94 CYTOLOGICAL TECHNIQUE 



amino-groups has been replaced by a phenyl group (that is, by a 

 simple, unsubstituted aryl group). 



Methyl blue is a pure blue anionic triarylmethane dye that owes 

 its colour to three extra aryl groups acting as modifiers. Despite 

 its misleading name, it possesses no methyl group. 



Haematein. This is a very small group of dyes that are derived 

 from natural products and are not made synthetically. As the 



HO HO 



HO^^f^^^Hs HO 



CH, 



HO^x^ 



Skeleton-formula for 

 haematein dyes 



O 



Haematein 



Catechol 



formula shows, this is not only an acid dye, but actually an acid, 

 related to catechol. It is of a brownish-red colour, but becomes 

 blue in alkaline solution. It is therefore used as an indicator of 

 pH, but it is not valuable by itself as a dye. When used with an 

 intermediary or 'mordant' (p. 110) between itself and the tissues, 

 it becomes one of the most important dyes used in micro- 

 technique. 



Haematein is derived from the heart -wood of a small legum- 

 inous tree, Haematoxylon campechianum, native to Central 

 America. This contains a non-quinonoid substance, haematoxylin, 

 which is not a dye and is indeed colourless, since it lacks a 

 chromophore; but it is readily oxidized to haematein by weak 

 oxidizing agents, including atmospheric oxygen. Haematoxylin is 

 often sold in a partially oxidized form, as a brown powder that is 

 really a mixture of haematoxylin and haematein. 



Anthraquinonoid dyes. These are related to anthraquinone, a 

 yellow, crystalline substance. The simplest is alizarine, the chief 

 coloured constituent of madder, a vegetable dye. Alizarine is 

 easily synthesized from anthraquinone. It is not used in micro- 

 technique. Its more complex relative, carminic acid, is a parti- 



