After fixation with glutaraldehyde for electron microscopy, tissues are typically postfixated in osmium tetroxide.

• A. Postfixation in osmium tetroxide after glutaraldehyde is recommended
• B. Postfixation in potassium dichromate is recommended
• C. No postfixation is used
• D. Postfixation in formaldehyde is required

Answer: (A)

Glutaraldehyde fixes proteins well by cross-linking them, but it doesn’t fix lipids effectively. Postfixation with osmium tetroxide addresses this by binding to membrane lipids, stabilizing membranes, and adding electron density that greatly enhances contrast in transmission electron microscopy. This combination preserves ultrastructural detail, especially membranes, making it the standard sequence for EM tissue preparation.

Potassium dichromate isn’t used as a routine postfixative in EM because it doesn’t fix lipids as effectively and can introduce artifacts or less reliable membrane preservation. Postfixation with formaldehyde doesn’t provide good lipid stabilization or the membrane contrast needed for electron microscopy. Skipping postfixation would leave lipids and membranes poorly preserved, resulting in weak membrane contrast.