16 CYTOLOGICAL TECHNIQUE 



moulds; and will render the autolytic enzymes inactive. Such a 

 fluid is a preservative. A fixative must do everything that a pre- 

 servative does, but in addition it must modify the tissues in such 

 a way that they become capable of resisting subsequent treat- 

 ments of various kinds. Of these treatments the ones that are 

 most likely to cause damage are embedding, sectioning, and 

 mounting (though the latter is not damaging if the tissue has 

 already been severely shrunken while being embedded). Apart 

 from the partial protection it gives against damage from these 

 processes, fixation usually makes many tissue-constituents 

 (especially chromatin) readily colourable by suitable dyes. 



Not every tissue-constituent requires fixation, and some are 

 not capable of being fixed. Chitin, cellulose, starch-grains, 

 scleroproteins, amorphous silica, and certain inorganic crystals 

 are examples of hard, stable substances, scarcely or not at all 

 subject to distortion or decay. Other substances, such as the 

 soluble sugars, cannot be retained in their natural sites in the 

 tissues by any fixative. Glycogen is the only soluble carbohydrate 

 that is at all frequently retained in finished microscopical 

 preparations. 



Though usually soft or indeed liquid, triglycerides do not re- 

 quire fixation if lipid-solvents are avoided in subsequent treat- 

 ment. Some fixatives have no effect on lipids, but this does not 

 by any means make them useless, for neither the cell nor inter- 

 cellular material is held together by lipids. If, however, the com- 

 mon proteins are not stabilized, tissues and cells fall to pieces. 

 The essential function of fixation is the stabilization of the 

 protein framework of the cell. 



Fixation can be achieved by heat, but this method is seldom 

 used except for blood-films. It tends to cause distortion, but 

 presents the advantage that nothing is dissolved out of the cell. 



Most fixatives are solids used in aqueous solution, but some 

 are organic liquids that can be used without dilution. The number 

 of chemical compounds that are useful as fixatives is very small. 

 The seven unmixed or 'primary' fixatives chosen for mention in 

 this book are listed below. All fixatives fall easily into two groups, 

 according to their obvious reactions with soluble proteins. Some 



