IN-SITU INVESTIGATION OF VITAMIN-D-RECEPTOR, ALKALINE-PHOSPHATASE, AND OSTEOCALCIN GENE-EXPRESSION IN OROFACIAL MINERALIZED TISSUES

The aim of this study was to investigate the expression pattern of 1,2 5-dihydroxyvitamin D-3 receptor (VDR) and vitamin D-responsive gene ex pression during the steps of hard tissue formation in ore-facial devel opment. In situ hybridization of VDR, alkaline phosphatase, and osteoc alcin transcripts was performed in the mandibles of growing rats. Oste oblasts were used as the internal positive control for in situ detecti on of VDR messenger RNAs. Transcripts were present throughout the stag es of differentiation and in differentiated osteoblasts and osteocytes , and showed some anatomical specificities in their developmental expr ession pattern. In dental tissues, VDR was strongly expressed in the i nner dental epithelium at the beginning of the presecretion stage and, after a transient decrease at the end of the presecretion stage, in s ecretion stage ameloblasts. VDR was continuously expressed in epitheli al supraameloblastic cells. During dentin formation, VDR was mainly pr esent in subodontoblastic cells and was down-regulated during the term inal differentiation of odontoblasts. In these cells, VDR expression a ppeared to be induced by 1,25-dihydroxyvitamin D-3 injection. These da ta confirm that VDR is expressed in cells directly involved in mineral ized tissue formation: ameloblasts, odontoblasts, and osteoblasts. Fur thermore, they extend the idea of vitamin D sensitivity to cells that are not directly involved in this process: supraameloblastic, subodont oblastic, and osteoprogenitor cells. The differential expression patte rn of VDR in odontoblasts and osteoblasts together with the similarity in the expression of potential vitamin D-responsive genes (osteocalci n in odontoblasts and osteoblasts, and alkaline phosphatase in osteopr ogenitor and subodontoblastic cells) suggest the existence of a tissue specificity for the genomic action of 1,25-dihydroxyvitamin D-3, whic h may involve cooperation with additional nuclear factors.