Glutaraldehyde is most frequently used for electron microscopy because it preserves ultrastructure, though it tends to overharden tissue; it is normally used at 2-4%.

• A. Glutaraldehyde is best for EM due to ultrastructure preservation
• B. It penetrates rapidly and is best for quick fixes
• C. It does not preserve ultrastructure
• D. It is never over-hardening

Answer: (A)

For electron microscopy, preserving ultrastructure—the fine details of membranes, organelles, and overall cellular architecture—is essential. Glutaraldehyde excels here because it crosslinks proteins, forming strong covalent bonds that stabilize those delicate structures. This deep fixation yields superior ultrastructural preservation, which is why glutaraldehyde is the preferred primary fixative for EM and is commonly used at 2–4% to balance fixation strength with tissue diffusion. A known caveat is that glutaraldehyde can over-fix or over-harden tissue if used too long or at higher concentrations, and it tends to penetrate more slowly than some fixatives, often requiring postfixation steps like osmium tetroxide to enhance contrast and lipid preservation. This combination of excellent ultrastructural preservation, despite the tendency to over-harden, explains why the statement about its use for EM is correct. It’s not the choice for rapid fixes, and it does preserve ultrastructure, so those contrasting claims don’t fit.