48 CYTOLOGICAL TECHNIQUE 



It is an unexplained fact that the conjugated Hpids of the tissues, 

 though usually highly unsaturated, are darkened by osmium 

 tetroxide much more slowly than the mixed triglycerides. 



If suitable antemedia (p. 73) are used, tissues fixed by osmium 

 tetroxide may be embedded in paraffin without loss of unsatur- 

 ated lipids. Benzene and chloroform are particularly suitable 

 antemedia for this purpose.^^ 



If unsaturated lipid that has been blackened by osmium tetrox- 

 ide is bleached by hydrogen peroxide, the lipid is set free and is 

 now soluble once more in lipid-solvents, including benzene and 

 chloroform. ^^ 



After the fixation of tissues by osmium tetroxide, the unfixed 

 (saturated) component of a lipid globule may be dissolved out 

 during dehydration-' ^^^ or embedding. The fixed (unsaturated) 

 lipid is then left in a spherical cavity, which it does not fill. It 

 applies itself to the wall of the cavity, and may be seen in optical 

 section under the microscope as a ring, crescent, or 'cap'. These 

 appearances have often been misunderstood.^^ 



Reaction with carbohydrates. There appears to be no reaction 

 in the ordinary circumstances of fixation. 



Rate of penetration. This is the only fixative, so far as is known, 

 that does not maintain a constant iC-value. It penetrates slowly. 

 The TiC-value during the first 16 hours is 1-0, but during the period 

 16 to 144 hours it is only 0-31. 



Shrinkage or swelling. Gelatine-albumin gel shrinks very 

 slightly. There are no satisfactory figures for tissues and cells, 

 but there seems to be little change of volume. 



Hardening. Leaves tissues rather soft. 



Method of washing out. Osmium tetroxide is usually washed out 

 with running water, to prevent subsequent reduction in the tissues 

 by ethanol. In preparations for electron-microscopy, however, 

 tissues are sometimes transferred directly from the fixative to 

 50% or 70% ethanol. 



Effect on the appearance of cells in microscopical preparations. 

 Preserves the structure of the living cell better than any other 

 primary or mixed fixative. It might almost be supposed that the 

 cell was still alive. 



