categorised into two groups depending on the fluoride concentration in the low fluoride group (250 and 500 ppm) and analysed separately. 250 PPM: Results of the meta-analysis were statistically significant (p = 0.002 and 0.0005) and in favour of the control group (1,000 ppm). DFS increments in the 250-ppm group were 0.6-0.7 greater than the 1,000-ppm group. 500 PPM: Only two studies were in this category, and one of them failed to present the baseline caries levels, so a meta-analysis was not carried out.

CONCLUSION

250 ppm fluoride dentifrice was not as effective in caries prevention in permanent dentition as dentifrice containing 1,000 ppm F or more. More studies have to be carried out to test the anticaries efficacy of 500 against 1,000 ppm and above.

Is the Fluoride Concentration Limit of 1,500 ppm in Cosmetics (EU Guideline) Still Up-to-Date?

The 1,500 ppm fluoride (F(-)) concentration limit set up for European cosmetic toothpaste needs to be considered in terms of effectiveness and toxicity. There is clear evidence for a dose-response relationship between F(-) concentration and caries-preventive effect (25-30% caries reduction achieved with a 1,000 ppm F(-) dentifrice). Clinical significance of higher F(-) concentration has not been totally proven. Low F(-) toothpastes (<500 ppm F(-)) have been marketed to reduce F(-) ingestion by young children in order to minimize the risk of dental fluorosis. Their effectiveness is, however, unclear. Fluoride intake and caries risk assessment should be performed by dental professionals and prescription of topical fluoride should be adjusted individually.

R-twist gene expression during rat palatogenesis

Palatal clefting is often associated with premature fusion of cranial sutures in human craniosynostosis syndromes, many of which are characterised by mutations affecting the fibroblast growth factor receptor (FGFR) gene family. In palatal fusion, epithelio-mesenchymal transition (EMT) contributes to the dispersion of the midline epithelial seam. EMT has also been observed in neoplastic epithelial cells in relation to the acquisition of malignant characteristics where morphological changes are accompanied by rapid switching in the expression of fgfr2 from the epithelial type (kgfr) to the mesenchymal type (bek). The twist gene codes for a basic helix-loop-helix transcription factor putatively involved in regulation of transcription of fgfr2. Mutations in the TWIST gene have been described as being responsible for the Saethre-Chotzen syndrome, an autosomal dominant craniosynostosis associated with cleft palate as well as other disturbances of the facial skeleton. In this study we have analysed the distribution of twist transcripts during rat palatogenesis in vivo from 14.5 to 17.5 days post coitum by in situ hybridisation with digoxygenin-labelled ssDNA probes. twist transcripts were found to be concentrated in mesenchymal cells beneath the epithelium at the tip of the palatal shelves immediately prior to, and during fusion as well as in a localised epithelial area at the tip of the shelves prior to fusion, thereby implicating twist gene expression in the process of palatogenesis. This pattern of expression illuminates the disturbances of maxillary growth that occur in human craniosynostotic syndromes.

Midline fusion in the formation of the secondary palate anticipated by upregulation of keratin K5/6 and localized expression of vimentin mRNA in medial edge epithelium

Secondary palatal fusion is dependent on targeted removal of the epithelium between the palatal shelves. Aseptically delivered rat embryos 15 through 18 days post coitum (dpc) were probed with DIG-labeled antisense and sense ssDNA probes for spliced exon sequences flanking intron E of cytokeratins K5/6 and spliced exon sequences flanking intron F of vimentin. Cytokeratin K5/6 expression was upregulated in the medial edge epithelium (MEE) prior to rotation of the palatal shelves and in the vomerine epithelium in the region of fusion with the palate. K5/6 expression continued in the medial epithelial seam (MES) and in epithelial islands during breakdown of the MES. Vimentin expression was not detected in the MEE prior to rotation but was specifically upregulated in the MEE following rotation and prior to midline contact and continued in the MES and in epithelial cells identifiable during the breakdown of the MES. Initiation of vimentin upregulation in the MEE prior to contact of the palatal shelves was tested by serum-free organ culture of palates from embryos at 15.5 dpc with the shelves separated by a biocompatible membrane. Vimentin upregulation occurred in the epithelium specifically in the region of anticipated contact. These results are interpreted as indicating that i) cytokeratin K5/6 expression may play a critical role in the integration of the epithelial layers of the MES to ensure subsequent merging of the mesenchyme and ii) epithelial cells in the MEE are specifically 'primed' to upregulate expression of mesenchymal genes prior to integration into and breakdown of the MES.

The variable expressivity and incomplete penetrance of the twist-null heterozygous mouse phenotype resemble those of human Saethre-Chotzen syndrome

Most targeted gene mutations are recessive and analyses of gene function often focus on homozygous mutant phenotypes. Here we describe parts of the expression pattern of M-twist in the head of developing wild-type mice and present our analysis of the phenotype of heterozygous twist- null animals at around birth and in adults. A number of twist -null heterozygous mice present skull and limb defects and, in addition, we observed other malformations, such as defects in middle ear formation and the xyphoïd process. Our study is of interest to understand bone formation and the role of M-twist during this process, as within the same animal growth of some bones can be accelerated while for others it can be delayed. Moreover, we show here that expressivity of the mouse mutant heterozygous phenotype is dependent on the genetic background. This information might also be helpful for clinicians, since molecular defects affecting one allele of the human H-twist ( TWIST ) gene were identified in patients affected with Saethre-Chotzen syndrome (SCS). Expressivity of this syndrome is variable, although most patients present craniofacial and limb malformations resembling those seen in mutant mice. Thus the mutant mouse twist -null strain might be a useful animal model for SCS. The twist -null mutant mouse model, combined with other mutant mouse strains, might also help in an understanding of the etiology of morphological abnormalities that appear in human patients affected by other syndromes.

Expression of p21(WAF1/CIP1) during mouse odontogenesis

p21(WAF1/CIP1) is a cyclin-dependent kinase (Cdk) inhibitor. This protein may function during development as an inducible growth inhibitor that contributes to cell cycle exit and differentiation. The expression pattern of p21 during mouse embryogenesis was correlated with terminal differentiation of multiple cell lineages including skeletal muscles, cartilage, skin and nasal epithelium. p21 expression was analyzed by in situ hybridization during odontogenesis as well as during in vitro tooth development in chemically defined medium with or without retinoic acid. p21 transcripts were detected in the restricted area of the inner dental epithelium during late cap and initial bell stages and then confined to the post-mitotic odontoblasts and ameloblasts. The replicating cells were devoid of any signal. The distribution of p21 mRNA in vitro, whatever the culture conditions, was similar to the in vivo pattern. p21 protein immunolocalization was superimposed on the transcripts distribution but more restricted in ameloblasts. TGFbeta1 is known to induce p21 expression. During dental cytodifferentiations, TGFbeta1 and p21 expressions overlap. Growth inhibition by TGFbeta1 may be associated with p21 induction.

Roles of retinoic acid receptors and of Hox genes in the patterning of the teeth and of the jaw skeleton

Retinoic acid receptors and transcriptional factors encoded by Hox genes play key roles in vertebrate development and belong to an integrated functional network. To investigate the actual functions of these molecules during ontogenesis and in particular in the patterning of the cranial neural crest cells giving rise to the teeth and to the jaw bones, we have generated null mutant mice lacking functional retinoic acid receptors or Hox genes by gene targeting in embryonic stem cells.

Lipids in predentine and dentine

Using two histochemical methods, malachite green-aldehyde and iodoplatinate, phospholipids were visualized in the predentine of rat incisors in the spaces located between collagen fibers and in dentine as needle-like structures located along individual or groups of mineralizing collagen fibers. The same staining pattern was seen with phospholipase A2-gold. Autoradiographic investigation using 3H choline as labelled precursor, visualized the incorporation of membrane-associated and extracellular choline-containing phosphatidyl choline and sphingomyelin. The cell and membrane-associated labelling decreased gradually between 24 and 4 days, whereas incorporation of the labelled precursor as stable extracellular matrix component was seen in dentine. In addition to these investigations, pharmacologically induced (suramine) and genetically (Krabbe's disease) lysosomal storage pathology was investigated. Defects due to lipid metabolism alterations were seen in predentine and/or in dentine. The major differences visualized here between the non-mineralized and mineralized compartments and interactions between phospholipids and proteoglycans, support the view that phospholipids as matrix components play an important role in the mechanisms of dentine formation and mineralization.

In vitro effects of retinoic acid on mouse incisor development

The developing dentition is known to express the complete set of retinoic acid (RA) nuclear receptors and cytoplasmic RA-binding proteins (CRABPI and II), and RA is required for in vitro mouse molar morphogenesis, so the role of RA during in vitro mouse incisor development was investigated. Histological procedures, immunocytochemical detection of proliferating cells, immunofluorescence detection of laminin, and in situ hybridization with RNA probes for CRABPI and II were done on the tooth-germ cultures either in the presence or in the absence of RA. RA appeared to control initial morphogenesis, particularly the asymmetrical growth of the cervical loop, and to regulate required differential mitotic activity. RA seemed also to be involved in asymmetrical laminin deposition. The distribution of the CRABP gene transcripts was similar during in vivo and in vitro incisor development. However, CRABPI gene transcript distribution in the labial part of the epithelial loop was detected in vitro only in the presence of RA. A direct role of the CRABPs during tooth development is, however, unlikely because Ch55, a synthetic RA analogue that does not bind to CRABP, had the same effects as RA on in vitro incisor development.

Expression of nuclear retinoic acid receptors during mouse odontogenesis

The developmental expression of retinoic acid (RA) nuclear receptors RAR(alpha, beta, gamma) and RXR(alpha, beta, gamma) was analysed during mouse odontogenesis by in situ hybridization on frozen sections and compared with the expression patterns of the cellular retinoic acid binding proteins CRABPI and II. The transcripts distribution of each RAR and RXR was basically similar in developing molars and incisors. RAR alpha and RXR alpha were preferentially expressed in dental epithelia, whereas RAR gamma and RXR gamma were transcribed in the dental mesenchyme. RAR beta, RAR gamma and RXR beta displayed both epithelial and mesenchymal expression. RAR beta expression was initiated during bell stage. RXR gamma transcripts were observed only at day 19.5 post coitum in the mitogenic mesenchyme facing the epithelial loops. Odontoblasts expressed RAR beta and RAR gamma, RXR alpha and RXR beta. Preameloblasts expressed RXR alpha and RXR beta and ameloblasts RXR gamma, RXR alpha and RXR beta. RAR alpha transcription in the incisor preameloblasts and ameloblasts was not observed in the first molar. The coexpression between RARs and RXRs might be important to form RAR/RXR heterodimers which are necessary to activate the transcriptions of target genes. CRABPI and CRABPII demonstrated graded variation of expression during odontogenesis in the mesenchyme and in the inner dental epithelium respectively. The pattern of CRABPI transcripts overlapped at least partially with expressions of all the studied nuclear receptors whereas CRABPII epithelial expression was superimposed with the transcription of RAR alpha, RXR alpha and RXR beta. These cytoplasmic proteins might participate in the storage and/or metabolism of RA and then distribute RA to colocalized nuclear receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Galactosylceramide lipidosis (Krabbe's disease) and deciduous dental tissues. A case report

The deciduous teeth of a 5-year-old girl affected by a protracted type of infantile globoid-cell leukodystrophy or Krabbe's disease, a galactosylceramide lipidosis, were examined with the scanning and transmission electron microscopes. The diagnosis was established on the basis of galactosylceramidase activity (0.04 mu katal/kg proteins versus 0.86 +/- 0.49 in the normal situation) and abnormalities in the white matter and trouble in myelinization assessed by magnetic resonance imaging. In spite of the fact that enamel, dentin and pulp are extraneural tissues, they were also affected by the disease. Ultrastructural observations of dental mineralized tissues revealed enamel hypoplasia as well as abnormal proteinaceous lamina and hypomineralized lines or bands. Inside the mantle dentin, dendritic inclusions of amorphous material were seen. Lysosomal storage inclusions were observed in all the cells of the dental pulp. The myelin sheaths of dental peripheral nerves displayed severe degenerative changes. Proliferation of dentinoclasts contributed to stimulate root resorption.

Effects of retinoids on tooth morphogenesis and cytodifferentiations, in vitro

The first embryonic lower mouse molar was used as a model system to investigate the effects of two retinoids, retinoic acid (RA) and a synthetic analogue, Ch55, on morphogenesis and cytodifferentiations in vitro. Exogenous retinoids were indispensable for morphogenesis of bud, cap and bell-stage molars in serum-free, chemically-defined, culture media. Transferrin and RA or transferrin and Ch55 acted synergistically in promoting morphogenesis from bud and cap-stage explants. Transferrin, per se, had no morphogenetic effect. Epithelial histogenesis, odontoblast functional differentiation and ameloblast polarization always occurred in RA-depleted explants. Comparison of the distributions of bromodeoxyuridine (BrdU) incorporation between explants cultured in the absence or presence of RA revealed that RA could modify the patterns of cell proliferation in the inner dental epithelium and dental mesenchyme. Inner dental epithelium cell proliferation is regulated by the dental mesenchyme through basement membrane-mediated interactions, and tooth morphogenesis is controlled by the dental mesenchyme. Laminin is a target molecule of retinoid action. Using a monospecific antibody, we immunolocalized laminin and/or structurally-related molecules sharing the laminin B chain in the embryonic dental mesenchyme and in the dental basement membrane and showed that RA could promote the synthesis or secretion of these molecules. Based on previous in situ hybridization data, it was speculated that CRABPs might regulate the effects of RA on embryonic dental cell proliferation. The fact that Ch55, a retinoid which does not bind to CRABPs, is 100 times more potent than RA in promoting tooth morphogenesis in vitro seems to rule out this hypothesis. On the other hand, the stage-specific inhibition of tooth morphogenesis by excess RA is consistent with the hypothesis that CRABPs might protect embryonic tissues against potentially teratogenic concentrations of free retinoids.

Involvement of cellular retinoic acid-binding proteins I and II (CRABPI and CRABPII) and of the cellular retinol-binding protein I (CRBPI) in odontogenesis in the mouse

The coordination of the activities of individual cells during development is regulated in part by epigenetic signals either encoded in the insoluble extracellular matrix or provided by small diffusible factors such as growth factors peptides and retinoids. Odontogenesis offers a suitable model to correlate the temporospatial distributions of such molecules, and of their cell receptors and ligands, with particular developmental processes. We have analyzed, by in situ hybridization, the distribution patterns of CRABPI, CRABPII and CRBPI transcripts during odontogenesis in the mouse. CRABPI transcripts were restricted to the mitogenic regions of the dental mesenchyme during late bell stages and were absent from post-mitotic odontoblasts. The only epithelial site of CRABPI transcription was the labial epithelial loop of the continuously growing incisor. CRABPII transcription was preponderant in the mitogenic zones of the dental epithelium: differential labeling of the dental epithelium occurred as early as the dental bud stage and during subsequent molar morphogenesis, this labeling became confined in the epithelial loops. The graded distribution of CRABPII transcripts along the anteroposterior axis of the continuously growing incisor was superimposed with the gradient of mitoses. CRABPII transcripts were absent from post-mitotic ameloblasts. It is concluded that during odontogenesis the expressions of the CRABPI and CRABPII genes are confined to regions exhibiting the highest rate of cell proliferation whenever differential mitotic activity is required. Moreover, the putative effects of retinoic acid on the regulation of cell proliferation kinetics in the dental epithelium and in the dental mesenchyme imply distinct CRABPs. CRBPI transcripts were restricted to the dental mesenchyme prior to the onset of CRABPI transcription. This observation supports the hypothesis that the two proteins might perform antagonistic functions in some retinoic acid-mediated developmental processes.