Topical retinoic acid for photoaging: clinical response and underlying mechanisms.

Photoaging is primarily composed of wrinkling, mottled hyperpigmentation and a tactile roughness of the skin, all three of these parameters improve following use of topical retinoids. It appears that smoothening of the skin results from a combination of epidermal changes including thickening, stratum corneum compaction and glycosaminoglycan deposition. Lightening of actinic lentigines and mottled hyperpigmentation correlates with a reduction in epidermal melanin content maybe resulting from inhibition of tyrosinase activity. Effacement of wrinkling in mice correlates with new collagen synthesis, and there is evidence that this is also the case in humans. An irritant dermatitis is a feature of retinoid-treated skin but this diminishes in severity during treatment despite continued improvement in photoaging. Thus it is unlikely that irritation per se is responsible for clinical improvement.