Demonstration of amelogenin in the enamel-free cusps of rat molar tooth germs: immunofluorescent and immunoelectron microscopic studies.

The enamel-free cusps of 1-4 day-old rat mandibular first molars were investigated using the monoclonal antibody En3 against rat amelogenin at light and electron microscopic levels in order to clarify whether the enamel-free cusp is virtually devoid of enamel. At 1 day after birth, there were presecretory ameloblast-like cells (PALCs), which were short and were not polarized, at the cusp tips. They were close to the outer enamel epithelium. Hematoxylin positive enamel matrix was not distinctly observed in the enamel-free cusp by light microscopy, but almost continuous immunofluorescence for amelogenin was detected at the interface between PALCs and dentin. The penetration of immunopositive material toward the dental pulp was also observed in the enamel-free cusp. At 4 day after birth, both in the frontal section and in the horizontal section, almost continuous immunofluorescence was recognized at the interface between PALCs and dentin in the enamel-free cusp. The penetration of amelogenin toward the dental pulp was not seen in the enamel-free cusp. By immunoelectron microscopy, immunolabelling was recognized in the Golgi apparatus of PALCs, in a layer of amorphous material at the interface between PALCs and dentin, and in stippled material-like substance in the intercellular space between PALCs. Although no basement membrane was observed beneath PALCs, they did not have Tomes' processes. These investigations suggest that PALCs in the enamel-free cusp differentiate into the secretory cells and that they can synthesize and secrete the amorphous material containing amelogenin at the interface between PALCs and dentin. The penetration of amelogenin toward the dental pulp might play a role in the interaction between PALCs and odontoblasts in the enamel-free cusp and/or the initiation of mineralization of predentin.