128 KEITH R. PORTER 



possibly valid to think of the cytoplasmic part of the system as an outgrowth 

 of the envelope. 



This is the system that has come to be known as the endoplasmic 

 reticulum, or for short, the ER [4]. In one structural expression or pattern 

 it adopts, there are many lamellar sacs or cisternae in parallel array and 

 this form corresponds to the cytoplasmic component, referred to for 60 

 years as the ergastoplasm [5]. But this is only one of many complex 

 organizations that are encountered and which are characteristic of different 

 cell types. In some other cells the system is represented in large part by a 

 tri-dimensional latticework of tubules. In cells of the early animal embryo 

 or the plant meristem, before differentiation is completed, the ER is 

 usually simple compared with the patterns and complexities achieved in 

 the completely differentiated unit. This suggests that it is designed to play 

 an important role in the particular activity of the differentiated cell, 

 whether this is secretion, contraction or impulse conduction. 



These and other observations make it evident, then, that we are dealing 

 with a complex, finely-divided vacuolar system which ramifies and extends 

 to all parts of the cytosome. This effectively creates within the cytoplasm 

 an internal phase which is separated from the continuous matrix phase of 

 the cytoplasm by a membrane. These facts of morphology, as depicted by 

 electron microscopy and supported by phase contrast observations on 

 living cells, have been the basis for diverse speculations regarding the 

 function of the system. It has, e.g., been suggested that the system satisfies 

 a need in larger cells for channelled diffusion and segregation of metabolites. 

 Its peculiar morphological relation to the myofibrils in striated muscle has 

 prompted the proposal that its limiting membrane is electrically polarized 

 and possibly capable of transmitting intracellular impulses. The large 

 surfaces provided by the system have been recognized as suitable for the 

 support and patterned disposition of enzymes within the cytoplasm. 



This information and speculation regarding the ER is now fairly well 

 known and probably needs no further comment. It can be found in a 

 number of recent reviews with ample illustration of the more important 

 points [5, 6, 7]. The system described has been an object of interest and 

 study in our laboratory for a number of years. Large blocks of what we 

 now know about the system have been discovered and described by 

 G. E. Palade and P. Siekevitz, and I am particularly grateful to them for 

 valuable discussions and permission to republish a few of their micrographs. 



It is the purpose of this review to present a morphological background 

 against which the subsequent biochemical papers on this programme may 

 be presented. I shall focus your attention principally on functions of the 

 ER — functions that are suggested or defined by (a) morphological varia- 

 tions observed in cells with similar and different functions (comparative 

 cytology), and {b) modulations related to naturally occurring or experi- 



