Fig. 4. Section through a microsome pellet isolated by differential centrifugation from a 

 pancreatic brei (guinea pig) prepared in 0.88 M sucrose. 



The microsomes are small, closed vesicles limited by a thin membrane which bears small 

 (~150 A), dense particles attached to its outer surface. The apparent heterogeneity of the 

 microsomal fraction is due primarily to sectioning. Some vesicles are seen in median sec- 

 tion {mv x ) and therefore clearly display their normally sectioned membrane and their 

 attached particles. Other vesicles are cut medially (mv z ) and show obliquely sectioned, 

 poorly defined membranes. Finally, in lateral sections (mv s ), the cavity of the microsomes 

 cannot be seen, and their particle-studded membrane appears in full-face view. 



The structural details described indicate that the microsomes are derived (by fragmenta- 

 tion) from the rough-surfaced part of the endoplasmic reticulum. A comparison with 

 figure 2 suggests that the fragmentation occurs spontaneously when the cell membrane 

 is ruptured during tissue grinding. 



Note that the microsomal content varies widely in density from light (rhv ± ) to medium 

 (raz/ 4 ) and high (mi/ 5 ). A ruptured microsomal vesicle (mv G ) contains the equivalent of 

 an intracisternal granule. 



Fixation: 2 hours at 0° C in 2 per cent Os0 4 in 30 per cent (0.88 M) sucrose. 



Embedding: n butyl methacrylate. 



Magnification: 72,000. 



