the lysosomes and the mitochondria; however, their precise morphologi- 

 cal identification remains to be determined. 



ENDOPLASMIC RETICULUM 



The first electron micrographs of cells were published in 1945 by 

 Porter, Claude, and Fullam, who noted the presence of a network or 

 reticulum of strands and associated vesicle-like bodies in the cytoplasm 

 of thinly spread tissue culture cells. Further electron microscopy obser- 

 vations of Porter and Thompson (1947) on the same kind of material 

 indicated that the strands were actually vesicular bodies interconnected 

 to form a complex network which was confined, in most instances, to 

 the inner or endoplasmic portion of the cytoplasm. This network, on the 

 basis of its reticular form and endoplasmic location, was termed the 

 endoplasmic reticulum. Some five years later, electron microscopy studies 

 of thin sections of cells generally confirmed the observations of Porter 

 and his colleagues. Since that time improvements in fixation, embedding, 

 and sectioning have led not only to further confirmation, but also to a 

 generally accepted concept of a structural organization of the cytoplasm 

 (Palade, 1956; Porter, 1957). Recently, Fawcett and Ito (1958) and 

 Rose and Pomerat (1960) have studied the form and distribution of the 

 endoplasmic reticulum in living tissue culture cells by phase-contrast 

 microscopy (Figure 3-21). 



Despite variations in patterns and differences in opinion concerning 

 details, the following description of the endoplasmic reticulum appears to 

 be reasonably valid. In almost all cells, the cytoplasm is traversed by a 

 three-dimensional network of cavities bounded by a system of membranes, 

 these cavities occurring either as vesicles or tubules. This is more extensive 

 than Porter's original observations suggested. This membrane system is in- 

 terconnected to produce a more or less continuous network which sepa- 

 rates two distinct phases in the cytoplasm. One phase is represented by the 

 material inside the cavities of the network, the other by the cytoplasmic 

 matrix surrounding the elements of the reticulum. In many types of 

 cells, especially those concerned with protein synthesis, small, electron- 

 dense granules are found associated with the outer (matrix) surface of 

 the membranes bounding the cavities of the endoplasmic reticulum. 

 Because these granules were first described in detail by Palade (1955) 

 they are frequently referred to as the "small granules of Palade," though 

 a more common term now is "ribosome." In size, these granules measure 

 about 100 A, occasionally occur freely dispersed in the cytoplasm, and 

 are reported to have a high content of ribonucleoprotein. Those ele- 



STRUCTURE AND FUNCTION OF CYTOPLASMIC ORGANELLES / 49 



