An electron microscopic study of experimentally-induced comedo and effects of vitamin A acid on comedo formation

The mechanisms of comedogenesis and comedolysis were investigated at the ultrastructural level after applying oleic acid (OA) only, or oleic acid together with vitamin A acid (VAA), to rabbit ears daily for two weeks. 1) In the follicular epithelium of the experimentally-induced comedo (EIC) by OA, several ultrastructural changes similar to those in human comedo were observed. EIC in rabbit ears appeared to be induced both by the hyperkeratinization of the follicular epithelium and by the delayed desquamation of horny cells due to the persistence of intercellular binding apparatus. 2) VAA strongly inhibited the formation of EIC. In the follicular epithelium, two different types of changes, non-cohesive hyperkeratinization and inhibition of keratinization, were observed. These represent the cell injury and recovery stages, respectively. VAA induced the disturbance of follicular epithelial keratinization and reduction of intercellular bindings between horny cells. These effects might prevent the cohesion and accumulation of horny cells and inhibit EIC formation. 3) The number of Odland bodies (Ods) showed an inverse correlation with the cohesion of horny layer. These findings support the theory that Ods have a desquamating function as extracellular lysosomes. The change in Ods would also contribute to both EIC formation by OA and the inhibition by VAA. In addition, VAA caused a characteristic increase in Ods with lamellar structures. It is suggested that Ods with lamellar structures have a desquamating function.