Diversity at the Subcellular Level and Its Significance 149 



such that the individual glycogen particles are essentially locked or 

 incapsulated by the smooth reticulum (Ashford and Porter, 1961; 

 Porter, 1961b). When glucose secretion is stimulated by glucagon 

 injection of the animal, the glycogen disappears rapidly, and con- 

 comitantly the tubules of the smooth ER dilate as though sequestering 

 the glucose. This interpretation is favored because at the same time, 

 vesicles derived from the smooth reticulum accumulate at the basal 

 and lateral cell surfaces and appear to fuse with the plasma mem- 

 brane, as though to discharge their contents. It is pertinent at least 

 in this regard that glucose-6-phosphatase is localized in membranes of 

 the microsomes. Other enzymes of the glycogenolytic sequence are 

 apparently not associated with the membranes but are soluble or are 

 part of the glycogen granules ( Luck. 1961 ) . 



The liver cell is not the only instance where smooth and rough 

 forms of the ER intermingle or are at least present in the same cell. 

 Philpott ( 1962 I , reports the presence of both in the chloride-secreting 

 cells of Fundulus; Christensen and Fawcett (1961) in the interstitial 

 cells; and Ito ( 1961 1 in the parietal cells of several forms. The extent 

 to which the two systems are continuous in these latter instances is 

 not indicated, but it is probable that the relationship found in the 

 liver cell will eventually be demonstrated in these other forms. 



A discussion of this smooth form of the endoplasmic reticulum 

 would be incomplete without some mention of the system found in 

 striated muscle (Fig. 7). Here a continuous system of tubules and 

 vesicles occupies the sarcoplasm between the myofibrils and differ- 

 entiates into a pattern of organization which repeats with each sar- 

 comere (Porter and Palade, 1957; Andersson-Cedergren, 1959; Faw- 

 cett and Revel, 1961; Revel, 1962). This precise association with the 

 contractile elements has convinced more than one observer that the 

 system must be involved in some phase of muscle physiology. Thus 

 the continuity of the system laterally in the muscle fiber led to the 

 early proposal that it might conduct the excitatory impulse to the 

 myofibrils located centrally within the fiber, sometimes at distances 

 of 50 microns from the sarcolemma. More recently it has been recog- 

 nized that one component of the system — the middle or intermediary 

 element of the triad ( Fig. 7 ) , the T system of Andersson-Cedergren— 

 is a derivative of the plasma membrane and hence a better candidate 

 for lateral conduction (Smith, 1961; Porter, 1961c). This leaves the 

 sarcoplasmic reticulum, or SR — the smooth-surfaced reticulum of the 

 muscle cell — with no assigned function. One might reason from the 

 above-reported observations on the smooth ER of the liver cell that the 



