Ultrastructure of spindles and tufts in human dental enamel.

A transmission electron microscope (TEM) study was made of spindles and tufts as identified in the light microscope, from samples prepared by selected-area argon-ion-beam thinning. Spindles in human dental enamel were found to be continuous with dentinal tubules across the dentin-enamel junction (DEJ) and usually appeared at the DEJ as electron-lucent, empty channels nearly circular in cross-section. The spindles were found to cross prism boundaries, branched rarely, and some were occasionally found to be occluded or partially occluded with small needle-like crystals (approximately 5 nm width and approximately 70 nm length), granular material (approximately 1.5 nm diameter) and/or amorphous material. Near the DEJ, the majority of spindles had a diameter less than 2 microns, while in the prismatic enamel away from the DEJ, spindles with a diameter greater than 2 microns were generally found. Spindle varicosity was characterized by an enlargement of their diameter. Tufts started at the DEJ and were not associated with dentinal tubules. Two types of ultrastructure were observed in the TEM: (i) disrupted regions of enamel incorporating large voids (up to approximately 0.1 microns in diameter), or, more commonly, (ii) channels within the enamel occluded or partially occluded, with small needle-like crystals and granular and/or amorphous material similar to that found in the enamel spindles. It was concluded that spindles and tufts represent areas of hypomineralization with increased void volume and partial remineralization.