278 KERATIN AND KERATINIZATION 



There would appear to be ample opportunity for these variations to arise 

 when the long train of enzymatically-calalysed reactions involved in pig- 

 ment formation is considered (p. 279). 



In their grafting experiments on guinea-pig skin, Billingham and 

 Medawar (1948) have demonstrated an actual diffusion of pigmentary 

 activity from a black graft into a white skin. They proposed an explanation 

 which is of some theoretical importance. They consider that the latent 

 melanocytes of the white skin are actually " infected " by the neighbouring 

 black melanocytes by a process which is related to the normal method of 

 transferring pigment from melanocyte to a keratinizing cell. They have 

 demonstrated microscopically that anastomoses between processes of 

 neighbouring melanocytes do actually occur and suppose that through such 

 contacts one melanocyte may transfer to another a sample of its cyto- 

 plasmic apparatus. Electron micrographs of the tips of dentritic processes 

 found in keratinizing cells show that in fact samples of cytoplasmic 

 membranes and particles are transferred along with pigment granules 

 (Birbeck et al, 1956). 



Such a transference of a cytoplasmic element capable of permanently 

 modifying the activity of an acceptor cell, if it proves not merely a peculia- 

 rity of the system of dentritic cells, could be of importance in the normal 

 developmental history of cell lines. It would imply first that inherited 

 differences between cells could have a cytoplasmic as well as a nuclear 

 basis, and second that a population of cells could establish and maintain 

 uniformity among their members by cytoplasmic exchanges. The actual 

 mechanics of the transfer of pigment from melanocyte to epidermal cell 

 needs further elucidation. Two broad possibilities may be envisaged: the 

 keratin cell may be actively penetrated by the tip of the melanocyte process 

 or alternatively, it may play the active role and phagocytize the tip. The 

 latter process receives some support from electron micrographs which 

 show small processes of the keratinizing cell in various stages of enveloping 

 the pigmented tips. 



The Pigment Granule 



The actual pigment granule contains both protein and pigment and is 

 often referred to as a melanoprotein, although almost nothing is known of 

 the nature of the linkage between the two components. It may be formu- 

 lated as a bipolymer in which polypeptides alternate with melanin, or 

 it may be a tanned protein, i.e. a network of protein chains cross-linked by 

 melanin polymer or other phenols. Whatever its structure, the granules 

 may exhibit remarkable chemical stability as is proved by the fact that their 

 separation from keratinized tissues by violent chemical destruction of the 

 keratin seems to leave them morphologically intact. Electron micrographs 

 by Birbeck demonstrate clearly the existence of a regular framework of 



