NEWER ASPECTS OF EPIDERMAL DIFFERENTIATION 17 The disintegration of cellular proteins goes hand in hand with keratin formation. The morphologic aspects of this disintegration have been mentioned previously. One of them, the frequent, though by no means constant, appearance of a zone of intensified sulfhydryl staining under- neath the horny layer, the so-called keratogenous zone, has been at- tributed to the unmasking of groups previously hidden inside the protein molecule. The end products of the decomposed proteins ascend with the keratin fibers to the surface of the skin from where they can be extracted and identified. Some of these end products, notably the free amino acids, perform an important physiologic function by giving part of the buffering power to the skin surface and thereby assisting in the neutralization of acids and alkalies applied on the outside of the skin (1). The horny layer, end product of epidermal differentiation, is not of uni- form composition, consisting mainly of keratin, but containing the end products of the broken cellular proteins as well. A number of protein fractions, including globular, mucous, and keratinous ones, were identified in calluses (4). In addition, the epidermis is also an organ for the excretion of endogenous and exogenous substances. In the former group belong the so-called melanoids, derivatives of melanin pigments. The excreted pre- cursors of melanin pigments ("premelanin") may reduce metallic salts to free metals on the skin surface (15). The number of exogenous sub- stances, discarded through the epidermis, is practically limitless. Some of these, such as arsenicals, may become incorporated into the keratin mole- cule and thereby may modify the process of keratinization others, like atabrine, may stain the epidermis and provide a screen against the effects of ultraviolet irradiation. The horny layer, though metabolically inactive, can be subjected to a number of chemical and physical reactions. The majority of cosmetic preparations act on the surface only, by modifying it through physical or chemical mechanisms. Absorption of topical agents, though often stressed by the cosmetic industry, is of minor importance for achieving the desired cosmetic effects. The foregoing discussion leaves little doubt that there are great gaps in our understanding of epidermal differentiation. The recent development of new methods for the study of human epidermal proteins offers promise to solve some of these questions. There is little excuse for the continued exclusive study of specimens obtained from animal sources and even less for implying that the results obtained are applicable to human skin. The in- vestigation of material like the cow's nose (7) or the horse burr (12) may be convenient because of the thickness of the epidermis and the relative avail- ability of these tissues nevertheless, recent progress in techniques, facilitating the study of human epidermal proteins, makes research in this area not only practicable, but imperative. These new methods include:

18 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS an easy way of separating the epidermis from the corium, by simply stretch- ing the skin and scraping off the epidermis without contaminating it with dermal tissue (16) a method for separating dried epidermal powder into cellular and keratinous fractions by differential centrifugation (17) and the isolation and purification of epidermal proteins by precipitation at the isoelectric point, dialysis, and chromatographic absorption (8, 9). The hope for a clearer understanding of epidermal differentiation through the isolation and analysis of well-defined epidermal fractions is, thus, within the realm of realization. Even with these improvements, the task is tedious and time-consuming, because of the small quantities of tissue available. In our laboratory less than 10 gm. wet weight of epidermis was obtained from almost the entire body surface of a newborn infant (8) the average weight of epidermis covering 1 cm. sq. is 1.8 mg. dry weight (4). or about 5 rag. wet weight (18). In spite of these difficulties, research of epidermal proteins is being actively pursued in several research centers and it is likely that these studies will ultimately shed more light on the most important and basic process of cutaneous metabolism. B•BL•OGRAPI4¾ (1) Rothman, S., "Physiology and Biochemistry of the Skin," Chicago, University of Chicago Press (1954). (2) Wassermann, F.,/ira. •./inat., 94, 399 (1954). (3) Pinkus, F., "Handb. der Haut- u. Geschlkrkh," Vol. 1, Pt. 1, Berlin, J. Springer (1927). (4) Maltoltsy, A. G., personal communication. (5) Mescon, H., et al., to be published. (6) Van Scott, E. J., and Flesch, P.,/irch. Dermatol. and SyphiloL, 70, 141 (1954). (7) Rudall, K. M.,/idvances in Protein Chem., 7, 253 (1952). (8) Anderson, D. S., personal communication. (9) Matoltsy, A. G., to be published. (10) Block, R. J., )t. Soc. Cosmetic Chem., 2, 235 (1951). (11) Van Scott, E. J., and Flesch, P., Science, 119, 70 (1954). (12) Giroud, A., and Leblond, C. P.,/inn. N.Y./icad. Sci., 53, 613 (1951). (13) Barrnett, R. J., in press. (14) Flesch, P., Brit. y. Dermatol., to be published. (15) Rothman, S., Rubin, L., and Houston, M.,/irch. Dermatol. and SyphiloL, 48, 400 (1943). (16) Van Scott, E. J., Investigative DermatoL, 18, 377 (1952). (17) Flesch, P., Proc. Soc. Exptl. Biol. Med., 85, 483 (1954). (18) Flesch, P., unpublished data. zlcknowledgment: The author wishes to express his thanks to Dr. D. S. Anderson for her help in the preparation of this paper.