Diversity at the Subcellular Level and Its Significance 159 



cytoplasm of all cells (excepting bacteria and certain blue-green 

 algae). This includes a nuclear envelope in the form of a thin, mem- 

 brane-limited vesicle, perforated at numerous points in the surface. 

 Many of the tubular and vesicular elements of the cytoplasm are 

 apparently derivatives of or morphologically continuous with this 

 envelope. These structures, along with mitochondria and other dis- 

 tinctive organelles, are bathed in the cytoplasmic matrix or continuous 

 phase of the cytoplasm, which supports as well such resolvable differ- 

 entiations as filaments or fibrils, and particulates like ribosomes and 

 ferritin. Diversity in the subcellular appearance of the cytoplasm is 

 largely a reflection of the degree to which one or another of these 

 structures dominates the picture. 



Special attention has been directed to the diverse forms adopted 

 by the complex membrane-limited system known as the endoplasmic 

 reticulum. It was emphasized that this newly defined component of 

 cells frequently appears in recognizable patterns in cells — patterns 

 which are constant in differentiated cells of a single tissue type. From 

 evidence available, one may conclude that the system functions in 

 the sequestration and transport of products of synthesis, especially 

 where these are destined for export from the cell. It supports on its 

 surfaces the ribosomes as specific sites of synthesis and one may infer 

 the same surface relationship holds for other enzymic dependent reac- 

 tions. Thus the ER provides a patterned distribution of functions 

 which extends to all parts of the cytoplasm and is integrated with the 

 nucleus, Golgi, mitochondria, myofibrils, and other structures in sup- 

 port of normal, coordinated cell function. Modulations in the appear- 

 ance of the system involve not so much a change of pattern as a change 

 in quantity and surface area and minor relationships to associated cell 

 components. The fluid anatomy of the system is evidenced by these 

 modulations. These various properties of the ER make it an excellent 

 candidate for the role of cytoskeleton which Rudolph Peters ( 1956 ) 



FIG. 14. A hepatoma cell with morphology typical for this transplantable tumor 

 (a rapidly growing hepatoma known as the Dunning). There are many features of 

 fine structure that one expects to see in tumor cells. The surface shows irregular- 

 ities, in this instance like microvilli; the cytoplasmic matrix contains numerous 

 free ribosomes; and the mitochondria (m) are small and abnormal or incom- 

 pletely differentiated. The features of particular interest for this treatise are the 

 extreme abnormalities of the ER. There are no lamellar cisternae such as one finds 

 in the normal. Instead the system consists of irregular vesicles, with particles un- 

 evenly distributed (er). There is evidence of unusual extensions from the nuclear 

 envelope (at arrows). The cell fails to store recognizable glycogen; there is no 

 system resembling the smooth ER of the normal cell. The endoplasmic reticulum 

 in general is highly disordered. (Courtesy of Carlo Bruni.) Magnification: 15.000 X. 



