An ultrastructural study of the effects of topical tretinoin on microcomedones.

It has been hypothesized that topical tretinoin prevents inflammatory acne lesions by loosening follicular impactions (microcomedones) and clearing the follicular canal of retained keratin. To lend support to this hypothesis, 15 volunteers applied 0.1% tretinoin cream once daily for 12 weeks to one side of the face and an emollient cream to the other side. During the 12-week treatment period, samples of microcomedones were obtained using the follicular biopsy technique. By 6 weeks, the number of microcomedones was reduced approximately 50% from baseline on the side treated with tretinoin and approximately 80% at the end of the study. Only a minimal reduction in the number of microcomedones was seen on the emollient side, even after 12 weeks of application. Morphologic examination showed a progressive loss of the cohesiveness of the microcomedones and ultrastructural alterations in the epithelial cells treated with tretinoin. The microcomedones changed from well-developed keratinous plugs containing many bacteria to a few wispy layers of keratin containing few bacteria. This study confirms the microcomedolytic activity of tretinoin and provides evidence that this activity is associated with changes in the differentiation of the follicular epithelial cells.