370 



PRINCIPLES OF EMBRYOLOGY 



iiito the nuclei if the cells are damaged, or treated with inappropriate 

 histological methods. The main physiological activity of the mito- 

 chondria seems to be the performance of co-ordinated sequences of re- 

 spiratory enzymatic processes. Electron microscope studies show them 

 to have quite an elaborate internal structure, which again consists largely 

 of closely opposed double membranes (Palade 1952). 



Figure 17.2 



Diagram of structures seen in the cytoplasm of mouse pancreas cells (elec- 

 tron-microscope studies of ultra-tliin sections). The circles to the right show 

 higher magnifications. The sausage-shaped bodies are mitochondria, which 

 have an internal structure consisting of double membranes. (From Sjostrand 

 and Hanzon 1954.) 



The other main class of cytoplasmic granules generally found in cells is 

 referred to by the term 'microsomes'. They are typically at or just below 

 the limit of visibihty in the ordinary microscope (diameter usually 60-150 

 mij.) but can be sedimented out of the clear cytoplasm by ultra-centrifuga- 

 tion. They contain little lipid, and are mainly characterised by their rich- 

 ness in RNA. In addition they contain protein, but usually show little 

 enzymatic activity, except that if the cell from which they are isolated is 

 of a kind in which a specific enzyme is found (as trypsin in the pancreas or 

 amylase in the salivary gland) then this enzyme may be demonstrated in 



