148 The Nature of Biological Diversity 



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and Yamada, 1960) . Its role here is not so easy to determine, hut 

 since these cells are deeply involved in supplying metaholites to the 

 adjacent photosensitive structures, it is prohahle that the system has 

 some role in the interconversion of vitamin A and retinene (Porter 

 and Yamada, 1960). Several other examples of the involvement of 

 the smooth ER in lipid metabolism could be cited, but for our pur- 

 poses here it seems more important to point out its association with 

 other activities in other types of cells. 



A striking example of the smooth endoplasmic reticulum is found 

 in cells engaged in the concentration and secretion of a specific ion. 

 Thus one finds unusual developments of smooth elements in oxyntic 

 (acid-secreting) cells of the gastric mucosa in a variety of animals 

 (Sedar, 1961). The concentration of these is especially striking near 

 the free surface of these cells, where chloride ion is presumably 

 being discharged into the gastric lumen. "Presumably" is inserted 

 here because a direct demonstration of this phenomenon at the surface 

 of oxyntic cells is still lacking. For present purposes the observation 

 of special patterns and developments of the smooth ER in these acid- 

 secreting cells is all that is stressed. Sedar speculates that substances 

 normally foreign or injurious to the cytoplasmic matrix could be 

 sequestered in the system prior to secretion. 



Another chloride-secreting cell is encountered in the gills of salt- 

 water fishes as well as in salt-secreting cells of other forms. In a recent 

 study of these cells as encountered in the gill filaments of Fundulus, 

 Philpott ( 1962 ) discovered compact reticula of smooth-surfaced ER. 

 The extraordinary character of these is depicted in Fig. 6. The role 

 of this system in chloride secretion is unfortunately not better under- 

 stood here than in the case of the parietal cell, but the morphological 

 association of the smooth ER with this type of cell is certainly indic- 

 ative of some involvement (Philpott, 1962 a, b) . 



Sequestration of small molecular species may be a function common 

 to smooth forms of the ER. This is suggested in the chloride-secreting 

 cells and also by a form of the smooth ER found in liver cells. In 

 these latter, it is common to find a lattice-work of tubules associated 

 with the large rosette granules of glycogen (Fig. 2). The amount of 

 this in a cell seems to vary from time to time in direct proportion to 

 the amount of glycogen. Structural continuities with the rough ER, 

 which is more nearly constant in amount, are perfectly evident and 

 this suggests that the smooth form may have its origins from the 

 margin or edges of the rough form (Figs. 12 and 13 ) . In its association 

 with glycogen, the development of tubular and vesicular elements is 



