When used as a primary fixative for electron microscopy, Millonig formaldehyde provides what compatibility?

• A. Allows both light and electron microscopy.
• B. Allows only light microscopy.
• C. Is incompatible with electron microscopy.
• D. Requires osmium tetroxide post-fixation.

Answer: (A)

Millonig formaldehyde is a buffered fixative whose pH stability and protein cross-linking preserve tissue morphology well enough for light microscopy, while also being compatible with electron microscopy workflows. The buffering helps maintain structure during fixation, yielding good LM staining and sectioning. For EM, tissues fixed with this solution can be prepared further with osmium tetroxide post-fixation to enhance membrane preservation and contrast, enabling the same specimen to be analyzed ultrastructurally. This dual suitability is why it’s described as compatible with both modalities. It isn’t inherently incompatible with electron microscopy, and while osmium tetroxide post-fixation is commonly used to optimize EM results, it isn’t an absolute requirement in every protocol.