THE KERATINIZATION PROCESS 271 



a useful distinction to be made between keratinized and non-keratinized 

 components, and to establish what fraction of the tissue actually consists 

 of keratins. 



The most satisfactory procedure is to oxidize the disulphide bonds with 

 peracetic acid and to extract thoroughly the oxidized material with dilute 

 ammonia. Thioglycollates at high pH (11-12) have also been used, but 

 effect a less complete extraction. The undissolved residue contains the 

 following components recognizable in the light microscope : 



(a) The ill-defined and obscurely altered remnants of the cell nuclei. 



(b) Many small particles distributed throughout the cytoplasmic space 

 which are the remnants of the various cell organelles, mitochondria 

 and Golgi membranes. 



(c) The external membranes of the cells which are still held together in 

 a foam-like formation by the intercellular deposits. 



Similar residues remain after extraction by other reagents and the findings 

 are the same for all types of keratinized tissues (Mercer, 1953; Matoltsy, 

 1958). In some cases, when the cells assume a specialized form, as in the 

 hair cuticle discussed earlier, the intercellular remnants may separate 

 during growth from the keratinized protein and thus form a distinct layer 

 within the cell which is visible (Fig. 110 (d)) after extraction. 



A picture which is almost the " negative image " of the above is obtained 

 when the material is digested with a proteolytic enzyme (trypsin or papain) 

 which removes everything except the keratinized proteins. The pro- 

 cedure is not very satisfactory with the soft keratin, epidermis; with all the 

 hard keratins (hair, horn, nails, etc.) after prolonged digestion, a clear 

 delineation of the resistant protein results. The tissue falls apart, since the 

 cell membranes are removed, and the residue is often referred to as 

 " cells", which they resemble superficially in shape, although they consist, 

 in fact, only of the keratinized protein, all other components visible in the 

 intact state being removed (p. 261). Such preparations should form the 

 material for analysis of " keratin". More prolonged digestion removes 

 part of the keratin itself, suggesting that this material is not entirely 

 uniform in composition. This suggestion, that the " degree of keratini- 

 zation " may be variable at a microscopic level, is compatible with the idea 

 that keratinization is a process which takes place gradually and may remain 

 incomplete and patchy. 



A " dissection " of the cuticle cell, as effected by various methods of 

 extraction, illustrated in Fig. 110, shows clearly the differing properties of 

 its several layers. 



The recognition that hardened tissues contain several components 

 differing in resistance to chemical attack was the basis of an earlier system 

 of classification, introduced by Unna (1926) in which keratins were to be 

 distinguished by the proportions they contained of " keratin A", insoluble 



