Amelin: an enamel-related protein, transcribed in the cells of epithelial root sheath.

Since 1974, when Slavkin and his collaborators proposed the epithelial origin of cementum, many experiments have been carried out to provide evidence for deposition of enamel-related proteins along the root surface. However, neither amelogenin nor other proteins have fully satisfied expectations. In previous studies, we have identified a novel mRNA coding for an extracellular-like protein which we called amelin. It was expressed at high levels in secretory and postsecretory ameloblasts in rat molars and incisors. In situ hybridization experiments described in the present study also localized the amelin message to epithelial cells adjacent to the peripheral surface of newly deposited dentin in the root end and to cells embedded in cellular cementum in molars. In incisors, the amelin RNA positive cells were detected in the area where cementum formation had been initiated. No amelogenin RNA signal was found in the cells at the root surface. We postulate that the epithelial cells of the root sheath as well as the ameloblasts are synthesizing amelin which might be one of the key proteins coupled to the process of cementogenesis.