PRIMARY fixatives: NON- COAGULANTS 129 



were darkened by unmordanted haematein; this was removed by 

 bleaching with alkaline potassium ferricyanide, which did not 

 affect the lake. 



Better results were obtained by short fixation in some other 

 fluid and subsequent 'postchroming' in potassium dichromate 

 solution. It is usual to postchrome tissues for quite a long time, 

 often days or weeks, sometimes at 37° or even 60° C. This process, 

 as a method of fixing particular constituents of cells, was intro- 

 duced by the celebrated German cytologist Benda,^^ who used it in 

 his pioneer research on mitochondria. He named it PostcJiro?ninmg. 

 It had previously been used only to harden tissues for easier 

 sectioning by hand. Benda sometimes used chromium trioxide in 

 the same way, but potassium dichromate is nearly always used 

 now^adays. 



Smith *^^' *^^ modified Weigert's method by introducing a pre- 

 liminary fixation in formaldehyde, followed by postchroming and 

 the cutting of frozen sections. This technique was adapted by 

 Dietrich ^"^^ and made into a histochemical test for phospholipids. 

 The acid haematein test ^^'^^ is its modern version. 



The chemistry of the action of potassium dichromate on lipids 

 has been studied especially by Kaufmann and Lehmann 260-262 

 and by Lison.^^^ It would appear that a wide variety of unsaturated 

 lipids can be rendered insoluble in lipid solvents by the prolonged 

 action of potassium dichromate. There is no action on saturated 

 ones. Different periods of postchroming are suitable for different 

 unsaturated lipids. 



The evidence suggests that three processes can be involved in 

 the action of potassium dichromate on lipids, and that they need 



H H H H 



— C=C— ► — C— C— ► — CH HC— 



\ \ II II 



0—0 o o 



Double bond Peroxide Aldehydes 



not all occur together. These three are simple oxidation, poly- 

 merization with loss of solubility in lipid solvents, and binding of 

 chromium (additive fixation). 



Simple oxidation at the double bond occurs particularly when 

 there is only one such double bond in a fatty acid radicle. The 

 fatty acid chain is split at the double bond, with formation of two 

 aldehydes. 



