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

0,04 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS rigidity that the additional bonds formed (a{ter removal of the hygroscopic substances) impart to the structure. Due to the reduction in swelling capacity of the corneum imposed by this structural collapse, water uptake will also be less. This is also indicated by water-binding isotherms (1, 13) and differential thermal analysis ( 90 ). Indirect evidence for the cell membrane system rather than the keratin being the rheologieally active material in the stratum corncure is provided by the fact that isolated keratin fihns swell considerably in wate ", almost to the point of dissolution, and display no detectable elasticity. This behavior con- trasts with that of untreated and extracted eorneum, both of which have finite elastic modulus values in water (45). The concept that the mechanical strength of the corncure resides in the cell membrane system is supported by the fact that the cell membrane protein (110) has a much higher cystinc con- tent (x 3-4) than the keratin filaments (91) leading to more permanent (with regard to disruption by water) disulflde erosslinks in the membrane (4•). W•n B•m• Transepidermal water loss is a small fraction ooe the water loss from a com- parable area of water. The skin thus has the property of a water barrier, and the stratum corneum is generally recognized as representing the principal skin barrier to water loss as well as to entry of environmental noxious agents (6, 7, 60). Removal ooe the skin barrier by cellophane tape stripping will en- hance the absorption ooe almost any substance. The barrier possesses a resistance to the transport ooe water molecules, and it is irrelevant whether these water molecules are passing from the inside to the outside of the skin or vice versa. It is also irrelevant whether the water is in the liquid or in the gas phase on one or both sides ooe the skin (57). The vapor barrier properties ooe the stratum corneum are virtually mechanical and not dependent on living cells, functioning enzyme systems, intact cell mem- branes, or attributable to an "electrical double layer" ( 1, 7, 8, 12, 14, 88, 92). The location ooe this epidermal barrier has been the subject ooe considerable controversy. At one time it was thought that the barrier layer resided between the stratum granulosum and stratum corneum (92-94). Later it was placed in the lower portion of the corncure (1, 95, 96). However, at present, the avail- able data offer no evidence for the concept that different layers within the main bulk ooe the stratum corneum have different diffusivities (10). Water permeability experiments on isolated stratum corncure (8) indicate that the barrier to penetration is the entire stratum corneum ( 10, 15, 50, 54). Water diffusion rate can be principally used to evaluate the functional state ot • the skin barrier. It is probably the most sensitive measure of barrier intact- hess (97). Ioe the skin barrier is removed, water will evaporate from the skin surface at approximately the same rate as from a free •vatcr surface. Water