In vitro comparison of water-holding capacity of the superficial and deeper layers of the stratum corneum.

We measured the electrical conductance at 3.5 MHz of a sheet of normal stratum corneum (SC) mounted with either the upper surface or the lower surface downward in simulated in vivo conditions. In this way, we assessed the water-binding capacity of the upper and lower portions of the horny layer. Measurements were made with the SC model in environments of various relative humidities. Between 30% and 90% relative humidity, the conductance of the upper surface was always significantly higher than that of the lower surface. In contrast no significant difference was observed in transepidermal water loss (TEWL) with the SC sheet placed upwards or downwards. After exposure to lipid extraction with acetone/ether, a significant decrease in conductance and increase in TEWL occurred, particularly in the upper surface. The amount of extractable amino acids was much higher from the middle layer than from the outer layers of the SC, and was lowest from the deepest part of SC. These results, indicating a lower efficiency of the lower surface of the SC for water-binding than the desquamating upper surface, suggest that newly formed immature SC does not have the water-holding capacity of the mid portion of the SC, which is probably the layer with the greatest water holding capacity.