74 



SUBCELLULAR PARTICLES 



600 



500 



C 4001- 



5 

 <-> 

 o 



a. 



cb 300 

 E 



r. 



o 



200 



JOO 



Fig. 5. Variation with time in 

 the specific activities of protein of 

 attached particles (AP), microsomal 

 content {MC), and zymogen gran- 

 ules (Z) of pancreas after injection 

 of DL-leucine-i-Ci^ into guinea pigs 

 I hour after refeeding. For further 

 details see (38). 



3 



JO 



20 



45 



temporarily stored in the apical region of the cell, are dangerous products which 

 could easily wreck the cell. To handle them without mishap, the cell uses a num- 

 ber of protective devices. One is represented by the fact that some of the enzymes 

 involved, namely the proteases, are produced and stored in the form of inactive 

 precursors. Another consists of the isolation of the enzymes from the rest of the 

 cytoplasm by a membrane barrier, which is clearly visible when the granules are 

 normally sectioned, measures -^70 A in thickness, and has a smooth surface: it 

 does not have attached particles on the side that faces the cytoplasmic niatrix. 

 This latter finding introduces a complicating element in our general hypothesis. 

 We have postulated, on the basis of the previous example, that the digestive 

 enzymes are produced by the endoplasmic reticulum, presumably by its attached 

 RNP particles, and are subsequently segregated in the form of intracisternal 

 granules. These granules were found within the cavities of the endoplasmic 

 reticulum surrounded by a membrane bearing attached particles and separated 

 from it by a relatively large, light halo. If the intracisternal granules are the 

 direct morphological precursors of the zymogen granules, then we must assume 

 that they grow within the cisternae by continuous accretion until they fill a whole 

 cisterna and reach the dimensions of the zymogen granules. At that time, we must 

 further postulate that the membrane becomes smooth by shedding its attached 

 particles. Such a sequence of events is, however, highly improbable because the 

 intracisternal granules usually remain below ^0.2 to o.3;u, in diameter and never 

 reach sizes comparable to those of the zymogen granules. An alternative course 



