PRIMARY fixatives: NON-COAGULANTS 135 



not even mention acetic acid. The softness of tissues fixed with 

 this substance probably counted against it, for the microscopists 

 of the time were more interested in hardening agents than in 

 fixatives. Corti,-^-^ who experimented freely with fixative fluids, 

 tried it in his study of the inner ear in 1851, and in the same year 

 Clarke ^^^ used it in a mixture with ethanol for the treatment of 

 tissues that had already been soaked in the latter fluid. It was 

 subsequently used by Remak ^^^ in 1854 and Auerbach ^^ 20 years 

 later. Flemming ^^^ mentioned that it made the nuclear membrane 

 very refractive and tended to distort it: he preferred chromium 

 trioxide and picric acid. Acetic acid appears to have been valued 

 in the seventies and eighties chiefly for showing nuclei clearly and 

 making connective tissue transparent; pyroligneous acid was 

 sometimes preferred, because it hardens somewhat. ^"^ 



Reactions with proteins. Acetic acid (at the standard concentra- 

 tion) does not coagulate albumin, does not set egg-white into a gel, 

 and has no fixative effect on gelatine/albumin gel or on haemoglo- 

 bin. Histone can be extracted from tissues by acetic acid. Its most 

 evident effects are to swell protein gels and fibres and to produce 

 a precipitate with nucleoprotein. 



The undissociated acid is thought to break the linkages between 

 amide groups of contiguous protein chains, by associating with 

 these: this would permit swelling. The dissociated acid splits the 

 salt-links (amino to carboxyl) that also hold protein chains together, 

 and this again permits swelling. Water is drawn into the protein 

 by attraction to the hydrophil groups exposed by these reactions. 

 The hydronium ion has a preservative effect, because it checks 

 autolysis and stops the growth of putrefactive bacteria. 



Acetic acid gives a thick precipitate with nucleoprotein solution. 

 This is attributed to the action of the acetate ion in splitting off 

 DNA from the protein. DNA is precipitated from solution by 

 acetic acid. 



Reactions with lipids. Certain lipids are miscible with glacial 

 acetic acid, or soluble in it: sphingomyelin, the ricinolein of castor 

 oil, and cholesterol are examples (though the latter is only 

 slightly soluble). These facts, however, are not of much significance 

 for microtechnique, since lipids are not ordinarily soluble in 

 acetic acid at the usual fixative concentration of 5% or thereabouts. 

 Phospholipids can form colloidal solutions in water, but their 

 solubility in acetic acid of fixative strength does not appear to have 

 been determined. Acetic acid is not known to fix any lipid. 



