Diversity at the Subcellular Level and Its Significance 157 



or almost fully differentiated cells. In some instances, the most prom- 

 inent expression of differentiation is to he found in the ER. In other 

 instances this system fades to morphological insignificance as the cell 

 differentiates — as for example, in erythrohlasts (Fig. 10) or in cells of 

 the epidermis (Fig. 8) (the retaining cells of Mercer, 1961). Regard- 

 less of the degree of its eventual involvement in the fine structure of 

 the differentiated cell, it is a constant structural component of the 

 undifferentiated cell, and it is a constant structural component of the 

 undifferentiated unit. It is represented from the beginning of develop- 

 ment by the nuclear envelope, and some evidence of it as a cytoplasmic 

 structure is found as early as the two-cell stage ( Sotelo and Porter, 

 1959). 



Since electron microscope studies of differentiating cells are still 

 rare (see Hay, 1958; Salpeter and Singer. I960; Slautterhack and 

 Fawcett, 1959; Waddington and Perry, 1960; Bellairs, 1959), gen- 

 eralizations are hazardous. One gets the impression, however, that the 

 system exists in embryonic cells as a relatively loose reticulum of 

 tubular and vesicular elements (Fig. 5). Recognizable patterns of 

 organization have so far been described only in the mature function- 

 ing cell. 



Conclusions 



The paper has sought to remind the reader of the wide diversity of 

 forms which may be found at the subcellular level of structure. These 

 ai - e all recognized as variations on a basic architecture found in the 



FIG. 13A. Part of a normal rat liver cell, showing a morphology typical for cells 

 around the central vein of the lobule. The parallel cisternae of the rough ER are 

 closely packed in parallel array and in this form coincide with the discrete baso- 

 philic bodies of these cells. The mitochondria normally retain a peripheral location 

 relative to such clusters. Adjacent areas, rich in glycogen, also show dense develop- 

 ments of the smooth reticulum. (For symbols, see Fig. 1.) Magnification: 18,000 X. 

 FIG. 13B. Area of liver cell similar to that in Fig. 13A, except that the animal from 

 which this tissue was taken had been deprived of approximately 15 cc of whole 

 plasma within the 24-hour period preceding sacrifice. The change in fine structure of 

 the ER reflects changes in the stained image in which, under the conditions of this 

 experiment, the basophilic material appears in less discrete, more diffuse masses. 

 This electron micrograph reveals that the cisternae in these cells are less closely 

 arrayed and show a larger and less uniform intcrcisternal space; most significant 

 of all, in accounting for the change in the light microscope image, the mitochondria 

 are here intermingled with the cisternae. Presumably the ER here is more active 

 in the synthesis of plasma proteins, and the changes in fine structure reflect this 

 fact. Despite the mitochondrial invasion, the general ER pattern, characteristic of 

 rat liver cells, persists. (Symbols as for Fig. 1.) Magnification: 1 8.000 X. 



