26 FIXATION 



confidence, but high claims have been made for the faithful 

 stabilization of cellular structure by f reezing- substitution. ^^^ The 

 necessity to use a non-aqueous fixative limits choice severely. 



A piece of tissue may be suddenly chilled and the ice completely 

 evaporated off from it while the low temperature is maintained. 

 This process of 'freezing-drying' is an old one.^ It is often 

 carried out in a very elaborate way, but quite simple apparatus is 

 nowadays available.^^^ After warming to room-temperature the 

 dry tissue may be fixed as desired. The main advantage of the 

 method, however, is that the tissue can be obtained in an unfixed 

 but fairly stable state, the stability being due to the absence of 

 water. This state is suitable for certain chemical and enzymological 

 studies. The subject does not fall within the scope of this book, 

 because neither fixation nor dyeing is directly involved. 



Although it is the purpose of fixatives, as a general rule, to leave 

 the structure of the tissue so far as possible unchanged, yet this 

 cannot apply to its chemical composition. Indeed, it may be said 

 that the whole purpose of fixation is to alter the chemical com- 

 position of certain tissue-constituents. If the chemical composition 

 of the tissue were unaffected, the fixative would be without effect: 

 the cells, in fact, would be still alive or like recently dead cells. 



It might be thought that the need to alter chemical composition 

 would render fixation inapplicable in histochemical studies, but 

 this is far from being true. Indeed, the service of fixation to histo- 

 chemistry is as important as its service to dyeing. Tests that would 

 smash a living cell are applicable when it has been fixed. Many 

 fixatives affect the proteins only, leaving DNA, RNA, lipids, 

 carbohydrates, and various inorganic constituents, unchanged; 

 and the proteins themselves, though radically altered, generally 

 retain reactive groups that respond to histochemical tests. The 

 retention of the original structure in the fixed tissue is nearly as 

 important in histochemical as in purely morphological studies, 

 for the purpose of histochemistry is to disclose the distribution of 

 particular substances in -^pace. 



Although it is ■ '-Oi \ation to stabilize the form of 



tissue-constituents ays desirable to preserve all of 



them. In studies c . .lOoi s, lOr instance, it is generally best 



to use a fixative i ...i will either destroy mitochondria or allow 

 them to be destroyed by subsequent treatment, for otherwise they 

 will obscure the view. 



With such exceptions as this it is the purpose of fixatives to 



