ELECTRON MICROSCOPY 



and the granules or particles have, therefore, 

 been called RNA-particles. The number of 

 membranes varies from tissue to tissue; In 

 cells engaged in heavy protein synthesis, 

 such as the exocrine cells of the pancreas, 

 the ergastoplasm dominates the cell with 

 its complicated pattern of membranes and 

 cisternae throughout the entire cell ex- 

 cept in the Golgi area. Evidently the mem- 

 branes and cisternae produce the precursors 

 of the zymogen granules which, at a later 

 stage appear within the Golgi apparatus. In 

 nerve cells, the Nissl bodies have the same 

 ultrastructure, participating in the produc- 

 tion of proteins needed within the cell itself. 

 Again in other cells the ergastoplasm may be 

 seen as scattered short pairs of membranes 

 with the RNA-particles attached. 



The terminology used in connection with 

 the ergastoplasm is somewhat confusing. As 

 mentioned, the term "ergastoplasm" refers 

 to basophilic areas which can be seen in the 

 light microscope. The term "endoplasmic 

 reticulum," introduced by Porter and Kail- 

 man (1952), originally referred to a structure 

 presumably vesicular or tubular w^hich was 

 observed in whole cells in tissue cultures. 

 Extended studies demonstrated that the 

 endoplasmic reticulum corresponds to the 

 ergastoplasm. When later structures like 

 infoldings of the plasma membrane, pino- 

 cytosis vesicles, and the membranes of the 

 Golgi apparatus were sufficiently analyzed, 

 it was found that some of these structures 

 could be in direct continuity with the er- 

 gastoplasm. It was, therefore, suggested 

 that all membranes wdthin the cell, whether 

 smooth or rough surfaced, may represent 

 diverse differentiations of one single mem- 

 branous system. Therefore, the ergasto- 

 plasm wdth its RNA-dotted membranes is 

 usually referred to as the rough-surfaced 

 endoplasmic reticulum, whereas the Golgi 

 membranes, cytoplasmic vesicles and in- 

 folded or invaginated portions of the plasma 

 membrane are called smooth-surfaced endo- 

 plasmic reticulum. For a more detailed dis- 



cussion of this problem, consult Haguenau 

 (1958) International Review of Cytology, 

 VII. Another example of a purely smooth- 

 surfaced endoplasmic reticulum is foimd in 

 striated muscle cells, here called sarcoplasmic 

 reticulum.Ii is an elaborate network of smooth 

 tubules around the mj^ofibrils with expan- 

 sions of the system specifically localized in 

 relation to the Z-band. It has been suggested 

 that this system might function in the inward 

 spread of the excitation impulse to contract. 



Microsonies. When using differential cen- 

 trifugation the membranes and cisternae 

 of the ergastoplasm break up to form small 

 spheres which can be isolated at a certain 

 cent rifugat ion speed. They then represent 

 the microsome fraction (Fig. 2). 



RNA-particles. In most cells, the cyto- 

 plasm displays an abundance of small 

 granules or particles with a diameter of about 

 150A (Fig. 9). They appear single or in 

 clusters of 3-5 and are freely dispersed 

 throughout the cytoplasm. They are iden- 

 tical in size and shape to the particles 

 which are attached to the membranes of the 

 ergastoplasm. It has been clearly demon- 

 strated that either type of granules contains 

 ribonucleoproteins and is, therefore, called 

 either RNA- or RNP-particles. 



Glycogen Granules. Particles have been 

 demonstrated in the cytoplasm of the stri- 

 ated muscle and in the heart muscle which 

 have a diameter ranging between 150A and 

 300A. Thus, they are somewhat larger than 

 the ribonucleoprotein particles with which 

 they can easily be confused. The larger 

 particles are more variable in their size and 

 shape and have less sharply defined margins. 

 Histochemical tests seem to prove that they 

 contain glycogen and it is, therefore, likely 

 that they represent a particulate form of 

 glycogen. One has not been able to demon- 

 strate similar granules in mammalian liver 

 cells. The glycogen-rich areas of the liver 

 cell cytoplasm usually show a diffuse, cot- 

 ton-wool texture of low density when using 

 solely osmium tetroxide as a tissue stain. 



104 



