yo SUBCELLULAR PARTICLES 



Spectacular as they are, these morphological changes are not entirely unex- 

 pected. It is already known from the initial work of Schneider (t,^) and of Leh- 

 ninger and Kennedy (i8), that mitochondria can oxidize long chain fatty acids, 

 and more recently Green (lo) and his collaborators have shown that the numerous 

 enzymes involved in fatty acid oxidation are located in the mitochondria. This 

 fact, and the limited solubility of the lipids involved (mostly triglycerids), can 

 well account for the peculiar relationship established between mitochondria and 

 lipid droplets. In this respect, the morphological findings represent only support- 

 ing evidence for well established biochemical data. Certain details of the struc- 

 tural changes described raise, however, a number of questions, for which— to my 

 knowledge — there is no answer at present. We may ask, for instance, by what 

 mechanism are the triglycerids hydrolyzed? If a lipase is involved, is it located, 

 like the oxidative enzymes, in the mitochondrion? We may also wonder about 

 the means by which structural damage by fatty acids and soaps is prevented, and 

 finally, we may note that the two mitochondrial membranes are not affected to 

 the same extent in this circumstance: the outer membrane apparently can be dis- 

 posed of, while the inner membrane is retained. This finding suggests that the 

 oxidative enzymes are located, as already postulated (24, 25), in the inner mito- 

 chondrial membrane and in its infoldings or cristae. 



Changes Incurred by Endoplasmic Reticulum. The second example of func- 

 tional modulation in the organization of the pancreatic exocrine cell is encoun- 

 tered at the level of the endoplasmic reticulum. As is known, the term describes 

 an extensive system of membrane-bound vesicles, tubules, and flat vesicles or 

 cisternae interconnected into a more or less continuous network (32, 28, 27). 

 The membrane limiting this system separates two distinct phases in the cytoplasm: 

 one is represented by the material inside the cavities of the network, the other by 

 the cytoplasmic matrix surrounding the elements of the endoplasmic reticulum. In 

 the acinar cell, the system is characterized by: a) a large relative volume; b) pre- 

 ferred orientation (its elements are disposed in successive, parallel planes at more 

 or less regular intervals); and c) an extensive association with small dense particles 

 which appear to be attached to the outer surface of the limiting membrane of the 

 system (27, 30, cf. also 44, 42, 7). The features mentioned can be seen to advan- 

 tage in the basal region of the exocrine cell, in which the endoplasmic reticulum 

 reaches its highest intracellular concentration. Light microscope studies have 

 shown that the cytoplasm of this basal region is intensely basophilic, and histo- 

 chemical tests have demonstrated that the basophilia is due to the presence of 

 ribonucleic acid (RNx\) in relatively high concentrations (2, 5). From cell frac- 

 tionation studies it is known that the endoplasmic reticulum breaks down during 

 tissue homogenization into closed vesicles which constitute the bulk of the micro- 

 somal fraction (30). And, finally, from experiments carried out with isolated 



