THE ENDOPLASMIC RETICULUM 1 35 



instance is for internal consumption rather than for export. Only in other 

 instances where particles are associated with the membranes of the ER, is 

 the product prepared for secretion and the external environment of the cell. 

 For observations on the role of the ER in this phenomenon one must 

 turn to studies on intact cells engaged in protein synthesis. In an early 

 paper, Weiss [20] noted that the cisternae of rat pancreas cells appeared 

 swollen and filled with a material which he interpreted as zymogen during 

 the synthetic phase of the cell's activity. The real nature of the intra- 

 cisternal material was not, however, demonstrated. Later, Palade [6], in 

 some observations on guinea-pig pancreas, observed that during the 

 period of gland recovery after secretion, relatively large granules, having 

 the density of zymogen granules, appeared within the cisterna of the ER 

 (Fig. 3). These granules were subsequently isolated and analyzed [21, 22] 

 and found to have the properties of zymogen granules as normally secreted 

 by the cell. In this instance, then, and probably in every other case of 

 ergastoplasm function, the product of synthesis is, so to say, separated or 

 segregated from the site of synthesis (the ribosome) by the membrane 

 limiting the ER cisterna. One can say that the product is externalized with 

 respect to the continuous phase of the cytoplasmic matrix, where, if it 

 were left, it would presumably diffuse away among the structural proteins of 

 the cytoplasm. 



Once sequestered within the cavities of the ER it appears to move, in 

 a dissolved or diffusible form, to the Oolgi region in the apical pole of the 

 cell, where it is apparently prepared tor secretion. Some evidence of at least 

 temporary continuity between elements of the ER and the Golgi com- 

 ponent, which would provide channels for transport, have recently been 

 described [23]. Thus, in addition to its role in sequestering the product of 

 synthesis, the ER appears to function in transporting metabolic products 

 from one point to another in the cell. It mav be recalled that this was 

 mentioned earlier as a logical function of the system. 



The smooth form of the ER 



In all the attention gi\en the more striking, rough form of the ER and 

 the ribosomes, other configurations of this intracellular system are seem- 

 ingly overlooked. Yet there are probably as many types of cells with 

 promment differentiations of the smooth ER as there are with the rough 

 form. In some instances, as mentioned earlier, the two are mixed in one 

 and the same cell and where this is the case, microsome fractions can be 

 shown to contain both types (Fig. 2). Thus in one of the earliest studies 

 of liver microsomes, note was taken of a substantial content of smooth- 

 surfaced vesicles derived, it was thought, from the smooth-surfaced 

 portions of the endoplasmic reticulum [14]. 



