12 Fixation (chap. 1) 



solution. Keep it moist with normal saline and wash off accumulated 

 blood with normal saline. Blood retards the penetration of a fixative. 

 Gently shake the solution and contained tissues several times to make 

 certain that the fluid can reach all surfaces and that pieces are not 

 sticking to the bottom or sides of the container. A chunk of glass wool 

 may be laid in the bottle to aid in keeping the tissue free of the bottom. 



Thin pieces of tissue that show muscular contraction or that may 

 turn inside out (tissues of the gastrointestinal tract are particularly 

 likely to do this) may be placed on thick filter paper, outside wall against 

 the paper, and dropped in the fixative. Tiny, easily lost specimens, bi- 

 opsies, bone marrow, etc., may be wrapped in lens paper or coffee filter 

 paper (page 228). 



The length of time required for complete fixation depends on the 

 rate of penetration and action of the fixative. Most coagulant fixatives 

 produce complete fixation as fast as they can penetrate the tissue. But 

 some fixatives, such as formalin, exhibit progressive improvement in 

 fixing action after the tissue has become completely penetrated. Pro- 

 longed action in this case improves the condition of the tissue and 

 rarely is harmful. Occasionally some type of post-fixation treatment as 

 noted on page 23 is advisable. 



Washing the Tissue 



After fixation is accomplished, the excess of many fixatives must be 

 washed out of the tissue to prevent interference with subsequent proc- 

 esses. Often washing is done with riuining water; sometimes the tissue 

 may be carried directly to 50% alcohol or higher. Some technicians 

 maintain, for instance, that Bouin's tissue washed in water may lose 

 some of the soluble picrates and that this tissue should be transferred 

 from the fixative directly to 50% alcohol. When a freezing step is 

 planned, formalin-fixed tissue may be washed a brief moment in water, 

 but in rush problems it can be taken from the fixing solution directly 

 to the freezing microtome. The presence of alcohol will prevent freez- 

 ing action and its use must be avoided, or it must be thoroughly ^vashed 

 out before attempting to freeze the tissue. 



When tissue has been fixed with a mercuric chloride solution addi- 

 tional treatment is necessary. After washing in 50% alcohol, transfer 

 the tissue to 70% alcohol containing enough iodine-alcohol (saturated 

 solution of iodine in 70% alcohol) to give the solution a deep brown 



