THE ENDOPLASMIC RETICULUM 129 



mentally induced changes in physiological function. If, as now seems 

 possible, these obseryations can be tied to biochemical obseryations on 

 fractions of cells identified as coming from this or that part of the ER, we 

 haye a good start on the oyer-all analysis of the role of the ER in the 

 intact unit. Needless to say, \ye are still yery ignorant about the reticulum. 

 Ho\ve^■er, some correlations of function and structure are being achieved 

 and with the possibilities provided by modern techniques (as yet un- 

 explored) the progress toward greater understanding should be rapid. I 

 am encouraged to believe that even the suggestive evidence provided by 

 observations on morphological associations of structural elements invohed 

 in different cell functions will be of interest to biochemists who are more 

 experienced in cell fractionation procedures and their potentials. 



Forms and modulations of ER 



The cytoplasmic part of this membranous system, as distinct from that 

 part comprising the nuclear envelope, appears in a wide variety of forms. 

 These are readily recognized as characteristic of particular cell types 

 whether found in one or different species of animal. It is apparent, more- 

 over, that cells performing certain similar functions, whether otherwise 

 related, show similarities in the predominant form of the reticulum in 

 their cytoplasms. 



In some cells the system is represented in large part by only one of 

 these differentiations or configurations. In other instances two or more 

 forms may be prominently represented in a single cell and obviously 

 linked into a single system bv morphological continuity. There are also 

 cells in which a predominant form of the ER shows local specializations 

 or difierentiations, designed, presumably, to perform a particular function. 

 The system under consideration is therefore one of great variability, 

 possessing infinite possibilities to accommodate or control subtle func- 

 tional activities of the cell. 



These \arious patterns or designs displayed by the system are of 

 course not fixed or rigid, but modulate within limits as the cell performs 

 its functions or responds to environmental stimuli. The pattern most 

 regularly encountered in microscopic examination of any cell type is 

 regarded or generally accepted as representing the preferred form of the 

 system. 



The granular or rough form 



The picture of the endoplasmic reticulum that is most familiar to bio- 

 chemists consists of arrays of parallel lines encrusted with small dense 

 particles (Figs, i and 4). In actual fact, of course, the lines represent 

 membranes which limit relatively large lamellar vesicles and it is charac- 



K 



