g0g JOUBNAL OF THE SOCIETY OF COSMETIC CHEMISTS ever, there is a significant decrease in both .the weight and the water-binding capacity when callus is extracted with water after the solvent treatment al- though the callus can still hold considerable quantities of water (1). The loss in weight is due to the extraction of a water-soluble fraction which markedly enhances the ability of the stratum corneum to hold water. The corncure con- tains about 25% of its dry weight of these water-soluble substances (11, 80, 81). It seems well established that much of the water-binding capacity of corncure is due to the presence of these hygroscopic water-soluble substances and that they are in some way protected by a lipid material which must be removed with solvents before they can be extracted by water (13, 24, 44, 49, 80-87). There is no apparent correlation between the loss of water barrier capacity ooe the skin and the total amount ooe lipid extracted from the skin by the varied solvents (88). Lipid solvents appear to have very little effect on the structural elements of the stratum eorneum. Neither the mechanical strength is changed nor is any change detectable in the birefringence after lipid extraction. On the other hand, large amounts of lipid material are removed and open mem- brane interstices are apparently formed which act as low energy diffusion pathways. The lower activation energy for diffusion and nonselective higher permeability for all solutes indicates liquid-like transport through these sol- vent-filled interstices. The capacity of the tissue to bind large amounts of water is also destroyed by treatment with delipidizing solvents. This suggests that the lipid material does not simply plug the interstices in the membrane but rather aids in stabilizing the water structure in concert with the other tis- sue components (28). The degree of alteration of water barrier capacity varies greatly among different specimens of skin extracted by the same lipid solvent for the same period of time (15). Water transmission was measured through isolated hu- man, rabbit, fish, and frog skins (89). The rate of transmission was much greater for fish and frog skin. Human and rabbit skins contain a hexane-solu- ble lipid which appears to be the maior regulator of passive water holding. This hexane-extractable lipid was not detected in the fish or frog skin. Burned mammalian skin contained 30% or less of the normal water-holding lipid and transmitted up to 4 times more water than intact skin. Water transmission by the skin surfaces was inversely proportional to the water-holding lipid content of the surface. The effect of relative humidity on water binding and extensibility indicates a close relationship between the two. It is interesting to note, however, that it is that fraction of the water which is held by the water-soluble substances which is responsible for most of the extensibility. Removal of the water-sol- uble substances with ether followed by water extraction results in a very large reduction in extensibility while there is only a comparatively small reduction in the water-binding capacity. The residual water which is bound by the sol-

WATER AND THE SKIN 203 vent-extracted corncure in hunfid atmospheres does not appear to contribute significantly to the extensibility (44). No change in elastic modulus is ob- tained if the corncure is extracted only with water (lipid left intact), but the elastic modulus incrcases significantly if the callus is treated with chloroform or detergent solutions followed by aqueous extraction (45). The osmotic properties of the stratum corncure itself depend upon the presence of watcr-soluble substances within the corncure and suggest that there is a semipermeable membrane system operating within the corncure. The loss of osmotic properties after extraction with solvents indicates that the semipermeable membrane system contains lipids, the removal of which alters the permeability of the membrane system so that the water-soluble substances responsible for exerting the osmotic forces can escape. The membrane system prevents the water-soluble substances from being washed out when the cornc- ure is immersed in water but does not prevent the hygroscopic constituents from holding water in humid atmospheres (44). The presence of cholesterol and phospholipid in solvent extracts of guinea- pig footpad corneum suggests that the membrane system protecting these water-soluble substances from dissolution in water is the corneum cell wall. The effect of powdering the corncure in liquid nitrogen shows that breaking the cell wall allows water to extract the water-soluble substances and reduce the water-binding capacity. Breaking the cell wall has the same effect as ex- traction with solvents. The evidence is therefore consistent with the idea that the semipermeable membrane system is the cell walls of the individual corne- um cells (44). The hygroscopic substances have been grouped together and called the "natural" moisturizing factor (NMF) (82). They consist of a mixture of amino acids, organic acids, urea, and inorganic ions. Sodium lactate (24) and the sodium salt of 2-pyrrolidone-5-carboxylic acid (86) appear to be the most hygroscopic components of the natural mixture. Collectively, their hygro- scopic properties have been found to be approximately equal to those of glycerine (85). Relatively little is known as to how the hygroscopical]y bound water plasti- cizes the corneum proteins which are responsible for its mechanical integrity. Park and Baddiel (45) postulate that aqueous plasticization of the corneum is due to direct protein hydration both in the presence or absence of hygro- scopic substances. Due to their ionic or dipolar nature, the hygroscopic sub- stances will be bound to the proteinaceous components of the stratum cor- neum. Location of these bulky molecules between protein chain segments will create a more open structure than would exist in their absence, leading to en- hanced protein hydration. Upon removal of the hygroscopic molecules the protein matrix collapses with the resultant formation of more protein-protein bonds (mainly hydrogen bonds and ionic interactions). The higher modulus values obtained for the hygroscopic-free corneum samples result from the