THE LOCALIZATION OF LIPIDS 

 IN CYTOPLASM 



IT HAS BEEN obvious for somc time that the usual histological methods for 

 the detection of intracellular fat are as a rule successful only in displaying 

 the fats present in the form of discrete globules and leave a large portion of the 

 lipid content undetected. It is even true that cellular organs which display 

 no visible fat may yield a considerable quantity to chemical methods of 

 analysis. This fact has been recently emphasized by the work of Grafllin,^ 

 and of Marble, Grafflin, and Smith/ who made coordinated histological and 

 chemical investigations on the fat content of the guinea-pig liver. 



The animals used by these observers were kept in a well-fed condition by 

 having food always available, and showed a substantial content of lipids 

 (1.94-3.16 per cent of wet weight equivalent to 6.5-10.5 per cent dry weight), 

 yet the microscopically demonstrable fat was small in amount and irregular in 

 distribution. The majority of hepatic cells were devoid of fat. 



Pathologists will recall that Rosenfeld* showed long ago that the fat con- 

 tent of kidneys containing little or no visible fat yielded on chemical analysis 

 as much as, or even more than, those in an advanced state of fatty change. 



Notwithstanding these facts and the warning of Leathes and Raper* that a 

 clear solution might yet contain a considerable amount of fat, histologists 

 have continued to accord to histological methods an undeserved measure of 

 confidence for the detection and localization of lipids. Among the latter the 

 phospholipids have particularly captured the interest of investigators and, 

 one after the other, the visible components of protoplasm, such as mito- 

 chondria, Golgi apparatus, secretion granules, etc., have been identified as 

 phospholipid in nature. 



Experimental cytologists, on the other hand, have paid little attention to 

 these pseudomicrochemical studies and have in general assumed that the 

 phospholipids were either dispersed in the cytoplasm or disposed on cellular 

 or intracellular surfaces. Since the publication of the Overton theory of the 

 lipoid nature of the semipermeable membranes this point of view has been 

 increasingly popular. The studies of Gorter and Grendel,^ who determined 

 that the amount of lipid extractible from erythrocytes was just sufficient to 

 form a bimolecular layer of lecithin on the cell surface; the observations of 

 Mudd and Mudd' on the ready entrance of erythrocytes into an oil layer with 

 which they were brought into contact; and those of Chambers' on the pene- 

 tration of oil droplets into marine eggs denuded of their ordinary coverings 

 and on the repair of broken cell surfaces— all support the idea that the surface 

 of the cell either consists of or contains lipids. 



The discovery by Bensley and Hoerr^ that mitochondria of the hepatic cell 

 could be separated from other cell components for analysis by diflcrcntial 



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