Index >> Microscopy Methods in Microbiology >>Fixation and Dehydration

However, this fixative does not reveal the finer details. For the routine method, tissue should be cut into thin slices and placed in large bulk of fixative for 48 hours with one changeWash the tissue in running water for 1 hour and then transfer to 50% alcohol. Formalin tends to become acidic during the process. Excess of magnesium carbonate is added to keep it neutral.

Zenker-formal fixative (S₁₀) is a general fixative for animal tissues. Warm the fluid to 37°C and to it, place a small piece of tissue. Fixation is completed in 24 hours. Wash the tissue in running water for 24 hours to remove potassium bicarbonate and mercuric chloride. Transfer the tissue in 50% alcohol. Most of the mercuric chloride is removed during the washing with water. To remove the left over mercuric chloride, iodine is added in the alcohol during the dehydration process. Keep the colour of dehydration series brown by addition of saturated solution of iodine in 90% alcohol drop wise.

‘Susa fixative (M. Heidentain) is one of the best fixatives for both normal and pathological specimens. A 10 mm thick tissue is transferred to Susa fixative (S₁₃) and kept for 3-24 hours. Transfer the tissue directly to 95% alcohol. Add saturated solution of iodine in 95% alcohol dropwise till brown colour of alcohol persists for a long time. This will remove mercuric chloride from the tissue. This method is less time consuming and gives better result.

Fixation of plant material is usually done in formalin-acetic acid –alcohol (FAA) mixture (S₁₄). Better results have been noted with fungi by substitution of propionic acid in place of acetic acid in the same amount. Keep the material for fixation in this solution for more than 18 hours.