132 Microscopic Histochemistry 



purple-black indicate the presence of mustard. This is an 

 untested method. 



F. PIGMENTS 



Pigments will be defined as substances visible in unstained 

 preparations by virtue of their own color. They form an ex- 

 tremely heterogeneous group as to both their origin and their 

 chemical nature. Formalin pigment, for example, is an arti- 

 fact, not originally present in the tissues but produced by 

 the action of formaldehyde on hemoglobin. Some of the pig- 

 ments are entirely foreign to the organism, introduced acci- 

 dentally ( "exogenous" pigments, such as carbon and various 

 metals); others are the products of normal or pathological 

 metabolic processes within the organism ("endogenous" 

 pigments ) . 



The number of colored substances found in various animal 

 and plant species is very large, and but a small fraction of 

 them have been identified chemically. In this section only 

 such pigments as are encountered in tissue sections of man 

 and the more common laboratory animals will be dealt with. 



I. Formalin pigment^ occurs in tissues fixed in acid for- 

 malin or formalin-containing mixtures; it is found mainly in 

 and around larger collections of blood. It forms brown-black 

 granules of a crystalline structure. Two methods of removal 

 are recommended. 



1 ) Barrett's method.^— Treat the section for 10 minutes to 

 2 hours with a saturated solution of picric acid in alcohol. 



2) Kar dose wit scKs method.^— Treat the section for 1-2 

 hours with a 2-3 per cent solution of concentrated ammonia 

 water in 70-80 per cent alcohol. 



The formation of formalin pigment can be prevented by 

 using a formaldehyde solution buffered to pH 6-7 with M/15 

 phosphate. 



4. Lillie, R. D., and Hershberger, L. R.: Bull. Intemat. A. M. Mus., 

 27:136, 1947; Hershberger, L. R., and Lillie, R. D.: Bull. Intemat. A. M. 

 Mus., 27:162, 1947. 



5. Barrett, A. M.: J. Path. & Bact, 56:135, 1944. 



6. Kardasewitsch, B.: Ztschr. f. wissensch. Mikr., 42:322, 1925. 



