Under-fixation occurs when the tissue is not exposed to the fixative for a sufficient amount of time, resulting in poor preservation of cellular structures. This can lead to autolysis and poor morphology. Over-fixation, on the other hand, can cause excessive cross-linking of proteins, making the tissue hard and brittle, which complicates subsequent processing steps. Both scenarios can be avoided by adhering to proper fixation protocols and using the correct concentration and volume of fixative.
