Possible role of heat shock protein (Hsp) 25 in the enamel organ during amelogenesis in the rat molar

The postnatal expression of heat shock protein (Hsp) 25 during the amelogen esis of rat molars was investigated by immunocytochemistry and confocal mic roscopy. The localization pattern of Hsp 25-immunoreactivity in the inner e namel epithelium and ameloblast cell layer of the rat molars was almost ide ntical to that in the rat incisors which we have previously reported: an in tense Hsp 25-immunoreactivity, which first appeared in the preameloblasts, was recognized in secretory ameloblasts and ruffle-ended ameloblasts with s tage-specific immunointensity. Confocal microscopy with Hsp 25-antibody and rhodamine-labeled phalloidin clearly demonstrated the co-localization of H sp 25 and actin filaments in the ameloblast layer, supporting our hypothesi s that this molecule might serve to reinforce the ameloblast layer during e namel formation as well as the formation and maintenance of the ruffled bor der in ruffle-ended ameloblasts. Interestingly, the enamel free area cells, which essentially lack the ability for enamel formation, showed the Hsp 25 -immunoreactivity during 4-11 days when they developed a ruffled border, bu t decreased in that immunoreactivity after postnatal 15 days following apop tosis. Since Hsp 25 has been shown to be a specific inhibitor of apoptosis, the enamel-free area cells contribute to determine the outline of dentin a t the cusped area. These data support our previous hypothesis on the divers e functions of Hsp 25 in amelogenesis.