Gene expression and immunolocalisation of amelogenins in developing embryonic and neonatal hamster teeth

Amelogenins are a group of related matrix proteins, synthesised and secreted by ameloblasts during the formation of dental enamel. We have examined expression patterns and the tissue distribution of amelogenins by in situ hybridisation and by immunohistochemistry of developing teeth of embryonic (E12-E15) and neonatal (1- to 4-day-old) golden hamsters. Amelogenin expression and (intracellular) immunostaining for amelogenins were first observed in late embryonic stages in E14 incisors and E15 first molars in partially polarised pre-ameloblasts located along a thin layer of predentine before any overt deposition of enamel. Expression of mRNA and protein staining for amelogenins increased with age and early pre-dentine became immunopositive. The highest mRNA levels and substantial immunostaining for amelogenins were noted in neonatal-stage secretory ameloblasts fully engaged in enamel matrix deposition. After completion of the secretory phase, amelogenin gene expression continued at a lower level in post-secretory stages and was seen in transition-phase and maturation-phase ameloblasts. No amelogenin transcripts were observed in odontoblasts at any stage of their development. However, young odontoblasts stained weakly with anti-amelogenin antibodies before they formed the first layer of dentine, although this staining disappeared in odontoblasts at later stages of development. We conclude that amelogenin gene transcription occurs as early as the polarisation stage of pre-ameloblasts and is closely followed by translation of mRNA into amelogenin proteins. Odontoblasts do not transcribe the amelogenin gene and probably endocytose and digest amelogenins from the pre-dentine. Amelogenins are also transcribed but at a low level in post-secretory stages of amelogenesis.