Ultrastructural and histochemical changes and apoptosis of inner enamel epithelium in rat enamel-free area

The formation of an enamel-free area (EFA), a region of the dentin without an enamel cap at the cusp tip of rodent molar, is thought to depend on the specific differentiation and function of inner enamel epithelium of EFAs (EFA cells). The authors attempted to clarify both the ultrastructure and alkaline phosphatase (ALPase) activity of EFA cells up until tooth eruption by using rat mandibular first molars. Apoptosis was also examined. The EFA cells differentiated into secretory cells resembling differentiating ameloblasts but without Tomes' processes (postnatal day 1-3). No reactivity for ALPase was observed in the EFA cells. Enamel-like crystals were detected in close vicinity to dentin crystals at this stage. Thereafter, EFA cells became maturative ameloblast-like with ruffled border-like structures (postnatal day 5-8) and exhibited a strong reactivity for ALPase. These findings suggest that EFA cells change from secretory to absorptive cells within a short period and become reduced enamel epithelium at the early stage of tooth development. Apoptosis occurs in EFA cells, as it does in ameloblasts, but its significance seems to differ between the two cell types.