122 KERATIN AND KERATINIZATION 



These instructions determine the sequence of amino acids and this 

 sequence determines the sort of intermolecular interaction which follows. 

 Some types of sequence produce strong intramolecular chain association 

 and a corpuscular type molecule with weak forces of intermolecular 

 association; these may form proteins soluble as discrete particles. Other 

 sequences may lead to the stronger form of witeraiolecular association we 

 recognize as fibrous. The several means by which protein fibres are 

 synthesized are classified in Fig. 52. 



micromolecules 

 (amino acids) 



I* 

 soluble microprecursors 

 (activated amino acids) 



[* 



condensed micromolecules 



(polypeptide associated 

 with RNP particle) 



I * 

 shed macromolecule 

 (non-fibrous precursor) 



I 



retaining-type cells 



I 



I 

 immediate intracellular 

 condensation in fibrous 

 form (fibrous hair keratin) 



secretory-type 

 cells 



temporary accumulation 

 of precursor followed 

 by a transformation into 

 the fibrous form 

 (keratohyalin) 



extracellular conversion 

 into the fibrous form 

 (fibroin) 



Fig. 52. Classification of types of protein fibre synthesis. 

 (*See also Figure 51.) 



Epidermal cells provide examples of most of these methods of forming 

 fibres. In the hair cortex and in the feather, filaments seem to appear 

 directly without the accumulation of substantial amounts of macro- 

 molecular precursor ; in the inner root sheath of the hair follicle trichohy- 

 alin accumulates as a precursor and undergoes a transformation into 

 fibrils. Skin cells may employ both methods at different times (p. 228). 

 The hair-cuticle cells are peculiar in that the keratin appears as amorphous 

 droplets and condenses without changing into fibrils. These cells remain 

 non-birefringent and yield no defined X-ray pattern. Among birds, 



