Differential Gene Expression in Primary and Secondary Dentinogenesis

Dentinogenesis is a continuous process of dentine secretion throughout the life of a tooth. Primary (before eruption) and secondary (after eruption) dentinogenesis are physiological processes, whereas tertiary dentinogenesis (reactionary and reparative) occurs in response to an injury. Histologically, no difference is noticeable between primary and secondary dentine, and they differ only in the rate of secretion (4µm/day and 0.4µm/day, respectively). Thus, odontoblasts are initially actively secreting cells, become quiescent and then may be up-regulated again in response to an injurious challenge. The expression profile of several genes (DSPP, DMP1, Collagen I) are commonly used as markers for odontoblasts and their levels may reflect cell secretory behaviour.

Objective: To investigate whether odontoblasts show differences in gene expression during primary and secondary dentinogenesis. Methods: Coronal pulps were removed from immature developing and mature erupted bovine teeth from 30month old animals and RNA from odontoblasts was selectively extracted (McLachlan et al – Arch Oral Biol 2003;48:373-83). After reverse transcription, expression of 20 genes was compared between the two populations of odontoblasts (Early stage (ES) and late stage (LS)) by RT-PCR (ADM, AMEL, BMP4, Clock Genes, Collagen III, Collagen I, DMPI, DSPP, MEPE, Msx1, Msx2, OSTEOCALCIN, PGAP I, TGFâ1, TGFâ1R, NESTIN, Alkaline Phosphatase, SHH, NaNog, LPR15. Results: Differential expression of some genes was observed in the 2 cell populations. For example, Amelogenin was only detected in ES, whereas osteocalcin was detected only in LS. Collagen I expression was however not differentially expressed, although Adrenomedullin and DMP1 were, up and down-regulated in LS, respectively. Conclusion: The differential changes in gene expression in primary and secondary dentinogenesis indicate modifications in transcriptional control of the cells and highlight the need to identify the nature of these control mechanisms both to characterize cell phenotype and to better understand how cell secretory behaviour can be controlled during tertiary dentinogenesis.