Improved in vivo and in vitro techniques for measuring cuticular water permeability are described. Air flowing over a cuticle disc mounted in a holder, permitted elimination of unstirred layers, or corrections for them, for the first time. Conditions inside the holder were incompatible with the long-term health of the epidermal cells. Significantly, mean permeabilities of these discs did not differ from values obtained in vivo on the same cuticular plate.
Overall cuticular permeability was apportioned between endocuticle and combined epicuticle and exocuticle on the basis of measurements made before and after solvent extraction of lipids. Under identical activity gradients, endocuticle permeability was 35 to 40 times greater than the value for the other layer. Permeability of both component layers showed strongly non-linear relationships with ambient activity, with empirical proportionality to the reciprocal of vapour pressure lowering.
Cuticle water contents measured in activity gradient conditions showed significantly higher values in vivo than in vitro. The amount of water contained in the combined epicuticle and exocuticle was too small to measure.
We conclude that neither permeability nor water content data support the existence of a significant water barrier in the region of the epidermis.