144 The Nature of Biological Diversity 



It should be evident from this brief review of diversity in cyto- 

 plasmic structure that certain components recur, and that they appear 

 as variants of a basic form. One can, for example, find in almost all 

 cells some evidence of fibrous structures in the continuous phase of 

 the cytoplasm; ribosomes of nearly uniform size are present to greater 

 or less extent in most instances; and finally, the cytoplasm shows mul- 

 tiple expressions of membrane-limited tubules and vesicles which 

 seem to comprise a single system. In what follows this will be defined 

 as a unit system of the cytoplasm and considered relative to its struc- 

 tural and functional diversity. There are certainly other components 

 which could be used for the same purpose but which as yet have not 

 been so completely analyzed for variations, occurrence, and function. 

 The majority recur in all cell types and will of course come eventually 

 to be understood. These include structures which are currently known 

 as microbodies, lysosomes, multivesicular bodies, Golgi, and associated 

 microvesicles. 



The miscellaneous profiles of tubules and vesicles, which frequently 

 appear to clutter up the cytoplasm, have been shown to represent 

 parts of a continuous or intermittently continuous system, commonly 

 referred to as the endoplasmic reticulum, or ER (Porter and Thomp- 

 son, 1948; Porter, 1953). As the technics of electron microscopy have 

 improved over the past ten years, the image of this system has grad- 

 ually emerged from the optically empty ground substance of the cell, 

 and it is now recognized as a commonly occurring component of the 

 cytoplasm (Hagenau, 1958; Palade and Porter, 1954; Palade, 1955; 

 Porter, 1961a). 



The unit structure of this system can be thought of as a vesicular 

 or tubular element which appears in a wide range of forms and dimen- 

 sions. It is a membrane-enclosed space, usually showing no evidence 

 of internal structure. When these component elements, which appear 

 in thin sections as line or membrane-limited profiles, are followed 

 through serial sections into the depth dimension, they are found to 

 be, in most instances, part of a complex tridimensional reticulum 

 which extends into all parts of the cell. 



A further generalization which emerges from electron microscopy 

 of many cell types is that the nuclear envelope is morphologically 

 similar to the unit elements of the ER (Figs. 2, 3, 5), and is found 

 to be structurally continuous with the system at a number of points 

 (Watson, 1955). It is proper, therefore, to include the envelope and 

 the cytoplasmic tubules and vesicles in a single system, the endoplas- 

 mic reticulum, or ER. In support of these statements we note that 

 the envelope consists of two membranes and an intervening space. 



