Stratum Corneum: An Ideal Osmometer?

Water and osmolyte homeostasis is an essential biological function. Remarkably devoid of aquaporin (i.e., water channel protein) expression, the stratumcorneumavoids a substantial transepidermal water loss, compulsory for the adaptation to terrestrial life. In spite of its heterogeneity, the stratum corneumexhibits a homogeneous water transport, whereas highly osmotic endogenous materials control its water-holding capacity and skin’s physical properties (e.g., stiffness, firmness, flexibility) under various conditions. However, the contingent interplay between water homeostasis of the stratum corneumand an exogenous osmotic stress has been not reported, although constituting a cornerstone of skin physiology. Here, we show that an osmotic shock reinforces the endogenous stratum corneumosmolality proportionally to the osmotic pressure exerted against it and, therefore, reduces the magnitude of outward water transfer and net evaporation. The strengthening of endogenous osmolality enhanced the water-holding capacity of the stratum corneum with respect to chemical potentials. This property was found for organic osmolytes, but questioned for mineral electrolytes characterized by minimal stratum corneumpermeability. Besides, straightforward experiments performed using a built-in house osmometer, a so-called ‘corneosmometer’, have confirmed water transfer through the stratum corneumsubmitted to a gradient of osmotic pressure. Thus, the ambivalent function of the stratum corneum, which firstly behaves as an ideal osmometer and secondly adapts its own osmolality to an exogenous gradient of osmotic pressure, has been clearly demonstrated. This duality influences the water-holding capacity of the stratum corneum, by regulating the water transfer proportionally to an initial osmotic stress, and, finally, conditions the further capacity of the stratum corneumto facilitate or impede skin hydration.