CHAPTER 7 



Stains and Staining 



PRINCIPLES OF STAINING 



Many objects, after being impregnated with mounting media ( Chapter 

 9), become so transparent that their structure cannot be observed under 

 the microscope. To overcome this difficulty, such objects are commonly 

 "stained"— that is, impregnated with a color— which renders them more 

 visible. In the case of materials of uniform chemical composition— for ex- 

 ample, a section of wood— it does not matter very much what dye is used. 

 Most sections, however, contain an assortment of components that may 

 be distinguished by their ability to retain dyes of contrasting colors. Thus 

 a section of a plant stem may be so stained that contrasting colors are 

 exhibited by the lignin, the cellulose, the protoplasm, and the nuclei. A 

 section of animal tissue may be so treated that bone, cartilage, muscle, 

 nerves, white connective tissue, and nuclei stand out in vivid color 

 contrasts. 



These triple, and even quintuple, stains are rarely necessary for the 

 everyday business of teaching and research. In plant anatomy it is usually 

 sufficient to distinguish woody from nonwoody tissue. In animal histology 

 it is enough to distinguish the nucleus clearly from the surrounding 

 cytoplasm. This simple distinction is produced by using so-called "basic" 

 and "acidic" dyes, a distinction of more historic than sceintific interest. 

 The first group comprises those dyes, like hematoxylin and safranin, that 

 may readily be made to stain nuclei and woody tissues; the second group 

 contains dyes, like eosin or fast green, that do not adhere to nuclei or wood 

 but which are retained in other parts of tissues. 



The adhesion of dyes to nuclei is rarely direct. In most cases some 

 other compound must first be applied to the nucleus and the "nuclear 

 stain" then caused to adhere to this compound. This "undercoat," as it 

 were, is referred to as a "mordant" (Latin mordere, to bite) since early 

 dyers thought that it caused color to bite into the fabric. The process re- 

 ferred to in biological literature as "mordant staining" involves first the ap- 



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