An ultrastructural study of the retention hyperkeratosis of experimentally induced comedones in rabbits: the effects of three comedolytics.

Abstract

The precise pathologic processes of comedo formation in acne are not well understood. Retention hyperkeratosis may play an important role. To evaluate the effects of three topical comedolytics, 20% azelaic acid, 0.1% tretinoin and 5% benzoyl peroxide, on the retention hyperkeratosis of experimentally induced comedones (EIC), an ultrastructural study was done. After formation of EIC with 50% oleic acid in paraffin oil on the external ears of rabbits, each comedolytic was applied for 4 weeks. Biopsies were taken every week and, using a Hitachi H-600 transmission electron microscope, morphologic observations were done in the upper portion of the follicular epithelium. In EIC, after application of each comedolytic, the markedly thinned horny layer was loosely adhered by extremely few desmosomes and desmosomal bodies. The number and size of tonofilaments and keratohyaline granules decreased, but the number of variable sized Odland bodies increased in the upper epidermis. These findings appeared 1 week after application of either azelaic acid or benzoyl peroxide, and 3 weeks after application of tretinoin. For the first 2 weeks of tretinoin application, EIC showed rather compact hyperkeratosis with more desmosomes and desmosomal bodies than before. Azelaic acid tretinoin and benzoyl peroxide increased the number of Odland bodies, and the horny cells became less adhesive. This lysis of retention hyperkeratosis resulted in comedolysis. During 4 weeks of treatment with these three comedolytics, only tretinoin normalized the keratinization process.