Ultrastructure of human sebaceous follicles and comedones following treatment with vitamin A acid.

Ultrastructural findings in normal human sebaceous follicles, of comedones from acne patients, and from comedones during topical treatment with vitamin A acid (VAA) are described. In normal human sebaceous follicles, the two segments of the infundibulum-the acroinfundibulum and the infrainfundibulum-show differing ultramorphological patterns of keratinization. The incomplete horny layer of the infrainfundibulum is physiological and seems to be necessary for its normal function. On the other hand, no effective barrier function is established in this region. During comedo formation, excessive keratinization takes place in the infra-infundibulum, leading to a compact and coherent horny core. VAA-treatment leads to profound alterations of keratin production, with the appearance of small, irregular, non-coherent and foamy horny cells. The comedonal wall resumes almost physiological conditions. These changes are in agreement with experimental work using VAA in vivo and in vitro and may explain the therapeutic mechanism of action of this compound in the treatment of acne.