Chapter 21 



Histochemistry 



By definition the field of histochemistry is concerned with the locali- 

 zation and identification of a chemical substance in a tissue. In a broad 

 sense this might include staining, combining chemical and physical 

 reactions, instances where acidic and basic methods demonstrate basic 

 and acidic properties of the tissue. But strictly speaking, histochemistry 

 is being applied only to chemical methods immobilizing a chemical at 

 the site it occupies in living tissue. These methods can apply to in- 

 organic substances: calcium, iron, barium, copper, zinc, lead, mercury, 

 and others; they also apply to organic substances: saccharides, lipids, 

 proteins, amino acids, nucleic acid, enterochromaffin substance, and 

 some pigments. Some substances are soluble and react directly, others 

 are insoluble and must be converted into soluble substances before a 

 reaction takes place. Occult or masked materials are part of a complex 

 organic molecule. This has to be destroyed by an unmasking agent be- 

 fore the chemical can react. Some chemicals may be fixed in place, others 

 which are soluble or diffusible have to be frozen quickly and prepared 

 by the freeze-drying method involving no liquid phase. Sometimes it is 

 advisable to make control slides, thereby preventing confusion between 

 a genuine reaction and a nonspecific one giving a similar effect. 



A sharp distinction between staining and histochemical methods has 

 proved difficult and not wholly practical. Some will disagree with the 

 present arrangement of methods, but a sequence according to similar 

 tissue or cell types seemed adaptable to general laboratory application, 

 the primary intent of this book. The section "Histochemistry," there- 



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