Comparison of gene expression related to maxillary incisor development between the wild type and the homozygous rsey rats with maxillary supernumerary incisors

This study aims to investigate the developmental process of the maxillary incisor tooth in the wild type SD and in the homozygous rat small eye. Rat small eyes; rSey; is inherited a dominant mutation in Pax-6 gene. The mutation leads to impaired migration of anterior midbrain neural crest cells as well as the developmental defects of central nervous system, resulting in lack of eyes and nose, facial cleft and excess maxillary incisor-like structure in the homozygote. Histological observation of serial head sections and whole mount in situ hybridisation of the genes involved in early tooth development were carried out between E12 and E15 wild type and homozygous embryos. The supernumerary incisor-like structure was characterized by histological analysis of twenty E20 homozygous rSey fetuses. In wild type, the initiation of maxillary incisor formation is composed of the fusion of the medial and lateral primary dental placodes (M-PDP and L-PDP) at E13-14 and it develops into bud stage of incisor at E15. In the rSey homozygous fetus, the fusion of PDPs does not take place and they remain separated. Eventually, M-PDP starts invagination and proceeds incisor development. L-PDP also starts invagination, then 25% of them develop incisor-like structures while the rests are arrested at bud stage. The PDPs fusion is concomitant with the fusion of facial primodia, which is interrupted in the homozygote. Whole mount in situ hybridization showed that the genes involved in early tooth development (Fgf-8, Pitx-2, Shh, Bmp-4, Msx-1 and Pax-9) are expressed in wild type maxillary incisor at the initiation stage. This expression pattern is also seen in homozygous M-PDP and L-PDP. These results suggest that Pax-6 mutated in homozygous rSey is not associated to the incidence of supernumerary incisor-like structure via gene controlling but rather via the failure of the fusion of facial primodia to form midface in the homozygote. The failure of the fusion leaves the potentiality to develop tooth of the separated PDPs and allows them to develop incisor as well as the supernumerary incisor-like teeth.