132 KEITH R. PORTER 



chromophilic material of the liver cell cytoplasm (1943). A definitive 

 demonstration of the intracellular origin of the microsome v^'as not, how- 

 ever, provided until about 12 years later when Palade and Siekevitz [14] 

 published micrographs of sections through the pellet representing the 



Fig. 2. Micrograph of thin section of microsome pellet from rat liver homo- 

 genization and fractionation. The long slender profiles are readily identified as 

 fragments of intracellular, particle-studded (rough) cisternae of the endoplasmic 

 reticulum (see Fig. 4 for comparison of rough ER in intact cell). There are also in 

 the field (at arrows) a few profiles of vesicles without particles (smooth or agranular). 

 These are thought to be derived from the smooth form of the reticulum observed 

 in liver cells. Mag. x 70 000. (Micrograph courtesy of G. E. Palade.) 



microsome fraction. These pictures showed profiles ot particle-studded 

 units which by their morphology could be unmistakably identified as 

 fragments or pieces of the endoplasmic reticulum of the intact rat liver 

 cell (Fig. 2). Thus the microsome became part of a well characterized 



