J. Soc. Cosmet. Chem., 31, 11-18 (.January/February 1980) Measurement of stratum corneum function R. MARKS, Department of Medicine, Welsh National School of Medicine, Heath Park, Cardiff CF4 4XN, Great Britain. Received June 12, 1979. Presented at Annual Scientific Meeting, Society of Cosmetic Chemists, May 1979. Synopsis MEASUREMENT of STRATUM CORNEUM (SC) FUNCTION has largely been neglected despite the potential benefits to the understanding of skin disease and the design and evaluation of new treatments. Measurement of transepidermal water loss using an evaporimeter is simple, rapid and accurate and can be conveniently used for clinical and pharmacological assessments. Permeation of the SC by certain substances applied to the skin can be traced morphologically and can be quantitated using electron probe X-ray microanalysis of serial skin surface biopsies in the scanning electron microscope. The barrier that the SC imposes to pathogenic microorganisms is difficult to test in vivo but can be done in vitro. The resistance to mechanical trauma is capable of assessment in vivo with an instrument especially constructed for the purpose. Two new techniques for the direct determination of the rate of desquamation have been elaborated and show a significant correlation. A technique for the measurement of intracorneal cohesion is described which gives useful information on the process of desquamation. INTRODUCTION The development of the cosmetic arts has a longer and more dignified history than the development of the cosmetic sciences. It is strange that the interface between the organism and the potentially hostile and injurious environment should be so poorly understood in a time when its importance socially and biologically is widely recognized. The lack of substantial advances in the understanding of skin disease and its treatment is compounded by several separate problems. They include, an obsequious worship by the skilled amongst us at the altar of biochemistry and the confusion caused by the mass of data presented to our eyes by the myriad appearances of skin disease (Kligman has frequently pointed out the desirability of a race of blind dermatologists!). To be fair, it seems that the intrinsic complexity and diversity of function is also partially to blame. We have not kept pace with our colleagues who have other organ systems as their responsibility. Whereas cardiovascular, respiratory, renal and gastrointestinal physicians and investigators have developed sophistication in assessment of function of their particular system, the dermatologists and skin scientists have not. It should be self evident that without tests of function, development of a rational skin pharmacology will be virtually impossible. It is important to distinguish the term function from properties. Although these two aspects of a tissue often are closely related, they need not be in all circumstances. In addition it is extraordinarily difficult to measure properties in vivo and unfortunately we are still ignorant of the detailed changes we produce in the stratum comeurn when 11

12 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS we remove it in isolation. For example, testing the mechanical properties of the stratum corneum in vitro tells us little about its capacity for mechanical protection in vivo. In this paper, tests of stratum corneum (SC) function will be reviewed and the progress of the Cardiff group in the development of these tests will be described. WHAT SC FUNCTIONS CAN BE USEFULLY ASSESSED? The SC is a barrier in many senses. It is arguable which particular aspect of its barrier function is most important. Certainly as a water barrier it is essential to our wellbeing. The technologies now exist for the rapid and accurate measurement of this function and they should be more often used by clinicians as a means of measuring the progress of skin disease during treatment. Allied to the water barrier function, but not identical with it, is the function which prevents foreign substances from permeating into the skin. Assessment of the ability of the SC to inhibit the permeation of such materials would aid in the design of topical treatments and in the understanding of the absorption of toxic materials via the skin. The mechanical barrier function has been largely ignored in the past, yet were we to be covered by a soft vulnerable mucosa, rather than a firm and comparatively hard horny membrane, we would be totally disabled. Measurement of this mechanical protection afforded by the SC would be useful for pharmacological evaluation of new topically active agents, and perhaps for preemployment assessment of potential workers in industry in an effort at preventing occupational dermatitis. Infection of the skin implies in an effort at preventing occupational dermatitis. Infection of the skin implies the prior penetration of the SC by pathogenic microorganisms. We know little about the variability of the barrier in its ability to prevent this penetration, but it seems likely that measurement of the variability will be important in categorizing patients' susceptibility to infection of the skin. Desquamation is not normally thought of as a function, but if the normal loss of horny cells is in some way impaired, other functions become deranged. In addition, measurement of the rate of desquamation is an important parameter for studies of epidermal cell population kinetics and for the evaluation of drugs acting on the SC. Desquamation results from the loss of binding forces within the SC and measurement of these binding forces (or intracorneal cohesion) tells us much about desquamation. MEASUREMENTS OF WATER BARRIER FUNCTION. The initial interest in this topic was stifled by the complexity of the techniques involved in measurement of the rate of transepidermal water loss (TEWL). The '•closed systems" devised (1,2) appeared to be accurate, but were cumbersome. Nonetheless, the data that derived from their use has withstood the test of time. Nilsson (3) has recently described an elegant apparatus for measurement of TEWL by determining the gradient of water vapor pressure at two points above the skin surface, using delicate humidity sensors. This evaporimeter does not require a closed system and has the added advantage of rapidity of response. In our studies with a similar instrument, we found that in a draught-free room, at a constant temperature of 20øC, and a constant relative humidity of 41%, readings taken at 5-min intervals (over a 30-min period) from