Electron microscopy of abnormal secretory products of rat incisor ameloblasts in tetracycline-induced lesions.

Lesions were created in the incisor enamel organs of adult rats by a single dose of tetracycline, and the secretory products in these lesions were examined with the transmission electron microscope. The following abnormal products were found: 1) circumscribed areas of a mottled uncalcified material within the enamel; 2) an uncalcified material consisting of tubules with dimensions similar to the crystal coat of normal enamel, 3) spherical calcified masses which at their periphery resemble normal enamel; 4) layers of a basal lamina-like material in the extracellular spaces between Tomes' processes, and an attachment site-like modification of the cell borders next to this material. It is suggested a) that the enamel tubule is a primary secretory product of the ameloblast; in normal enamel it would support the growth of the crystal; b) that, in the lesion, enamel can grow by diffusion of precursors from distant ameloblasts, while rat enamel normally grows only in direct contact with the ameloblast cell membrane.