Measurement of retinoid effects on epidermal renewal

Retinoids have become important therapeutic tools. They are used to treat a wide array of skin diseases such as acne, ichthyosis, and photoaging. Changes in epidermal turnover always accompany the therapeutic use of retinoids, irrespective of the pathogenesis of the disease being treated or the biochemical mechanism(s) by which the retinoids produce their therapeutic effect. This chapter describes methods for measuring epidermal turnover. Epidermal turnover is continuous and spatially oriented. Renewal or turnover occurs to replace the most highly differenced keratinocytes (squames) that separate (desquamate) from the outermost layer of the epidermis (stratum corneum). The general principles of renewal as well as the measurements made to quantitate renewal are equally applicable to human epidermis, the epidermis of commonly used experimental animals, and cultivated human keratinocytes. Three parameters are most often measured to assess epidermal renewal: proliferation, transit time, and horn production or desquamation.