THE KERATINIZATION PROCESS 233 



bulb. This evidence shows clearly that sulphur compounds do not enter 

 through the papilla and advance up the follicle. They must enter in some 

 way through the walls of the follicle at the level of the keratinization zone. 

 The transport of sulphur compounds could be one of the functions of the 

 extensive vascular network surrounding the follicle. There is also evidence 

 of an absolute increase in labelled compounds in the upper levels of the 

 keratinizing levels according to Ryder (1958). Rudall, who separated 

 and analysed the several layers of the growing tissues of horn, established 

 the same fact (1955). 



On account of its shape, the hair follicle enables sulphur absorption to 

 be separated from fibrillar growth, thus facilitating interpretation of the 

 phenomena. In the epidermis on the other hand, the sulphur compounds 

 must diffuse upwards from the basal layers into the keratinization layers, 

 and this may have some influence on the lower uptake. Comparable 

 experimental work has not been carried out on other keratinizing tissues. 



Sylven (1950), commenting on the presence of acid sulphate-containing 

 mucopolysaccharides in the papilla (see also Montagna (1956)), believed 

 these might be active in transferring their sulphur to the growing hair. In 

 view of the demonstration that labelled sulphur-containing amino acids 

 enter at the level of keratinization while labelled phosphate enters the 

 papilla, this would now seem unlikely. These acid mucopolysaccharides 

 (substances staining metachromatically with thiazine dyes) are most obvious 

 during active hair growth and seem clearly connected with the proliferation 

 of the bulb, but their sulphur is certainly not transferred to the growing 

 cells at this level. 



No definite intracellular structure has been yet shown to be associated 

 with the absorption of the sulphur acids or their change into cystine. 

 Enzymes are of course suspected but not isolated. " Microbodies " (Plate 

 22), single-walled dense bodies, are often common in the keratinizing zone 

 and may contain a special enzyme system. 



Soluble products of partial keratinization 



However the process of keratinization is viewed, it certainly involves in 

 the first place the synthesis of one or more polypeptides which sub- 

 sequently become stabilized by the formation of cystine bridges. Clearly 

 some insight into the nature of keratinization would be obtained if the 

 state of aggregation of the proteins in the cells at various levels in the 

 keratinization zone were known. The concept of the molecular weight of 

 keratin in the hardened tissue is itself meaningless, since the protein 

 is extensively united into large and indefinite heterogeneous formations 

 by covalent cross-linkages. It would, however, be valuable to know in 

 the first place the number, composition and molecular weight of the 

 primary polypeptide chains, i.e. chains containing only peptide links, which 



