ENAMEL FREE AREAS IN RODENT MOLARS - ULTRASTRUCTURE OF BASEMENT-MEMBRANE IN RAT TOOTH GERM

At the cusp tip of rodent molar, there is a region of dentin without a n enamel cap. This region is called enamel free area (EFA). The surfac e collagen arrangement has been reported to differ between the EFA and the dentin covered with the enamel (DCE). To clarify the cause of thi s difference, we observed the ultrastructure of the basement membrane and the distal ends of the inner enamel epithelium (IEE) in rats. At 2 0 days prenatal, distal ends of IEE were relatively flat on both the D CE and EFA. Ultrastructurally, there was no difference between the bas ement membranes. At newborn, no marked changes were observed in the mo rphology of the distal end of IEE on the DCE or the EFA, but aperiodic microfibrils perpendicular to basal lamina were denser and longer on the DCE than the EFA. At 2 days postnatal, cytoplasmic extensions from distal end of IEE penetrated through basal lamina, and these extensio ns were more developed on the DCE than the EFA. On the DCE, collagen f ibrils ran into and between cytoplasmic extensions and were arranged p erpendicular to the surface. On the EFA, collagen fibrils ran parallel to the surface, and few collagen fibrils ran into and between cytopla smic extensions. These findings suggested that the differences in the collagen arrangement between the EFA and DCE are associated with the d evelopmental state of aperiodic microfibrils in the basal lamina benea th IEE and the morphology of the distal end of IEE.