Cell Structure and Metabolic Regulation 19 



pancreatic acinar cell, and in the endothelial cell of a blood 

 capillary. In some cases, as in the macrophages, these in- 

 foldings reach deeply into the cell, and it is thought that the 

 probing tip gets broken off, that the membrane reforms around 

 it, and that the fluid which is thus enclosed and which came 

 from the extracellular space is now within the cell, enclosed 

 within a membrane-bounded vesicle. Epstein (1957) thinks 

 that in the Rous sarcoma ascites cell the membranes of such 

 infoldings are catenated with the membranes of the ER and 

 thus the extracellular fluid is continuous with the fluid in the 

 intraluminal spaces ; but this point is still under investigation. 

 However, all workers agree that the invaginations of the cell 

 membrane do exist, and the physiological meaning of this 

 existence is discussed below (p. 20). 



When sections of cells from various tissues are examined 

 with the electron microscope, it is observed, particularly in 

 the liver cell, that many of the mitochondria seem to be 

 enclosed in folds or caps formed by the curving around of some 

 of the membranes of the ER. Only a small distance thus 

 remains between one of the ER membranes and the outer 

 mitochondrial membrane. Bernhard and Rouiller (1956) 

 have made a detailed study of this relationship and believe 

 that the close structural apposition portends a close functional 

 relationship between the mitochondria and the ER. It 

 should be understood that the mitochondria do not sit in the 

 intraluminal cavities of the ER, but lie outside, in the cyto- 

 plasmic matrix. 



As we go inward and reach the nucleus, we come upon 

 another sort of relationship. This link has been amply shown 

 (Watson, 1955; Epstein, 1957; Palade, 1956a) particularly by 

 Watson, who has come to the following generally accepted 

 conclusion: cells from many tissues have nuclei which are 

 bounded by two successive membranes. Upon closer examina- 

 tion it was noted that the inner membrane curves back to 

 merge with the outer one and is continuous with it, and that 

 the inner one is devoid of particles, while the outer one is 

 clearly a membrane of the ER and can be identified as such 



