490 RADIATION BIOLOGY 



very paiiil'ul and di.sabliiif^. This .suhjcct will not he discussed here, the 

 reader being referred to an earher review (Blum, 1945).* 



ERYTHEMA 



The ciylhcnia. which is the first grossly observable manifestation of 

 sunburn, appears oiilx on the area exposed to the ultraviolet radiation, 

 being (juite sharply delimited from the surrounding normal skin. Unless 

 the dosage is very severe or prolonged, the erythema docs not appear 

 immediately; with moderate doses there is usually an interval of an hour 

 or more. The erythema may persist for a few days, fading imperceptil)ly 

 into the brown color of suntan. Histological examination shows no 

 changes in any laj'cr before the gross appearance of reddening.^ The 

 erythema itself is revealed as the enlargement and engorgement of the 

 minute vessels of the dermis (corium), which lie just below the epidermis 

 (see Fig. 13-1). The red color results from the increased amount of blood 

 in these vessels. Intracellular edema and the migration of leukocytes 

 into the surrounding tissues begin at about the same time as the erythema. 



Although these first detectable changes involve the vessels of the dermis, 

 this is not the site of the photochemical reaction that initiates them. 

 Only a very small fraction of the incident ultraviolet — virtually restricted 

 to the longer wave lengths — ever reaches the most superficial vessels (see 

 Fig. 13-7), yet shorter wave lengths, e.g., 0.28 ^x, which are completely 

 absorbed in the epidermis, do elicit erythema.'" Ultraviolet radiation 

 that is absorbed in the epidermis must, then, cause changes there that 

 lead in some way to dilation of the vessels in the layer beneath. It seems 

 necessary to conclude that photochemical changes in the epidermis lead 

 directly or indirectly to the elaboration of some mediating substance or 

 substances which move down to the superficial vessels of the dermis and 

 cause their dilation. The time between dosage with ultraviolet and 

 appearance of erythema is presumably consumed in the elaboration of 

 these dilator substances and their penetration into the dermis. The 

 horny layer of the epidermis, the corneum, is a nonliving structure built 

 from the viable cells of the malpighian layer beneath it. There is a tran- 

 sition zone between the two layers whei'c it may bo difficult to distinguish 



* It has been claimed that ultraviolet radiation raises the threshold of scotopic 

 vision, but this seems to have been definitely ruled out by recent studies by Wald 

 (1952) who shows conclusively that radiation which might damage the retina does not 

 reach it. 



9 The lustological studies of Keller (1924a, b), Miescher (1930), and Ilamperl et al. 

 (1939a) may be cited as the most complete. There are points of apparent divergence 

 which probably result from differences in the spectral quality and dosage of radiation 

 and the times at which the biopsies were made, t)ut the picture is similar in all three 

 studies. 



'" Evidence that the longer wave lengths that iK-netnitc sligiitly to the dermis may 

 have a direct effect there will be presented a little later on. 



