6 ' Fixation (chap. 1) 



with acetic or trichloracetic acid solutions, transfer the tissues to an al- 

 coholic washing solution in preference to water. 



Acetone, CHJZOCH^ 



Acetone is used only for tissue enzymes, such as phosphatases and 

 lipases. It is used cold and penetrates slowly. Only small pieces of tis- 

 sue wall be fixed in this chemical. 



Chromium Trioxide (chromic acid), CrO^ 



Crystalline chromium trioxide forms chromic acid when added to 

 water, usually a 0.5% solution. It is a valuable fixative, but rarely used 

 alone. It penetrates slowly, hardens moderately, causes some shrinkage, 

 forms vacuoles in the cytoplasm and often leaves the nuclei in abnormal 

 shapes. It is a fine coagulant of nucleoproteins and increases the stain- 

 ability of the nuclei. It oxidizes polysaccharides and converts them into 

 aldehydes — an action forming the basis of Bauer's histochemical test 

 for glycogen and other polysaccharides. Better fixation, however, is ob- 

 tained with acetic acid, which ^vill fix water-soluble polysaccharides; 

 later these can be post-treated with chromic acid. Fat can be made in- 

 soluble in lipid solvents by partial oxidation with chromic acid, but 

 the action can go too far. Potassium dichromate, which reacts in a simi- 

 lar fashion, is safer and is therefore more commonly used. 



Excess chromic acid must be washed out, because later it can be re- 

 duced (undesirably, for our purpose) to green chromic oxide, Cr20o. 

 Formalin and alcohol are reducing agents and must not be mixed with 

 chromic acid until immediately before use. 



Ethyl Alcohol (ethanol), CM:,OH 



Ethyl alcohol hardens tissue but causes serious shrinkage. It is a 

 strong cytoplasmic coagulant, but does not fix chromatin. Nucleic acid 

 is transformed into a soluble precipitate and is lost in subsequent solu- 

 tions and during staining. Alcohol cannot be a fixative for lipids be- 

 cause it does not make them insoluble in lipid solvents. It does not fix 

 carbohydrates, but neither does it extract mucins, glycogen, iron and 

 calcium. Alcohol seldom is used alone; occasionally it is used alone for 

 fixation of enzymes. 



