226 KERATIN AND KERATINIZATION 



beneath the cell membrane. The reasons for the delay in synthesis and for 

 the different type of keratin produced are not known. It is possible to 

 speculate that the denser and closer opposed membranes of the cuticular 

 formation deprive the cells of some essential factor, and that synthesis 

 proceeds under limiting conditions which tend to produce amorphous 

 peptides richer in cysteine. Similar conditions may prevail in the upper 

 keratinization zone of the cortex where an increase in cystine also occurs. 

 A sufficiently-high frequency of cystine residues in a polypeptide would 

 seem to preclude both the possibility of assuming the regular folding 

 necessary for crystallization and of long-range extensibility. 



The birefringence, which can be observed in cross-sections of the cuticle 

 cells, particularly in swollen hairs, may be due either to the fact that the 

 molecular chains of the keratin are compressed into one plane or to the 

 compression of the largely membranous cell apparatus (Schmidt, 1925). 



The Inner Root Sheath (i.r.s.) 



The characteristic product of the inner root sheath is trichohyalin, sl 

 substance as yet imperfectly characterized. The name is due to Vomer 

 (see historical review in Auber) (1950) who wished to emphasize by it the 

 clear structureless character of the granules and a distinction from similar 

 granules (keratohyalin) found in the skin. It is a protein and when in the 

 fibrous form is strongly birefringent and yields an a-type X-ray pattern. 

 It is quite distinct chemically from fibrous keratin, notably in being 

 deficient in cystine. Rogers (1959) has shown further that it is peculiar in 

 containing the amino acid citrulline. The similar keratohyalin occurs in 

 the cells of the epidermis proper where its appearance, as strongly staining 

 granules, gives rise to the name, stratum granulosum. In spite of some 

 differences in stainability (see Rothman, p. 376 (1954)) the two substances 

 seem essentially similar in nature. In this sense the inner root sheath is a 

 soft keratin ; and, like the epidermis itself, it desquamates half-way along 

 the follicle. It is this event which actually frees the advancing hair shaft 

 from its enveloping sheaths. 



The synthesis of trichohyalin commences first and proceeds further in 

 the cells of the layer of Henle, the outer-most layer of the i.r.s. contiguous 

 with the outer root sheath. Small dense granules of trichohyalin appear 

 free in the cytoplasm and grow rapidly. They have no internal structure, 

 are not birefringent, stain heavily (in the electron-microscopic sense) with 

 phosphotungstic acid and with basic dyes as judged by light microscopy. 

 The fact that the granules absorb basic dyes, as does nucleic acid, has led 

 some to think that they contain nucleic acid. This is not now thought to 

 be the case since digestion with ribonuclease has only a small effect on the 

 basophilia (Leuchtenberger and Lund, 1951) and electron-microscopically 

 the trichohyalin droplets are free from the small dense particles usually 



