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Methods for the assessment of the effects of topical retinoic acid in photo-ageing and actinic keratoses.

Quantitative techniques for assessing the effects of tretinoin on photo-aged skin mainly involve humans, although hairless mice were used in initial studies. Ideally techniques should be non-invasive but occasionally biopsies have to be taken, especially when studying the effects of tretinoin on different skin compartments. Characteristic features of photo-aged skin, including the development of fine and coarse wrinkles, skin discoloration, rosy cheeks and telangiectasis, have been assessed subjectively using a visual analogue scale. Effects of tretinoin on wrinkle depth have been measured non-invasively by quantifying contours of silicone rubber replicas using either a mechanical tracking device or an optical technique employing an image-analysing computer. Changes in skin colour have been measured using an erythema meter and the stimulatory effect of tretinoin on blood flow has been established by laser doppler flowmetry. Skin thickness has been measured non-invasively using pulsed A-scan ultrasound which showed that tropical tretinoin increased thickness. Biopsies have also been used but no changes in the thickness of the dermal repair zone have been noted in humans, in contrast to the situation in hairless mice. Epidermal dysplasia has been measured by a visual analogue scale or by objective image analysis.