SUNBURN 507 



by the formation of dopa itself, which they have found to be catalytic in 

 trace amounts. The photochemical formation of dopa meets, of course, 

 the same difficulty mentioned, since, presumably, oxygen must be present 

 during the exposure to ultra\dolet radiation. It may be pointed out that 

 this is not an objection to the in vitro reaction scheme as a whole since 

 oxygen is present during the time when the melanin is actually formed, 

 but only to the acceptance of the photooxidative step as the initial one. 



But it seems that none of these schemes, except possibly that of 

 Rothman, takes cognizance of what happens in suntanning. None 

 accounts adequately for the delay of several days between the exposure to 

 ultraviolet radiation and the first observable formation of new melanin. 

 Whatever the ultra\'iolet radiation does, it must be thought of as setting 

 off a chain of events that leads to melanin formation at a later time. The 

 removal of inhibiting substances as suggested by Rothman might act in 

 this way, but it is difficult to fit the other schemes into this picture, 

 although the formation of small amounts of dopa, which then catalyze the 

 formation of larger amounts, has attractive aspects. It may also be 

 pointed out that the initial part of the melanization process — migration of 

 pigment already formed — is not accounted for in any way by these 

 schemes. 



The objection may be le\aed against a good many of the in vitro studies 

 that massive doses have been used which bear no relation to those which 

 produce in vivo effects. There is more and more evidence to indicate that 

 profound changes may be brought about in living cells by very small 

 quantities of ultraviolet radiation. Thus, while ultraviolet may bring 

 about in living skin the formation of a small amount of melanin according 

 to reactions such as have been demonstrated in vitro, the skin might be 

 pretty thoroughly cooked before the quantity reaches the proportions of 

 suntan. On the other hand, a very small amount of ultraviolet radiation 

 might, by eliciting changes in the cell, lead to the ultimate production of 

 a good deal of melanin. 



In this chapter the point of view is adopted that the primary effect of 

 the ultraviolet radiation is injury to cells of the malpighian layer and that 

 melanization, like other aspects of the sunburn mechanism as a whole, is 

 secondary to this injury. Melanization is a common response of the epi- 

 dermis to injury of any kind, which maj^ represent an over-all reaction 

 similar to or identical with that by which melanin is formed in vitro. But 

 the initiating of such a reaction need not be specifically related to the 

 agent that brings about the injury. In this regard the experiments of 

 Peck (1930) may be cited, which show that melanization brought about by 

 radiation from thorium closely parallels melanization from ultraviolet 

 radiation, there being, in both cases, first pigment migration and then the 

 elaboration of more pigment. There is little reason for believing that the 

 ionizing radiation from thorium would bring about the same kind of 



