134 KEITH R. PORTER 



A?^ 



. •y^^'^nl 



Fig. 3. Micrograph of part of an acinar cell of the guinea-pig pancreas. The 

 basal surface of the cell crosses the figure at the lower left ; the apical pole, with a 

 few elements of the Golgi (G) showing, is at the upper right. The specimen was 

 fixed during the recovery phase following secretion of zymogen granules. 



The objects of greatest interest here are the large, dense, spherical granules 

 within the cisternae of the ER. Now known as intracisternal granules, these units 

 have been shown to possess the enzymic activities of purified zymogen granules. 

 It is thought that in acinar cells of guinea-pig pancreas the zymogen portions, 

 sequestered in the cavities of the ER, condense out as granules before being trans- 

 ported to the Golgi component in the apical pole for packaging as secretory 

 granules. The intracisternal granules are indicated at icg; the mitochondria at m. 

 Small (150 A) dense granules are abundant in the cytoplasm of these cells and 

 particularly on the surfaces of the ER. The specimen was fixed in OSO4 and stained 

 with lead. (Micrograph courtesy of G. E. Palade.) 



activity of the ribosome is suggested by the fact that in growing cells, e.g., 

 the particles seem to perform their synthetic role without membrane 

 support. The distinction to be made is that protein production in this latter 



