The Ultrastructure of Enamel from the Kangaroo Macropus giganteus: Tubular Enamel and its 'Black Spots' Defect

Abstract

A study of the enamel of M. giganteus by light microscopy and transmission electron microscopy on specimens prepared by argon-ion-beam thinning showed that normal tubular enamel consisted of closely packed long lath-like crystals of hydroxyapatite, thickness range 30-50 nm and width range 50-90 nm, and a micropore structure similar to that in human enamel. Enamel tubules, roughly circular in crosssection of approximately 0.2-1.5 µm diameter. and spaced 2-9 µm apart, were a common feature of tubular enamel and were not confined to any particular prism. Within the tubular enamel there were pockets of defective enamel containing: (1) big crystals (~ 250 nm diameter) of hydroxyapatite, whitlockite and/or octacalcium phosphate; (2) clusters of small (~ 15 nm diameter) crystals which were predominantly hydroxyapatite; (3) some amorphous material. These were poorly packed together, resulting in a porous (void diameter 25 nm) structure. The Rayleigh scattering of light due to the porosity of the pockets of defective enamel caused the so-called 'black spot' appearance under light microscopy. The defective enamel, which occurred typically in the interior, exhibited an ultrastructure consistent with progressive demineralization of the normal lath-like enamel crystals, followed by remineralization. The ultrastructure suggested that the tubules provided: (1) the pathways for dissolution materials to reach the sites of defective enamel: (2) the space for nucleation of crystals during remineralization.