THE EMERGING COSMETIC INDUSTRY 29 COMPOSITION AND FUNCTION OF TISSUES If cosmetic preparations are to be so formulated as to protect the skin effective]y, or to improve its condition, the fullest possible know]edge of the structure of the skin and the chernical composition and function of its com- ponents is desirable. Present knowledge of skin structure is unfortunately far from adequate, and many morphological features are known only vaguely. The chemical composition of important components is even ]ess completely known. Rothman's recent compendium, "Physiology and Bio- chemistry of the Skin, "• performs admirably the function of summarising and organising data published up to 1952 on these topics, and on the functioning of skin tissues. Connective Tissue. The tendency of ageing skin to wrinkle and sag is apparently connected with a loss of elasticity, which is in turn, at ]east partially, traceable to changes in the structural elements of the corium or curls. In the connective tissue of the cutis, three types of material can be distinguished, namely, cells, fibres and matrix. The matrix appears homo- geneous under the light microscope, but the electron microscope shows that it contains a fine network of fibrils. Collag•n. The fibres of connective tissue are of three types: Co]lagen, reticu]ar and elastic. Co]lagen fibres are greatly preponderant there is no agreement as to how they differ chemically from the branching reticulin fibres. Recent evidence 2 indicates that co]lagen of leather contains large numbers of extremely small pores, with radius of the order of 100A. Kennedy-' recently obtained electron micrographs of sections of co]]agen fibrils from various sources showing tubular structure in the fibrils. Such evidence adds support to the view that co]]agen may serve not only as a structural unit, but also as a conductor for transporting substances toward nomvascularised tissues. • The method of formation of collagen is very incompletely understood its prominent presence in new repair tissue, and the recent report that injections of Irish moss extract stimulated growth of repair tissue in rats • contribute to our desire to know more about co]lagen formation. Knowledge of the chemical composition of collagen is incomplete. Pauling and Corey, • in working out a proposed helical structure, had used the sequence -P-G-R-, where P is proline or hydroxyproline, G is glycine, and R is one of the other amino acid residues, as the typical repeating unit of collagen. Kroner, Tabroff, and McGarr, • however, recently isolated 28 new peptides from collagen hydrolysate and showed that in the 68 collagen peptides now known only 15 contain the P-G-R- sequence. They found the proline-hydroxyproline linkage to be strongly represented. Gustarson • cited the apparent relation between the hydrothermal stability of collagen and its hydroxyproline content as an indication that the hydroxyproline

30 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS residue may play the same cross-linking role in collagen as cystine plays in keratin. Various "procollagens" have been suggested as possible precursors of collagen. Boedtker and Doty • found in electron micrographs periodicities which suggest that the different collagen fibres have as a common unit rigid, rod-shaped particles of uniform size, with diameter of 14J• (hydrated), length 2,900 A, mol. wt. 300,000. Elastin. Although elastin constitutes only about 2 per cent of dry human skin, it appears to be of great importance in imparting the supple, wrinkle- free quality characteristic of youthful skin. In terms of amino acid composi- tion, elastin differs from collagen principally in its lower content of polar residues. Gross •0 considered this low polarity, with consequent paucity of cross linkages, to be the reason for the rubber-like properties of elastin. Slack n showed, by means of C•4-õlycine fed to rats that elastin of the aorta has a very slow rate of metabolic turnover, as has also most of the collagen. The status of elastin chemistry is illustrated by Hall's TM recent conclusions that (1) all existing methods for freeing so-called elastin from associated proteins are either inadequate or so drastic as to start the degrada- tion of the fundamental unit of elastic tissue itself, and (2) standard elastase preparations contain two enzymes which act on two components of an elastic tissue preparation which of itself is a two-component system. Banga and Balo•* found mucoprotein to be a component common to elastic and collagen fibres, and Lansing, et al.,•4 presented evidence indicating that elastin contains fat. The ground substance of connective tissue is more complex and less understood than the fibrous components. It is undoubtedly involved in maintenance of tone in the superficial tissues. Hartmann and Ziems •* found that connective tissue which contains hyaluronic acid showed changes of elasticity when treated with hyaluronidase. There is every reason for assigning to elastic fibres, and probably to other portions of connective tissue, vital roles in the changes in appearance of skin with increasing age of the individual. Much remains to be done before we can hope to understand or control these processes. CELLULAR FUNCTION Many questions encountered in cosmetic technology are at present unanswerable because so many phases of cellular function are only vaguely' understood. We do not know by what mechanism various applications affect the skin, nor how we smell perfumes, and we shall not know until we learn more about cell permeability, metabolism, and sensitivity. The cell is a complicated mosaic of structural units endowed with specific but interdependent biochemical properties. • A single microscopic cell may