Chapter J_ 



Fixation 



When a tissue is removed from a living condition, several changes are 

 initiated in its cells. Bacteria begin to mtiltiply and destroy them. Also 

 autolysis (self-digestion), a lysis or dissolution of the cells by contained 

 enzymes, sets in. The enzymes appear to reverse their action; instead of 

 synthesizing amino acids into cell proteins, they begin to split proteins 

 into amino acids. These amino acids diffuse out of the cells, and as a re- 

 sult proteins no longer are coagulable by chemical reagents. The above 

 changes lead to so-called post-mortem conditions. For purposes of lab- 

 oratory examination it is necessary to treat the tissue to prevent these 

 post-mortem effects; it is also necessary to convert the cell parts into 

 materials that will remain insoluble during subsequent treatment, and 

 to protect the cells from distortion and shrinkage when subjected to 

 fluids such as alcohols and hot paraffin. Further important objectives 

 of tissue preparation are to improve the staining potential of tissue 

 parts and to change their refractive indices toward improved visibility. 

 The procedure used to meet the above requirements is called fixation 

 and the fluids are called fixatives or fixing solutions. Fixing solutions 

 should meet the following principal objectives: 



1. Penetrate rapidly to prevent post-mortem changes. 



2. Coagulate cell contents into insoluble substances. 



.S. Protect tissue against shrinkage and distortion during dehydration, 



embedding and sectionino. 

 4. Allow cell parts to become selectively and clearly visible by means 



of dyes and improved refractive indices. 



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