66 SUBCELLULAR PARTICLES 



Under normal feeding conditions, the exocrine cells contain none or very few 

 lipid inclusions and their elongated mitochondria appear distributed at random 

 throughout the cytoplasm without any preferred relationship with the other cell 

 components (cf. 40). After a fast of 48 hours, however, most cells contain large 

 lipid inclusions which appear to be in close relationship with one or more mito- 

 chondria (29). Usually an incomplete mitochondrial ring is found around a lipid 

 droplet (fig. i),^ but frequently the ring is complete (fig. 2), and occasionally com- 

 plex conglomerates occur in which more than one lipid droplet and more than one 

 mitochondrion are involved. If the fast is prolonged for 3 to 4 days, the number 

 of lipid inclusions with associated mitochondria increases noticeably. Refeeding 

 slowly brings back the original disposition of the mitochondria, apparently by the 

 gradual disappearance of the associated lipid droplets. 



By comparison with the topographical changes just described, structural modifi- 

 cations incurred by the mitochondria are less striking. Sometimes the organelles 

 in contact with lipid inclusions increase in size, take bizarre forms (lig. 3), and 

 lose the regular arrangement of their cristae, but many retain a normal appear- 

 ance. Changes of probably greater significance occur at the periphery of the mito- 

 chondrion along the zone of contact with the adjacent lipid inclusion. In some 

 cases there is no resolvable space left between the mitochondrion and the inclu- 

 sion, the lipid mass abutting directly against the outer mitochondrial membrane. 

 In other cases, the outer membrane is no longer visible and the lipid extends 

 across the outer mitochondrial chamber up to the inner mitochondrial membrane. 

 Finally, the periphery of the lipid droplet usually differs in density and texture 

 from the rest of the inclusion and appears as a denser shell with punctate deposits 

 of high density scattered throughout it. 



What we have in this close association of lipid inclusions and mitochondria is, 

 in all probability, an expression of the fact that the fasting animal is obliged to 

 oxidize its reserve fat to cover the energy requirements of its important cell types, 

 i.e., the cells that constitute its 'vital organs.' Indeed, a comparable association is 

 encountered under similar conditions in many other cells, especially in the muscle 

 fibers of the heart and of the diaphragm (29) and, to a lesser extent, in the pa- 

 renchymal cells of the liver and in the nephron epithelium of the kidney. As ex- 

 pected, the animals lose weight during the fasting period and their adipose tissue 

 dwindles. In capillaries throughout the body the blood plasma appears loaded 

 with small (^-'50 m/x) lipid droplets comparable to the chylomicrons found in 

 the circulating blood during intestinal fat absorbtion. Such observations suggest 

 that at least part of the mobilized fat is transported in relatively large droplets 

 from the adipose tissue to the sites of oxidation. 



1 All figures are electron microjiraphs of thin sections of tissue fixed in buffered osmium 

 tetroxide (pH 7.6) and cmbeddeil in w butyl methacrylate. For general preparatory techniques, 

 see references 24 and 28. 



