SECTION FIVE 



against a brown background and due to the omission of lipid sol- 

 vents in this technique, smaller and more numerous fat droplets 

 can be seen than in the case of the sudan techniques as used in 

 ordinary microscopy. With methylene blue fat gives a blue 

 fluorescence. 



Method B. Solution required: 



Thioflavine S i% aqueous. 



Technique: 



1. Frozen sections are stained for two minutes; then rinsed 

 in water. 



2. Examine in water. 



Results: 



Fats appear violet against a dark background. 



Note: With this method fewer lipids are stained than with 

 phosphine 3R. 



3. Intravital Staining with Fluorochromes 



Dyes used for this purpose must be water soluble, non-diffus- 

 ible in the living body, non-toxic in the workable dilutions re- 

 quired, and highly fluorescent even in greatly diluted solutions. 

 Uranin possesses all these qualifications and is one of the most 

 useful fluorescent dyes for intravital work, as stated earlier in this 

 chapter. Acriflavine is another useful dye for this purpose, al- 

 though it is not so intensely fluorescent as uranin. The fluor- 

 escence of uranin is impaired in basic solution so that it appears 

 most readily in organs of an acid reaction. It is used in o-i% 

 solution in physiological saline, in which form it should be injected 

 into the animal's blood stream or into the organ to be studied. 

 The colour of its fluorescence varies with hydrogen-ion changes 

 and consequently it is of great value as an intravital hydrogen-ion 

 indicator. The colour changes are easily visible in dilutions to the 

 order of one part in ten millions and in dilute solutions the inten- 

 sity of the fluorescence has a definite relation to the concentration 

 of the dye and consequently the intensity of the fluorescence 



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