EXPRESSION AND LOCALIZATION OF SULFATED GLYCOPROTEIN-2 MESSENGER-RNA IN THE RAT INCISOR TOOTH AMELOBLASTS - RELATIONSHIPS WITH APOPTOSIS

The expression of sulphated glycoprotein-2 (SGP-2) is associated with the onset of cellular atrophy and death in many rodent tissues. This g ene has a multifunctional involvement that includes apoptosis, spermat ogenesis, promotion of cell-cell interactions, modulation of complemen t systems and tissue regeneration and remodelling. Using decalcified m andibles, mRNA for SGP-2 in rat incisor tooth ameloblasts was examined by in situ hybridization using S-35 riboprobes. The rat incisor is un ique in that, at one time, all stages of the complex life cycle of the ameloblasts are represented along the length of the enamel forming as pect of the tooth. The pre-ameloblasts only secrete enamel matrix afte r mitosis. When the full thickness of the enamel has been formed, a re markable transition in phenotype takes place in the ameloblast This tr ansition is accompanied by apoptosis or programmed cell death of appro ximately 25% of ameloblasts. An additional 25% of ameloblasts undergo apoptosis when maturation of enamel matrix takes place with removal of water and protein from the increasingly mineralized matrix. In the pr esent study, expression of SGP-2 was localized most often in the post- secretory transition and maturation ameloblasts. In contrast, the pres ecretory and secretory ameloblasts did not demonstrate specific hybrid ization signals. Consistently, neither the odontoblasts nor the pulp d emonstrated hybridization signals. Hence our results support other pub lished results which show that increased expression of SGP-2 is associ ated with apoptosis. The exact function of the SGP-2 gene and its prod ucts is not fully defined. However, the results of our study show that expression of the SGP-2 gene may provide an early indication of prese nce of apoptosis in rat incisor ameloblasts.