CHAPTER 8 



The Causes of Differential Dyeing 



When a section is soaked in a solution of a single dye, the 

 various tissue-constituents react differently. There is differential 

 uptake, and the varying intensity of coloration serves to dis- 

 tinguish the constituents. The latter may also react differently to 

 different dyes, for a particular object may take up much of one, 

 little of another. These facts make it possible to produce striking 

 colour contrasts in what had been a transparent, colourless 

 object. That, indeed, is the purpose of dyeing. We are now con- 

 cerned with the causes that produce these effects. 



Whether a particular object in a microscopical section is 

 strongly or weakly coloured by a particular dye depends on three 

 factors : the chemical ajfinity between the object and the dye, the 

 density of the object, and the permeability of the object by the 

 dye. 



Chemical affinity 



Some of the constituents of the tissues, in the form in which 

 they appear on the slide in sections, are acidic. Examples are 

 DNA and chromatin, RNA and ribonucleoprotein, the matrix of 

 cartilage, the mucous secretions of certain gland-cells, and certain 

 conjugated lipids. Other constituents, such as the ground- 

 cytoplasm of most cells and the contractile substance of muscle, 

 are neither particularly acidic nor particularly basic. They are 

 markedly amphoteric ; that is to say, their electric charge varies 

 easily with the pH of the fluid in which they lie. Others again, 

 such as collagen, the cytoplasm of red blood-corpuscles, and the 

 granules of eosinophil leucocytes, are basic. 



DNA, RNA, and phospholipids owe their acidity to their 



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