THE INDIRECT ATTACHMENT OF DYES TO TISSUES 219 



There is a contrast between the uptake of iron on one hand, and 

 of chromium and aluminium on the other. The two latter metals 

 attach themselves only slightly to the proteins of the cytoplasm, 

 and scarcely at all to elastin.^^^ They also have much less tendency 

 than iron to attach themselves to lipids (though anionic chromium 

 makes bonds with these; see pp. 107, 128). It is for these reasons 

 that we choose iron haematein to show mitochondria and certain 

 other cytoplasmic inclusions, but aluminium haematein or chrome 

 oxazine when we want to colour chromatin and little else. 



The attachment of the chromic cation to tissue-constituents has 

 not been studied in detail. We have some information about its 

 behaviour in the dyeing of textiles, although, as was mentioned 

 above, most of the research on the chemistry of mordanting 

 has been done with anionic chromium. It is thought that the 

 attachment is by covalent linkages. Various groups in the protein 

 of wool are thought to displace water or OH or both from their 

 attachment to chromium in the cationic chrome complex, and to 

 co-ordinate with the metal in their place. The hydroxyl, amino-, 

 and amido-groups of the protein are mentioned in this con- 

 nexion. ^^^ It is also claimed that where two protein chains are held 

 together by an -S-S- bond, this may be split apart with produc- 

 tion of two -SH groups, and these, reacting with the chromium 

 atom, may cause the latter to act as a new link between the protein 

 chains. ^^2 It will be remembered, how^ever, that the dyeing of wool 

 is carried out at high temperatures. 



In biological preparations it is certain that the phosphoric 

 groups of the nucleic acids and the acidic groups of certain muco- 

 substances make attachment with the chromium mordant more 

 readily than proteins, especially at low pH,^^^ and this also applies 

 to aluminium. 



It will be noticed that the attachment of mordants to tissues is a 

 very complicated process, of which we have as yet only an imper- 

 fect understanding. The statement is commonly made in chemical 

 textbooks that the mordant metals are deposited in textile fibres as 

 gelatinous, insoluble hydroxides. This is not only an extreme over- 

 simplification, but quite untrue. Neither in textiles nor in ordinary 

 biological material are the metals deposited in this way. Had the 

 statement been true, there could be no question of that delicately 

 differential tying up of the metals with particular tissue-constitu- 

 ents that makes mordants so invaluable in microtechnique. 

 There is no gelatinous mass pervading our sections. If there had 



