DIFFERENTIATION AND PROTEIN SYNTHESIS 121 



enter on a more-or-less prolonged life in which their activity is of a 

 different order. They perform some specialized function based upon the 

 properties and behaviour of their accumulated products, e.g. muscle cells 

 accumulate actomyosin and their subsequent functional behaviour is 

 based on the contractile properties of this substance; the reticulocyte 

 becomes the mature erythrocyte filled with haemoglobin and enters the 

 blood stream as an oxygen carrier; the cells of the stratum corneum harden 

 and form a protective layer to the organism. In this phase their RNA 

 content (dense particles) falls. 



Synthesis in Fibre-forming Systems 



No comparable biochemical studies have been made on cells whose 

 function is the formation of protein fibres. Nevertheless, a survey of the 

 cytology of such cells reveals their essential similarity to that of cells which 

 produce soluble, non-fibrous proteins (Mercer, 1958). The same dis- 

 tinction exists between cell systems secreting a precursor of the fibre, 

 which then forms extracellularly, and systems in which the fibres form 

 intracellular^. The cells secreting fibres have as before elaborately- 

 developed particle-studded membranes of which perhaps the most 

 developed examples are the cells of the silk gland of the silk worm (Bombyx 

 mori) (Mercer, 1958). The cells of the hair cortex are, of course, the 

 typical retaining cells. The dense particles found in all fibre-forming cells 

 have the same range of dimensions 120-200 A as those found in cells 

 forming soluble proteins, and may be identified on the same grounds as 

 particles containing RNA. Histochemical tests confirm this picture (see 

 p. 220). 



Further, no special cytological features are found to be associated with 

 the production of fibres having the different basic types of molecular 

 structure indicated by X-ray analysis of fibres. The a-type proteins occur 

 intracellularly as keratin or myosin (Astbury, 1947) in retaining cells, and 

 extracellularly as fibrinogen (Bailey et ai, 1943). The secreted fibres 

 (collagen-type) and fibroin (|8-type), originate in cells having the familiar 

 pattern of membranes and particles of the secreting cell. Thus there 

 would seem every reason again to associate the RNA system only with the 

 original link-up of amino acid in polypeptide chains and to consider that 

 the specific folding determined by this sequence occurs after the release 

 from the RNA particle. The subtle, genetically-determined differences 

 between an RNA particle producing a collagen sequence and one pro- 

 ducing an a-keratin sequence remain unknown. 



The novel feature about a cell, which produces a protein capable of 

 forming a fibre, is not then to be sought in the basic machinery used to 

 effect the synthesis which is common to all, but rather in the genetically- 

 determined instructions or information associated with the RNA particle. 



