As soon as a tissue is removed from the body, it begins breaking down through enzyme action. Fixatives prevent putrefaction (decay) and autolysis (enzyme attack). It is important to note that autolyzed tissue components will not stain properly, thus hindering diagnosis. As defined by Freida Carson, a fixative alters tissue by stabilizing the proteins so that they are resistant to further changes. The primary purpose of tissue preservation is to keep the cellular morphology intact through various processes to ensure the specimen is diagnostic.
Time in fixative is dependent on the tissue size. Larger samples require more time in fixative, simply because the fixative has more tissue layers to penetrate. Fixation occurs from the outside of the tissue inward. Under-processed tissue is often fixed on the surface, while the center remains soft and mushy. The fixative volume should be 15 to 20 times greater than the tissue volume to keep the fixative from depleting and cover the sample completely.
