THE KERATINIZATION PROCESS 277 



layer cells of the keratinizing system. According to Medawar (1953) the 

 melanocytes comprise from 5 to 15% of the total cell population of the 

 germinal layers of the epidermis. The epidermis thus really consists of two 

 entirely different classes of cells the members of which have distinct 

 morphologies, functions and embryonic origins. Keratinizing cells arise 

 embryonically from the ectoderm; melanocytes have, however, been 

 traced back largely to the neural crest (Rawles, 1947) (see also Niu, 1959) 

 and they enter the epidermis only after this has been clearly differentiated. 

 They are an amoeboid type of cell with several long arborescent processes 

 called dentrites, and are perhaps best referred to as dentritic cells. In their 

 dispersion from their site of origin they seem impelled by a mutual 

 repulsion which leads them ultimately to colonize the dermis and epidermis 

 and there to adopt a dispersed distribution, each cell occupying a small 

 domain determined by the extreme reach of its dentritic processes. They 

 also accumulate densely in a few other sites, such as the pigmented layer 

 of the eye. Each epidermal melanocyte pigments the small group of 

 keratinizing cells within reach of its dentrites. The granules of pigment 

 are formed in the perikaryon of the cell, pass along the processes and enter 

 the keratin cells. Owing to their situation in the basal layer attached to the 

 basal membrane, pigmentation occurs before the formation of keratin and 

 the subsequently-formed fibrils may lead to an orientation of the granules. 

 The presence of melanocytes seems in no way essential to the well-being 

 of the keratinizing system since some epithelia naturally lack melanocytes 

 and others may be deprived of them, accidently or by experiment, without 

 appearing to be at a disadvantage. 



The distribution of pigment cells is under genetic control and, since 

 changes in the integument are easy to observe, much attention has been 

 given to the genetics of skin and hair pigmentation. The value in terms of 

 natural selection of pigmentary patterns is obvious; but for all that, little is 

 known of the underlying causes determining the distribution of pigment 

 cells (Du Shane, 1944). 



According to Billingham and Medawar (Billingham, 1948; Billingham 

 and Medawar, 1948 and 1953; Billingham, 1958) not all dentritic cells 

 produce pigment. White skin patches are said to contain a full complement 

 of dentritic cells although special means are required to demonstrate these, 

 since they contain no pigment. The skin of white human beings is said to 

 contain as many melanocytes as that of negroes, for example. It would seem 

 that melanocytes differ in their response to the influences which provoke 

 pigmentary activity. This would be an inherited difference distinguishing 

 different sub-races of melanocytes even on a single skin. Some never fail 

 to begin production once they reach the epidermis; others may be pro- 

 voked into activity by exposure to actinic radiation; others normally 

 remain latent. The hormonal balance can also cause changes in activity. 



