138 KEITH R. PORTER 



sequestered as droplets within the cisternae. This scheme would provide 

 for the production of triglycerides at all levels in the ER — a possibility 

 suggested as well by the appearance of large quantities of fat globules in 

 the ER in deep parts of the cell within a short time after fat digestion 

 begins. The sequence of synthesis, sequestration and transport is obviously 

 analagous to that encountered in zymogen granule formation as depicted 

 in the guinea-pig pancreas. 



Glycogen metabolism 



The instances already cited in which the smooth or agranular ER is a 

 prominent component of cell fine structure do not exhaust the known 

 occurrences. Other reports include observations of agglomerations of 

 agranular vesicles and tubules in liver cells of the rat [36, 6] ; and a remark- 

 able tri-dimensional lattice of small tubular elements as part of the gly- 

 cogen-rich paraboloid found in the inner segment of the turtle cone cell 

 [37, 38]. Continuity between the smooth elements and adjacent rough cister- 

 nae describe them as parts of a single system, the endoplasmic reticulum. In 

 each of these instances, unlike those previously noted, the smooth ER is 

 specifically associated with glycogen deposits. 



One of the earliest descriptions of this smooth form of the ER in liver 

 cells appeared in a paper by Fawcett [36]. In the liver cells of rats, fasted 



Fig. 4. The image shown here represents parts of two adjacent liver cells (rat) 

 plus a neighbouring sinusoid (s). In the cell at the bottom of the picture, one can 

 identify the nucleus (A^) with its typical envelope ; mitochondria (w) ; long profiles 

 of cisternae comprising the granular or rough form of the ER or ergastoplasm (er) ; 

 and a few profiles of elements belonging to the agranular or smooth ER (ers). The 

 rough ER (within the rectangle) is particularly suitable for identifying the source 

 of the microsomes in Fig. 3. 



The part of a cell at the upper left, beside the bizarre mitochondrion, contains 

 two elements of particular interest here. The first is very dense (following staining 

 with Pb(OH)2) and represents the glycogen (gl). The unit structure, which appears 

 as a cluster of granular subunits, usually does not exceed a maximum size of about 

 150 m/ti. From this, the size ranges downwards to individual subunits. This is the 

 liver cell component that fluctuates with physiological conditions leading to 

 glycogen storage or depletion. It varies directly with intensity of PAS staining and 

 is removed by diastase. A number of investigators have referred to it as glycogen. 



These dense bodies are interspersed with less dense, line-limited elements, 

 which are interpreted as vesicles and tubules of the smooth ER (ers). They seem 

 to form an irregular lattice. While the diameters of these elements are not uniform, 

 it is obvious that they fall within a fairly narrow range of variation. 



This is the picture of association between ER and glycogen found during the 

 storage phase. The specimen was taken 2 hr. after re-feeding following a 24-hr. 

 fast. When taken later at 4 or 6 hr., the cells show much more glycogen and the 

 ER profiles intermingling with the glycogen are relatively fewer. The specimen was 

 fixed in OsOj, embedded in Epon 812, and, after sectioning, stained with Pb(OH)2. 



