Like intrinsic ageing, photoageing is reflected at the cellular levet. Compared with control cells derived from sun protected sites (upper inner arm or post auricular area), keratinocytes or dermal fibroblasts derived from paired habitually sun-exposed sites (upper outer arm or pre auricular area of the same donor) maintained in medium containing 10% calf serum have shorter in-vitro lifespans. The discrepancy increases linearly as a function of donor age and severity of clinically apparent photoageing changes. 

In addition, there is a highly consistent negative effect of photoageing on dermal fibroblast growth rate and confluent density at early passage as well as an influence on cellular response to retinoic acid, which is a physiological and pharmacological modulator of skin function. The behavioural differences between previously sun exposed and sun-protected cells of the same donor are indeed far greater and more consistent than those observed between sun protected cells of young adult vs those of old adult donors, perhaps because the large, presumably genetic inter-donor variability is avoided.

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