SUNBURN 509 



violet radiation by repeated doses of this agent, an observation also made 

 a few years later by Meyer (1924). Keller (1924b) noted that the first 

 darkening of the skin results from migration of the pigment rather than 

 from increase in amount. Others pointed out that sensitivity to ultra- 

 violet radiation returns to normal before the tan disappears (Perthes, 

 1924; Schall and Alius, 1928,a,b). 



The first to recognize the major factor contributing to the reduced 

 sensitivity that develops after exposure to ultraviolet radiation was 

 Guillaume. In a short paper in 1926 and in a book published in 1927 he 

 called attention to the thickening of the corneum that results from the 

 active epidermal proliferation which is one of the aspects of sunburn, and 

 pointed out that this must greatly reduce the penetration of ultraviolet 

 radiation to the malpighian layer. Histological studies by Lovisatti 

 (1929) and by Miescher (1930) soon gave strong support to Guillaume's 

 idea. As early as 24 hr after exposure to ultraviolet radiation, degenera- 

 tive changes in the prickle cells of the malpighian layer are detectable. 

 Later the whole layer including the basal cells may be involved, depending 

 on the severity of the dose. When the acute stages of this reaction sub- 

 side both the malpighian layer and the corneum are left thickened. An 

 idea of the difference in penetration that this makes is illustrated in Fig. 

 13-7. The thickening is eventually reduced and in about two months 

 may have returned to normal if there has been no further exposure to 

 ultraviolet radiation. The erythemal threshold returns to normal at 

 about the same time. 



As mentioned, vitiliginous areas of skin do not tan, but do increase their 

 erythemal thresholds after exposure to ultraviolet radiation (With, 1920; 

 Meyer, 1924). The same is true for albino skin (Lovisatti, 1929). That 

 decreased epidermal penetration resulting from thickening is the expla- 

 nation in these cases is also supported by the decrease in transmission of 

 the skin of the albino mouse after repeated exposure, which is illustrated 

 in Fig. 13-7 (Kirby-Smith et al, 1942; Blum and Kirby-Smith, 1942). Li 

 none of these cases is there any melanin pigment formed. Such evidence 

 does not, of course, preclude the possibility that increase in melanin plays 

 some part in reducing the amount of ultraviolet radiation reaching the 

 malpighian layer, but this factor may be of only minor importance. It is 

 easy to be misled by the observation that the skin becomes darker to the 

 eye when tanning takes place, because this is not directly dependent on 

 absorption by melanin within the erythemal spectrum. Presumably, 

 absorption in the latter region would be due largely to the phenolic ring, 

 as in the case of protein. It seems doubtful that the increase of absorp- 

 tion due to melanin could be nearly as great as that due to increase of the 

 protein layer by thickening of the corneum, and that the total attenuation 

 of the radiation should be influenced to a much greater extent by the 

 latter. 



