A light and scanning electron microscopic study of the development of enamel-free areas on the molar teeth of the rat

Abstract At the late bell of enamel organ development (about 20 days in utero) segments of the internal dental epithelium (IDE) at the future cusp tips of first mandibular molars appeared to make contact with the external dental epithelium (EDE); this relationship was maintained until about 3 to 4 days after birth. Dentinogenesis began at about birth; enamel formation began about the third day on the cusp slopes. However, those cells of the IDE at the enamel-free areas (EFAs) on the cusp tips which were in contact with the EDE remained low columnar and many of their nuclei failed to polarize; enamel was deposited only in sporadic fashion on the EFAs. Later, the EFA epithelial cells became shorter and less regularly arranged. The patches of enamel on EFAs, and even the dentine surface, showed resorption and, just before tooth emergence, these areas were virtually free of enamel. The enamel forming capacity of the EFA epithelium appears to be inhibited, in most cases totally but in some cases partially. The hypothesis is proposed that the EDE layer inhibits the enamel-forming capacity of those IDE cells which come into contact with it in early crown morphogenesis; EFA cells later resorb any enamel they deposited and some of the dentine beneath.