236 Biological Stains 



The red color produced on reduction was observed, and utilized 

 biologically, before the chemistry involved was understood, be- 

 cause the coloring thereby of certain parts of the embryo in seeds 

 was found to be an indicator of viability of the seeds. It was later 

 learned that other forms of living tissue could bring out the red 

 color; and histochemists discovered that the presence of certain 

 enzymes systems (dehydrogenases) could be visualized by their 

 ability to reduce the tetrazolium salt to formazan. See, for in- 

 stance, Pratt, Dufrenoy and Pickering (1948). 



The compound has, therefore, found considerable histochemical 

 use; but it has certain drawbacks (among them photosensitivity) 

 which have led to investigation of other derivatives of tetrazolium. 

 Among them are: 



2- (p-iODOPHEN yl) -3- (p-nitrophenyl) -5-phenyl 



TETRAZOLIUM CHLORIDE 



=N— N_/— \_N0.> 



/-\_c \_/ 



\_/ \ 



CI— N=N /— \ I 



This salt is less photosensitive than the preceding, and has the 

 additional advantage (Atkinson, Melvin and Fox, 1950) of slower 

 diffusion into ui>stained tissue. 



NEOTETRAZOLIUM CHLORIDE 



Synonym: Ditetrazolium chloride 



/-\_c \_/ \_/) c_/-\ 



CI— N=N_/— \_/— \_N.=N— CI 



The reduced form of this salt is deep purple to black, instead of 

 red as in the case of ordinary tetrazolium salt, which makes it 

 easier to distinguish in tissue. Antopol, Glaubach and Goldman 

 (1948) report that it stains malignant tissues rapidly and intensely. 



DIMETHOXY NEOTETRAZOLIUM CHLORIDE 



Synonym: Blue tetrazolium 



C 



,N— N_/— \ N— N, 



C 



Cl_N=N_/~\_/~\_N=N_Cl 



\_/ \_/ 

 CH3O/ \OCH3 



