TIMING OF THE EXPRESSION OF ENAMEL GENE-PRODUCTS DURING MOUSE TOOTH DEVELOPMENT

In order to understand the mechanisms involved in tooth development it is important to define the timing for tissue-specific gene expression . A consequence of ameloblast cell differentiation is the sequential e xpression of tissue-specific genes whose products form the enamel extr acellular matrix. The ameloblast phenotype has been characterized as c onsisting of two major classes of proteins: amelogenins and non-amelog enin proteins such as anionic enamel proteins (enamelins, tuft protein s, tuftelin, sulfated proteins) and enamel proteases. The postulated f unctions for the anionic enamel proteins are as nucleators for hydroxy apatite crystal formation while amelogenins control the crystal size, growth and orientation. While the amelogenins have been well character ized, detailed knowledge for anionic enamel proteins has been sparse. In the present study, we designed experiments to characterize one of t he anionic enamel proteins from mouse molars, tuftelin, and to determi ne the timing of expression of this protein during molar tooth develop ment. Our results showed the initial detection of tuftelin transcripts within proliferating inner enamel epithelial cells at very early stag es of tooth development (13 days of embryonic development equivalent t o the bud stage of tooth development). These data provide direct evide nce that invalidates previous dogmas that enamel proteins were synthes ized by polarized, non-dividing, fully differentiated ameloblast cells . In addition, tuftelin was found to be synthesized also by dental pap illa mesenchyme cells suggesting that this protein is not enamel-speci fic. These data taken together open the possibility that the tuftelin present in the dentino-enamel junction could be secreted by both, preo dontoblast cells and preameloblast cells. It might also suggest a poss ible different role for tuftelin than nucleator of hydroxyapatite crys tals.