What size tissue blocks are recommended for osmium tetroxide fixation in electron microscopy?

• A. 1 mm cubed
• B. 5 mm cube
• C. 0.5 cm cube
• D. 2 cm cube

Answer: (A)

Osmium tetroxide has limited diffusion into tissue, so using small blocks helps ensure rapid and uniform postfixation of membranes and lipids for electron microscopy. About a 1 mm on each side block allows the fixative to reach the center during typical postfixation times, giving consistent ultrastructure for embedding and imaging. Larger blocks, such as 5 mm or 2 cm cubes, would not fix evenly throughout, leading to gradients and potential artifacts in the preserved membranes.