232 KERATIN AND KERATINIZATION 



increase in the intensity of the SH reaction on passing from the inner 

 mucosum to the outer, suggesting that an increase in the amount of 

 cysteine occurs during the later stages of hardening. This was confirmed 

 by an actual chemical analysis of the several layers which in this material 

 can be separated mechanically and chemically. This conclusion is in good 

 accord with the suggestion to be developed later that cysteine-rich pep- 

 tides are added during hard keratinization. 



Follicular nutrition and the entrance of sulphur 



There seems little doubt on histological grounds that the greater part of 

 the material supplies for the growth of the hair are conveyed by the papilla 

 whose dimensions control the output of keratinized cells. This impression 

 is given a quantitative basis by Rudall's (1956) extensive survey of wool 

 follicles referred to on p. 150. 



The elaborate vascularization of the middle region of the shaft (Dur- 

 ward and Rudall, 1949; Ryder, 1958), which fluctuates with the hair- 

 growth cycle, remains to be explained. Possibly it is involved in the 

 transport of glucose, mobilized from the glycogen of the outer-root sheath 

 to the papilla or it could be associated with keratinization since the 

 evidence (see below) is that the sulphur enters at this level. 



At the biochemical level it has been shown by the use of radioactive 

 tracers that methionine and not cystine in the diet contributes its sulphur 

 to the hair of rats (du Vigneaud, 1947; Marston, 1946). It is thought that 

 methionine is converted to homocystine and then linked to 1-serine to 

 form the compound : 



X 



NH 2 



NH 2 

 HOOC' 



CH.CH 2 — 



— S.CH 2 .CH S 



COOH 



This compound is split in vivo at X-X and 1-cysteine is produced, the S 

 having been transferred to the serine to give cysteine. However, in other 

 animals the sulphur metabolism may differ (Ryder, 1958). 



The site of entry into the follicle would seem at first sight to be the zone 

 of keratinization since here for the first time SH can be detected (Figs. 95, 

 96 and 97). It would be possible on this histochemical evidence alone to 

 suppose that below this level the sulphur is present in a concealed (non 

 SH) form, but further work on the uptake of radioactive sulphur following 

 injection has dispelled uncertainties. Injection of radioactive cystine is 

 followed by the rapid appearance of radioactivity in the keratinization 

 zone, but not in the bulb (Ryder, 1958 ; Bern et al., 1955 and 1957). On the 

 other hand labelled carbon and phosphate compounds enter through the 



