Leukoedema: ultrastructural and histochemical observations.

The ultrastructural features of 12 cases of leukoedema were investigated and compared with clinically normal buccal mucosa. Histochemistry was undertaken to try to resolve some of the observed ultrastructural changes. The characteristic "intracellular edema" of the epithelial cells in leukoedema is due to vacuolation in the cytoplasm of cells. Abnormal mitochondria were observed in these cells. The vacuoles contained a granular material somewhat like clumped glycogen granules, but histochemistry failed to identify this material as glycogen. Towards the surface of the epithelium, the vacuolated cells collapsed into a compact layer of flattened cells. The outer cells of this layer abruptly swelled again to form the characteristic superficial layer of "ballooning" cells of leukoedema. The latter cells were not vacuolated but contained remnants of organelles, membraned vesicles with remnants of organelles, keratohyalin granules and structures apparently related to keratohyalin granules. We propose that the vacuolation represents a limited reversible form of cellular degeneration resulting from cell damage and that impeded mitochondrial function may be the cause of the vacuolation. The superficial "ballooning" cells are degenerated cells. The flattening of the vacuolated cells into a compact layer and the presence of keratohyalin granules and keratohyalin-like structures in the superficial cells are regarded as features of an aborted form of keratinization.