SKIN MORPHOLOGY AND WRINKLE FORMATION 305 UV-B irradiation. On the other hand, our results show a slight delay in the formation of longitudinal wrinkles after temporary fixation, compared with the formation of original transverse wrinkles. Thus, there remains a slight possibility that the latter hypothesis described above may explain wrinkle formation. The connective tissue elements collagen and elastin are known to be degraded by collagenase and elastase produced by inflammatory cells (14,15) and fibroblasts (16,17). Collagenase mRNA levels in the dermis (18) and cultured fibroblasts (19) were reported to increase after UV-A irradiation. It is well known that exposure of keratinocytes in culture (20) or of the skin in vivo (21,22) to UV irradiation induces marked elevation of release of several cytokines, including interleukin-1 (IL-1) and tumor necrosis factor (TNF). UV-inducible IL-1 has also been shown to stimulate the synthesis of elastase (23) and to increase collagenase activity (24) in normal human fibroblasts. On the other hand, recent studies have shown up-regulation of the elastin mRNA level by IL-i[• (25) and its down-regulation by TNF (26). Thus, it is conceivable that the synthesis and degra- dation of connective tissue elements are regulated by cytokines. In addition, recent studies have shown the expression of collagenase by normal human fibroblasts in re- sponse to singlet oxygen radicals generated by UV-A irradiation (27,28). Active oxygen species induce cross-linkage of collagen in vitro (29) and in vivo (30). Oxidative autoac- tivation of latent collagenase by human neutrophils has also been reported (32). Appli- cation of a radical scavenger has been reported to inhibit wrinkle formation in animals (31). Considering the expression of proteins such as collagenase, there may be a time lag between UV-B exposure and expression of enzyme activity, supporting the uneven irradiation theory. However, cross-linking due to active oxygen species and activation of latent-type collagenase (32) suggests that the skin morphology at the time of UV-B irradiation is important for wrinkle formation. Although it is unclear which mechanism is involved, wrinkles can be formed in any direction by temporary fixation followed by UV-B irradiation. Further studies are needed to determine whether this phenomenon occurs in other animal species and whether uneven irradiation, such as that by slit rays, is also the same effect. REFERENCES (1) A.M. Kligman, P. Zheng, and R.M. Lavker, The anatomy and pathogenesis of wrinkles, Br. J. Dermato/., 113, 37•42 (1985). (2) T. Tuji, T. Yorifuji, Y. Hayashi, and T. Hamada, Light and scanning electron microscopic studies on wrinkles in aged persons' skin, Br. J. Dermato/., 114, 329-335 (1986). (3) G. E. Pierad and C. M. Lapiere, The microanatomical basis of facial frown lines, Arch. Dermato/., 125, 1090-1092 (1989). (4) H. M. Daniell, A study in the epidemiology of "crow's feet," Ann. Intern. Med., 75,873-880 (1971). (5) G.L. Grove, M.J. Grove, and J. J. Leyden, Optical profilometry: An objective method for quantifi- cation of facial wrinkles, J. Am. Acad. Dermato/., 21, 631-637 (1989). (6) C. E. Griffiths, T. S. Wang, T. A. Hamilton, and J. J. Voorhees, A photonumeric scale for the assess- ment of cutaneous photodamage, Arch. Dermato/., 128, 347-351 (1992). (7) P. Corcuff, J. Rigal, andJ. L. Leveque, Skin relief and aging,J. Soc Cosmet. Chem., 34, 177-190 (1983). (8) D.L. Bissett, D.P. Hannon, and T.W. Orr, An animal model of solar-aged skin: Histological, physical, and visible changes in UV-irradiated hairless mouse skin, Photochem. Photobio/., 46, 367-378 (1987). (9) D. L. Bissett, D. P. Hannon, and T. W. Orr, Wavelength dependence of histological, physical, and visible changes in chronically UV-irradiated hairless mouse skin, Photochem. Photobio/., 50, 763-769 (1989).

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