120 KERATIN AND KERATINIZATION 



Experimentally, the fact that no special intracellular apparatus is 

 necessary to induce the polypeptide chains to adopt the specific foldings 

 which are responsible for the characteristic fibre patterns, is evident since 

 both proteins and synthetic polypeptides (Bamford et al., 1949, 1956) form 

 the structures spontaneously in solutions in vitro. Newer methods of 

 demonstrating the presence of specific structures in solution (p. 194) have 

 shown that only portions of the chains may adopt the folded form. This 

 probably also occurs in vivo and, surviving into the solid state, is probably 

 one of the sources of the non-crystalline fraction (Chapter 5). 



Synthesis in Retaining Systems 



Most of the evidence described above relating to protein synthesis has 

 been obtained from secretory cells; retaining cells such as epidermal cells 

 have been less studied. Their cytoplasm is, however, as rich in RNA and, 

 electron-microscopically, it is crowded with dense particles (Plates 7, 9, 

 11 and 12A and B) apparently identical with the RNA-containing particles 

 of secretory cells. It is perhaps desirable to mention the experimental 

 evidence which shows that in cells with this cytological pattern that 

 synthesis also involves RNA and up to a point is identical with the process 

 in secretory cells. Reticulocytes, which synthesize and accumulate 

 haemoglobin becoming erythrocytes, are retaining cells and in them the 

 course of synthesis seems to follow the lines indicated above. Bacterial 

 cells, although less easily classified, resemble in some respects the retaining 

 cells of higher organisms and in them protein synthesis also follows the 

 same course (Loftfield, 1957). 



The same general picture of diffuse cytoplasmic basophilia in the light 

 microscope and of vast numbers of dense particles with a scanty develop- 

 ment of membranes, when seen in the electron microscope, is found in 

 the cells of certain anaplastic tumours. These again lend themselves 

 more easily to biochemical study and make it possible to confirm that 

 synthesis is associated with the particulate elements of the cytoplasm. 

 Littlefield and Keller (1957) for example, using ascites tumour cells 

 showed that the most rapid incorporation of radioactive leucine occurred 

 in the cell fraction containing the dense, ribonucleic acid-containing 

 particles. 



There is then no reason to doubt that protein synthesis in retaining cells 

 in general follow the same course as in the more commonly studied secreting 

 cells. But whereas in secretory or glandular type cells protein synthesis 

 continues more or less indefinitely in a succession of cycles, the products 

 being (continuously or on demand) removed from the cell, in accumu- 

 lating cells there is a single phase, the product is retained within the cell 

 and synthesis comes to a halt as the cell fills, possibly as a result of mass 

 action or the simultaneous production of an inhibitor. Such cells then 



