DIFFERENTIATION AND PROTEIN SYNTHESIS 



113 



cytoplasmic synthesis is active, the diffuse cytoplasmic RNA increases. 

 (See also Braun-Falco (1958).) 



The nuclear membrane of epidermal cells also is double-layered and 

 exhibits pores (Plate 4C). There is perhaps a difference in the mechanism 

 by which the RNA particles reach the cytoplasm since clearly they are not 



Type of cell 



Disposal of 

 protein 



Lifetime of 

 synthetic phase 



Cytoplasmic* 

 features 



Examples 



Secreting 



passed through 

 cell membrane 

 usually as a 

 granule S 



long, periodic 



mitochondria m 

 Golgi cluster G, 

 reticulum R and 

 bound particles P 



Pancreatic and 

 thyroid cells, etc., 

 silk gland cells 



Retaining 



retained within 

 cell (/) 



short, single phase 



mitochondria m 

 Golgi cluster G 

 little reticulum, many 

 dense free particles P 



Epidermal cells, 

 myogenic cells, 

 early embryonic 

 cells, anaplastic 

 tumours 



Fig. 



50. Comparative characteristics of cells forming protein (*see also 

 Fig. 49). 



shed along with the external nuclear membrane to form a particle-studded 

 cytoplasmic membrane as pictured for glandular cells. They may simply 

 diffuse through the pores which are larger than particles or they may 

 leave in the small vesicles V to be seen in Plate 9. 



There are many other cell-types with these same characteristics, and some, 

 along with examples of cells of glandular cells are mentioned in Fig. 50. It 

 will be seen that two classes of protein-forming cells are to be distinguished 



