The influence of different strains and age on in vitro rat skin permeability to water and mannitol.

Water and mannitol were used as test penetrants to study the effect of age on the skin permeability of the Wistar-derived Alderley Park (AP) rat and Sprague-Dawley (SD) rat. Whole-skin membranes were prepared from rats aged 10 to 120 days, while epidermal membranes were prepared from rats aged 24 to 32 days. The results indicated that the skin permeabilities of the two strains were very similar for either whole-skin or epidermal membranes. The influence of age on skin permeability was found to be negligible for the AP rat, and a small decrease in whole-skin permeability was observed for SD rats above 80 days of age. A statistically derived expression ("the separation efficiency factor") was used to determine the optimum age for preparing intact epidermal membranes; these were 26 days for AP rats and 28 days for SD rats. Histological examination of whole-skin membranes for both strains revealed that the stratum corneum and epidermal thickness did not alter significantly with age (10 to 120 days old). Dermal thickness, hair follicle depth, and, to a lesser extent, the surface area occupied by hair follicles all appeared to be influenced by age, although these changes had no detectable effect on skin permeability.