FIXATION PROCEDURES 



17 



be affected to any serious degree. The choice of fixing agent has 

 to be made with the careful consideration of its effect upon the 

 group which is to be studied. It commonly happens, therefore, 

 that what to the cytologist has been the best type of fixative 

 is not available to the cytochemist. The common ingredients 

 of fixatives include strong oxidizing agents, such as osmic acid 

 and chromic acid ; strong reducing agents, such as formaldehyde 

 and formic acid; cross-linking substances, such as formaldehyde 

 and osmic acid; and acids, bases, and heavy metals which can 

 precipitate such substances as nucleic acids and proteins. In 

 addition, anhydrous fluids such as alcohol and acetic acid are 

 quite commonly used; they exercise their effect very largely as 

 denaturing agents. The anhydrous solvents cause denaturation 

 mainly by breaking up the lipoid adhesions which assist in 

 maintaining the native state in many proteins. Table I sets out 



TABLE I 



The action of various chemical reagents commonly employed as com- 

 ponents of fixatives upon chemical groups in tissue sections, -f- indicates 

 that a group is unaltered by a reagent. - indicates that it is unsafe to 

 use this component if it is desired to keep a particular group intact. 



a list of fixing agents and indicates their action upon various 

 groups which may be of interest to the cytologist. It will be 

 seen that there are many of the ingredients of the better chem- 

 ical fixatives which commonly should be avoided. For example, 

 if a study is to be made of NH 2 groups or of phenols such as 

 tyrosine, acetic acid in anhydrous solution must be avoided be- 

 cause of the danger of acetylation of the groups concerned. If 



