216 Biological Stains 



Orcein and Litmus c. i. no. 12.42 



Both orcein and litmus are obtained from certain lichens, Lecan- 

 ora tinctoria and Rocella tinctoria. These lichens are colorless, but 

 when treated with ammonia and exposed to the air, blue or violet 

 colors develop. The colors are due to certain phenolic com- 

 pounds, one of which is orcinol: 



OH 



_/ 

 CH3_/ \ 



\ 

 OH 



Orcinol, acted upon by air and ammonia, becomes orcein. 



u30 ORCEIN 



The exact formula of orcein is unknown. It is a weak acid, 

 soluble in alkalies, with a violet color. It is possible to synthesize 

 this dye from orcinol by the above-indicated process. As a 

 matter of fact very good synthetic products, for some purposes 

 better than the natural product, are now on the market. The 

 synthetic orceins from different manufacturers, however, are pre- 

 pared by different methods and are not the same in their behavior. 

 The reason for these differences are still unknown. The method 

 of manufacture has never yielded exact information as to its struc- 

 tural formula; but it is believed to contain in its molecule the resi- 

 dues of four orcinol molecules. It has some indicator properties 

 both for pH and oxidation-reduction; but is not as useful in that 

 respect as its closely related litmus. It is a weak acid, violet in 

 alkaline solution; in acid solutions it is orange-red. 



Unna (see Lee, 1937, p. 451) used orcein in alcoholic solution for 

 staining elastin tissue; he employed it for connective tissue, follow- 

 ing polychrome methylene blue; and for plasma fibrils in the 

 epithelium, following anilin blue; also with anilin blue or acid fuch- 

 sin in studying the process known by him as chromolysis. It has 

 found less frequent use among other histologists; but has been 

 employed in acetic acid solution for staining sections or dissolved 

 in weak hydrochloric acid, for staining sections of embryos; also 

 in combination with various other dyes for bulk staining or as a 

 connective tissue stain. Mollier (1938) calls for it in a quadruple 

 tissue stain, with iron-hematoxylin, naphthol green B and azo- 

 carmine G. Kornhauser (1943) uses it in a somewhat different 

 quadruple tissue stain with acid alizarin blue, orange G, and fast 

 green FCF. LaCour (1941) employs it in a stain-fixative for 

 cytological work, either for plants or animals. For this last men- 

 tioned purpose the synthetic orcein has not worked as well as that 



