Diversity at the Subcellular Level and Its Significance 147 



of parallel close array. Thus such diversity of pattern as is displayed 

 by this type of ER is in amount and degree of association rather than 

 in the form of individual elements. 



The function of this rough form has been more thoroughly explored 

 than any other form of the ER. As is well known, the combined 

 evidence from cytochemistry, comparative cytology, histochemistry, 

 and cytophotometry links the system to protein synthesis. This evi- 

 dence has been so frequently reviewed that it needs no recital here 

 (see, for example, Lotfield, 1957; Siekevitz and Palade, 1960 a, hi. 

 The important aspect of it to note for present purposes is that protein 

 is synthesized in the particle or ribosome and is thence moved through 

 the supporting membrane and segregated, or sequestered from the 

 matrix of the cytoplasm to be transported toward export mechanisms 

 represented usually by the Golgi component I Palade 1956b; Caro, 

 1961). Thus studies on protein synthesis have established functions 

 for the membrane and cavity of the ER. functions of segregation and 

 transport which might reasonably be extended to other forms of the 

 system. 



The smooth or agi'tinuluv Eit 



As in the case of the rough ER, this represents a basic form of the 

 system rather than a distinctive pattern for any cell type. Also, as in 

 the case of the particle-studded ER. the smooth appears more fre- 

 quently in one form, that of slender tubules (50 to 100 millimicrons 

 in diameter) arranged in a three-dimensional lattice. Various de- 

 partures from this can be found, but the unit elements remain tubular 

 or small vesicular (Fawcett. 1955; Porter and Bruni, 1959; Porter and 

 \amada, 1960; Christensen and Fawcett, 1961 1 . 



The distribution of this type of ER among cells is less constant 

 in its association with any single function than is the case in the 

 rough form. Thus it is not possible to say that the smooth ER is de- 

 signed for steroid synthesis as opposed to protein synthesis for the 

 rough ER, although it is customary for steroid-producing cells to 

 show this type of ER. Christensen and Fawcett ( 1961 ) , for example, 

 have described the interstitial cells of the oppossum testis as showing 

 only a smooth ER, in a close lattice. This they relate, by virtue of its 

 occurrence in other steroid producers, as common to cells engaged in 

 this activity. Biochemical studies by Lynn and Brown (1958) point 

 in the same direction. The inference is that enzymes active in steroid 

 synthesis are associated with the microsomal membranes. A very simi- 

 lar configuration is found in the pigment cells of the retina I Porter 



