184 



J. D. LEVER 



Fig. 4. In this field a vesicular component can be seen at (A) 

 and is partially masked by the osmiophile material in the 

 Jipid droplet. Magnification 47,000. 



Figs. 5-6. Fig. 5 is a small representative field of a normal 

 brown fat (control) cell while fig. 6 is a comparable field 

 from the same animal after 7 days starvation. There is a 

 marked increase in the number of mitochondria per unit 

 area during starvation (see text). Both figs. 5 and 6, magni- 

 fication 24,000. 



(figs. I and 2). Between the internal membranes is a 

 semiopaque matrix material and should the plane 

 of section be intercristal the entire mitochondrial 

 outline is then occupied by this matrix. 



In addition to obvious lipid droplets and mito- 

 chondria, other bodies of an intermediate appearance 

 are frequently found in proximity to the lipid drop- 

 lets. There is evidence of an enclosing membrane to 

 some but not all of these bodies, which resemble 

 mitochondria in size and shape and contain intensely 

 osmiophile internal vesicles (fig. 2). Fat droplets are 

 unconfined by a membrane and lie freely within the 

 cytoplasm (fig. 1). They are often seen to contain 

 vesicular elements (fig. 4) resembling those already 

 described within the "intermediate" bodies. 



Changes in starvation and after adrenalectomy: The 

 reaction of brown fat is similar in both these circum- 

 stances but the observed changes are more profound 

 and of more rapid onset in starvation. These changes 

 are: {a) A reduction in total lipid within the tissue 

 and a concurrent diminution in droplet size as 

 judged by light and electron microscopy. After 7 

 days starvation and in 21-28 days after adrenalec- 

 tomy, there is: (/>) a reduction in cell volume (esti- 

 mated with the light microscope by direct measure- 

 ments of cross-sectional cell diameters) of from 25- 



50 



and (c) a two-fold (approximately) increase 



over normal in the number of mitochondria per unit 

 area of fat cell (figs. 5 and 6). This last estimate was 

 based on direct counts in electron micrographs of 

 similar magnification. 



Discussion. — It is of considerable significance that 

 the size range, as measured by the light microscope, 

 of \-\ /I for the phospholipid-positive bodies and 

 fuchsinophile bodies (mitochondria) and the smallest 

 sudanophile droplets, should be comparable to mito- 

 chondrial average size (0.86 /n d.) as estimated from 

 electron micrographs. 



It is conceivable that the beading (figs. 1 and 2) of 

 mitochondrial cristae in brown fat cells may precede 

 Iheir fragmentation into filaments or vesicles. This 



would account for the mixture of these two types of 

 internal membranes in many mitochondria. It has 

 been suggested that lipid appears within the mitochon- 

 dria of the adrenal cortex (6), the corpus luteum (7) 

 and possibly the parathyroid (9). From the evidence 

 of the present investigation it is considered probable 

 that mitochondrial internal membranes in the brown 

 fat cell become modified from a predominantly 

 crystal to a vesicular form. The appearance of lipid 

 within the mitochondria is concurrent with this 

 modification. 



As already stated the lipid droplets within brown 

 adipose tissue lie freely within the cytoplasm. There- 

 fore if, in fact, lipid does appear within the mito- 

 chondria then these bodies either discharge it freely 

 into the cytoplasm through breaches in their limiting 

 membranes or these membranes disintegrate around 

 a contained droplet. Deficiencies in mitochondrial 

 enclosing membranes have previously been reported 

 (8) in the rat and hamster adrenal cortices. The 

 series of events within mitochondria that has been 

 described results in considerable alteration of their 

 fine structure and, in all probability, in their complete 

 destruction. An expendibility of mitochondria such 

 as is postulated has already been described in the 

 adrenal cortex (8). The observed increase in the 

 number of mitochondria during starvation and 

 following adrenalectomy must be viewed in the light 

 of a concurrent cell shrinkage. In ordinary white 

 adipose cells the cytoplasm and nucleus are displaced 

 peripherally by the mass of fat and the crescentic 

 shape of the nucleus suggests some degree of com- 

 pression. In brown adipose tissue, if it be assumed 

 that owing to the presence of intercellular fat droplets 

 the other cell constituents are displaced or "com- 

 pressed"; then, if most of this fat material is released 

 from the cell, as occurs for example during starva- 

 tion, the other cell constituents will become less 

 "compressed". The explanation does not take into 

 account the elasticity of the plasma membrane nor 

 does it allow for any other change of state within 



