PRIMARY fixatives: COAGULANTS IO7 



histidine present side-groups that are capable of oxidation, and it 

 is noteworthy that after prolonged fixation by chromium trioxide, 

 histochemical tests for these amino-acids give feeble results. ^^^ 

 This suggests that oxidation has in fact occurred, but it does not 

 explain the firm binding of chromium. 



Chromium trioxide is a coagulant of nucleoprotein. DNA is 

 precipitated from its solutions in an insoluble form; ^^^ it is 

 hydrolysed, with conversion of the pentose sugar to aldehyde, and 

 Schiff's reagent (p. 308) therefore gives a positive reaction with- 

 out the necessity for hydrolysis by hydrochloric acid on the 

 slide.iii 



Reactions with lipids. It was stated long ago by Smith and Mair ^^^ 

 that the fat of adipose tissue (triglyceride) can be rendered in- 

 soluble in lipid-solvents by prolonged treatment with chromium 

 trioxide. This has recently been confirmed. ^^^ The reactions with 

 lipids have not been fully investigated. Unsaturated fatty acids are 

 first oxidized at the double bonds, with production of hydroxy- 

 acids.^2^ Oleic acid is eventually split, with production of pelar- 

 gonic and azelaic acids ; these are somewhat soluble in cold water. 

 Presumably the reactions with lipids in general are similar to 

 those of potassium dichromate (p. 128), but quicker and apt to go 

 too far. It may be for this reason that potassium dichromate is 

 nearly always used when tissues are 'postchromed' (p. 129) to 

 render lipids insoluble in lipid-solvents by partial oxidation. 



Reactions with carbohydrates. When chromium trioxide acts on 

 polysaccharides, it works as an oxidizer, converting them to 

 aldehydes. The exact site of this reaction in the polysaccharide 

 molecule has not been determined. ^^^ The aldehvdes formed in 

 this way can be exhibited by means of SchiflP's aldehyde-reagent 

 (p. 308). This is the basis of Bauer's *^ histochemical test for 

 glycogen and other polysaccharides. It is usual to fix tissues in a 

 mixture of ethanol and acetic acid or some other fixative that will 

 not dissolve water-soluble polysaccharides, and then to treat 

 sections with a solution of chromium trioxide. If a simple aqueous 

 solution of chromium trioxide be used as fixative, a certain amount 

 of glycogen can subsequently be revealed in sections by the direct 

 application of Schiff's reagent. ^^^ 



If the proteins of a cell are fixed in such a way that the escape of 

 glycogen is hindered, and a solution of chromium trioxide is 

 subsequently allowed to act, the aldehyde produced will not dis- 

 solve out if the section be placed in distilled water. Nevertheless, 



