Chemicals Commonly Used in Fixing Sohitions 9 



ability of chromatin in basic dyes it reduces the stainability of the 

 cytoplasm. Fats and other lipids reduce osmium tetroxide, or combine 

 with it to form a dark compound. Thus fat sites become black and in- 

 soluble in absolute alcohol, cedar oil and paraffin; but they remain 

 soluble in xylene, toluene and benzene, and if left too long in xylene 

 or toluene (more than 5 minutes) they become colorless. Osmic acid is 

 not a fixative for carbohydrates. 



When action is complete excess osmic must be ^v'ashed out of the tis- 

 sue or it may be reduced during treatment in alcohol, forming an in- 

 soluble precipitate. Since osmium tetroxide is also easily reduced by 

 light and heat, it must be kept stored in a cool and dark place. 



Picric Acid, C,H.^{NO:)^OH 



Picric acid is usually used in a saturated aqueous solution, 0.9-1.2%. 

 It is an excellent protein coagidant, forming protein picrates having a 

 strong affinity for acid dyes. However, it penetrates slowly, causes ex- 

 treme shrinkage, and offers no protection against subsequent shrink- 

 age. (There is no tendency to swell in this case, because of acidity.) The 

 shrinkage has been foiuid to be close to 50% of the original tissue vol- 

 ume by the time it has inidergone paraffin infiltration. It does not dis- 

 solve lipids, does not fix carbohydrates, but is recommended as a fixative 

 for glycogen. It is a desirable constituent of many fixing fluids because 

 it does not harden, biu it cannot be used alone because of the inifavor- 

 able shrinkage it produces. Acetic acid frequently accompanies it to 

 counteract this poor quality. 



Potassium Dichromate, K.^Cr.^0- 



This substance, often incorrectly called potassium bichromate (which 

 ^\ould be potassium hydrogen chroma te), is a noncoagulant of proteins, 

 making them more basic in action, biu it dissolves nucleoproteins. 

 Chromosomes, therefore, are poorly shown, if at all. Potassiiun dichro- 

 mate leaves tissues soft and in poor condition for paraffin sectioning. 

 One valuable use, however, is the fixation of mitochondria (as with 

 Champy fluid); the lipid coinponents are made capable of resisting so- 

 lution in lipid solvents. After fixation, tissues may be soaked in potas- 

 sium dichromate to insure that lipids are well preserved. 



Potassium dichromate can be mixed with mercuric chloride, picric 

 acid and osmic acid, but it reacts with formalin and must not be mixed 



