Differential staining of biological structures by ruthenium red

After the application of a ruthenium red (RR) solution to smears of chicken and human blood for 15 min, thrombocyte and leucocyte nuclei showed a blue-grey colour, contrasting with the red-stained erythrocyte nuclei. Extracellular matrix in frozen sections of cartilage showed the blue-grey colour after 1 h of staining. After the application of RR for a prolonged time (24 h), goblet cell mucin, granules of salivary glands and starch granules in Epon-embedded tissues were coloured blue-grey, blue-green and brown-green respectively; although they appeared red after shorter staining times. Microspectrophotometric measurements of differentially stained structures, and correlation with the spectral behaviour of a related ruthenium compound (ruthenium violet), are presented. The formation in situ of this latter compound by interaction of RR with certain substrates and the capacity of RR to distinguish different cellular structures are discussed.