Formalin pigment (acid hematin) and related pigments.

Black to brown amorphous to microcrystalline granules are encountered in histologic sections prepared from tissues fixed in formalin having a low pH. This pigment is produced by acid acting upon hemoglobin and is known as formalin pigment or acid hematin. A similar pigment is also observed in sites of bleeding ulcers in areas of acid production such as the stomach. These pigments exhibit many physical and histochemical properties similiar to pigments produced by some animal parasites as in malaria, schistosoma and pulmonary mites. These parasites disintegrate erythrocytes in an unknown manner, and liberate an acid hematin-like pigment which is phagocytized by the reticuloendothelial system. Since formalin pigment can be considered as an artifact, confusion with other pigments can be avoided by the use of neutral buffered formalin for the fixation of tissues.