An unexpected black precipitate appears that is non-removeable after contact with alcohols during tissue processing. Which fixative and pigment, and how to prevent it?

Study for the CAMLPR Histology Practice Test. Study with flashcards and multiple choice questions, each question has hints and explanations. Get ready for your exam!

When a fixative containing osmium tetroxide is used, a black, insoluble pigment called osmium pigment can form in the tissue. Osmium tetroxide reacts with lipids to create osmium-containing complexes that appear dark and can persist through the dehydration steps, since the pigment is not easily dissolved by alcohols. The key to preventing this troublesome dark precipitate is to remove excess osmium after fixation. Washing the tissue in buffer after fixation helps flush out unbound osmium and reduce the amount that can precipitate as osmium pigment during subsequent alcohol dehydration. This buffer rinse minimizes the chances of a persistent black deposit that resists alcohols and interferes with staining and visualization.