SUNBURN 503 



former results from the increased blood flow in the minute vessels of the 

 dermis, the latter represents the change in position and increase in quan- 

 tity of melanin pigment in the epidermis. That the color of suntan is 

 principall}^ due to melanin is indicated by studies of the spectral distri- 

 bution of the radiation reflected from tanned and untanned skin (Edwards 

 and Duntley, 1939a, b). The difference in position of the erythema and 

 tan may be readily demonstrated by pressing a sheet of glass against the 

 skin. This squeezes the blood out of the minute vessels, causing the red 

 color to disappear, but the tan remains visible since the melanin pigment 

 in the epidermis is not affected. In this way the time of disappearance of 

 erythema and the appearance of tan may be judged. 



In untanned skin the melanin pigment is located chiefly in the basal cell 

 layer (e.g., Masson, 1948). According to histological studies, about the 

 time that tan first becomes grossly observable, this pigment begins to 

 migrate toward the surface, where it should have a greater effect on the 

 spectrum of the reflected light from the skin's surface. The pigment 

 migration begins about 24 hr after the initial exposure to ultraviolet radi- 

 ation. It was first described by Keller (1924a) (Pigmentverschiebung). 

 It was also noted by Peck (1930) in the case of pigmentation brought 

 about by ionizing radiation from thorium (pseudohyperpigmentation) 

 and is suggested in earlier studies by Lutz (1917-18). The mechanism 

 of this migration of melanin does not seem to be clearly explained. 

 Recent spectrophotometric studies by Jansen (1953) suggest that there is 

 early pigment formation preceding the migration. 



Later there is increase of melanin in the region of the basal cell layer, the 

 amount depending on the severity of the dose and whether repeated (Lutz, 

 1917-18; Keller, 1924a, b; Hamperl et al., 1939a). While it has been 

 maintained that all the basal cells can form melanin, the point of view that 

 only certain dendritic cells accomplish this gains favor (e.g., Masson, 

 1948). The pigment may be found in the prickle cells of the malpighian 

 layer and in the corneum of skin that has been subjected to repeated doses 

 of ultraviolet radiation. ^^ Once formed, the pigment persists in the epi- 

 dermis for months and may still be grossly observable after several years. 

 In later discussion the total of changes involving the melanin pigment will 

 be referred to as melanization, pigment migration and pigment formation 

 being distinguished as parts of the complex. 



Pigment Darkening. Formerly it was thought that tanning is brought 

 about by ultraviolet wave lengths somewhat longer than those which elicit 

 erythema. But in 1939 Henschke and Schultze published a series of 

 studies which showed that, while such wave lengths do cause darkening of 

 the skin, the mechanism is ((uite different from that of the primary melani- 



'" What has hoeii .said applies to tlie skin of the white races. In Xegro skin the pig- 

 ment is more uniformly distributed throughout the epidermis, including the cornevim, 

 but comparable histological studies are lacking. 



