THE ENDOPLASMIC RETICULUM 1 37 



some significance. The story comes from recent investigations of Palay 

 and Karlin [30]. These authors, in following the uptake of fat by epithelial 

 cells of the intestinal villus of the rat, observed that small fat droplets 

 ( ~ 50 m/.t in diameter), which appeared to traverse the cortex of the cells 

 in pinocytotic vesicles, reappeared in the cavities of the ER. Thev were 

 then transported within the channels of the system to the lateral margins 

 of the cell and thence, presumably through intermittently established 

 openings, into the inter-cellular spaces. All compartments of the ER 

 including the nuclear envelope were seen to contain droplets of fat within 

 30 min. after intragastric instillation of corn oil [31]. Besides providing 

 evidence of transport within the ER, these observations are descriptive of 

 continuity within the system. The authors express the opinion that the fat 

 particles which appear in the cisternae of the ER are transported directly 

 there by pinocytotic vesicles. It follows from this opinion that the passage 

 of fat through the epithelial cell is to all intents and purposes an extra- 

 cellular phenomena. 



This interpretation of e\ents in fat absorption is introduced into this 

 discussion because it bears on the question of the relation of the ER to 

 the plasma membrane and the cell surface, and also because there is an 

 alternate interpretation which seems to more nearly fit the observations 

 and related information. According to the Palay-Karlin interpretation, 

 materials taken up in small bits or sips (quanta) at the cell surface are 

 discharged helter-skelter into the lumina of the ER. This means that a 

 space designed to sequester the products of synthesis mav also receive all 

 manner of metabolites and non-metabolites from the environment. These 

 concepts have grown out of observations on pinocvtic activity at the cell 

 surface [6] and some e\idence of occasional and probablv intermittent 

 continuity between membranes of the ER and the cell surface [32]. They 

 remain, however, as concepts and much of the evidence a\ailable on 

 uptake of particulate material visible with the E.AI. fails to support them 

 [33, 34]. As a rule, a barrier of two membranes (plasma and ER) and a 

 layer of cytoplasmic matrix always lies between the extracellular space and 

 the cavities of the ER on the uptake side. Even the Palav Karlin observa- 

 tions may be interpreted to fit this principle. Thev admit in their report 

 that definite exidence of continuity between pinocytotic vesicles and the 

 ER is not available and that furthermore it is difficult, through the apparent 

 pinocytotic actix ity, to account for the rapid fat absorption. There is good 

 evidence, moreover, that long-chain fatty acids proxided in dietary 

 triglycerides are rearranged in the fats in the chylomicrons on the other 

 side of the epithelium [35]. These points support an alternate interpreta- 

 tion that only the products of triglyceride hydrolysis pass the plasma 

 membrane and enter the cytoplasmic matrix ; that these are reassembled 

 at the membrane limiting the ER and resynthesized into fats which are 



