The Endoplasmic Reticulum: Some Current 

 Interpretations of Its Forms and Functions 



Keith R. Porter 

 The Rockefeller Institute, Neiv York, U.S.A. 



Among the several discoveries in cell morphology which must be 

 credited to electron microscopy, one of the most significant is that which 

 has described the cvtoplasmic ground substance as extraordinarily rich in 

 membrane limited elements. The presence of these in such amounts and 

 in such complicated forms was not really suspected from light microscopy. 

 Their presence in the ground substance was first recognized in the electron 

 microscope image of cultured cells about 15 years ago, though it must be 

 admitted that the early images were not very convincing [i]. Subsequently, 

 and with increasing vigour, they have been studied in a wide variety of 

 animal cells and more recently in plant cells until, at the present time, they 

 are looked for and recognized as a standard element of cell fine structure. 

 The unit of structure — if such can be defined — is vesicular or tubular in 

 thin section profiles; i.e. a membrane-enclosed cavity or vesicle, itself 

 usually devoid of any internal structure. When followed through serial 

 sections into the depth dimension, these profiles can be seen to represent 

 two-dimensional images of structures which are really part ot a complex 

 three-dimensional system of tubules and vesicles extending into most 

 regions of the cytoplasm. This statement holds true for the majority if not 

 all cell types. 



It is also now recognized and accepted as a law of fine structure that 

 the nuclear envelope is part of this general system. This conclusion is 

 supported by the structural similarity evident between the cytoplasmic 

 elements of the system and the nuclear envelope as well as by the im- 

 pressive continuity between the envelope and the cytoplasmic units [2, 3]. 

 The envelope is essentially a large flattened or lamellar vesicle wrapped 

 around the nucleus. It is reasonable, therefore, to regard the nuclear 

 envelope and the cvtoplasmic formations as part of a single unit system of 

 the cell, in the same sense that we regard the mitochondria or plastids as 

 representing unit systems. The nuclear envelope is the most constantly 

 occurring part of the svstem, being found in all forms from yeast cells on 

 up the scale of plant and animal species. It is conceptually useful and 



