CHA PTER 



The Reactions of Fixatives with 

 Proteins 1. The Visible Effects 



For reasons that have already been mentioned (p. 23), fixation 

 is primarily the stabilization of proteins. It will be best to study 

 the reactions between fixatives and proteins before turning to 

 the more complex events involved in the fixation of tissues and 

 cells. 



Some of the long chains of amino-acids that make up the pro- 

 teins of plants and animals are wound into submicroscopic balls, 

 others are extended in long fibres, often interlacing with one 

 another and held together by chemical bonds of various kinds ^^^ 

 to form elaborate networks in three dimensions. The proportion of 

 globular to fibrous proteins varies widely in different tissues, the 

 globular existing alone in certain body-fluids, the fibrous pre- 

 dominating in certain elements of the connective tissues of animals. 

 In the present state of knowledge it is not possible to assess the 

 relative amounts of each in protoplasm (cytoplasm and nuclear 

 sap), but it is supposed that the fibrous here predominate and 

 form a 'cytoskeleton' in the form of a submicroscopic net- 

 work.^^i 



When fixatives are added to protein sols, there is usually (though 

 not always) a change in visible appearance. Sometimes there is an 

 aggregation of protein molecules into microscopic granules, which 

 are seen as a cloudiness in the fluid. Stronger action produces a 

 flocculus of visible particles, which may remain suspended or 

 fall gradually as a precipitate. Stronger action again may convert 

 the whole of the protein into a single, coherent clot. The appear- 

 ance of new interfaces between solid protein and surrounding 

 water results in the scattering of light and thus the production of 

 whiteness or colour: transparency is in some degree lost. Cloudi- 

 ness, flocculation, and clot-formation are grades or stages in a 

 single process of coagulation. The more concentrated the protein 



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