a) Types of fixatives
There are three common ways of classifying fixation: acidic fixation vs. basic, coagulant vs. non-coagulant, and additive vs. non-additive.
Acidic fixation preserves chromosomes, nucleoli, and the spindle apparatus, while dissolving nucleoplasm and mitochondria. Conversely, basic fixation preserves nucleoplasm, mitochondria and dissolves chromatin and the spindle apparatus.
Coagulant fixatives (such as ethanol, methanol, picric acid, murcuric chloride, acetone, and chromium trioxide) cause cytoplasm to become opaque and congeal into a netlike structure. Non-coagulant fixatives do not disrupt the cytoplasm ut transform it into a transparent gel. Examples of fixatives of this kind are glutaraldehyde, formaldehyde, acrolein, potassium dichromate, acetic acid and osmium tetraoxide. Non-coagulant fixatives very often stabilize tissues to such an extent that they are protected from disruption by the use of subsequent coagulant reagents during post-fixation or dehydration.
Additive fixatives cross-link with proteins in cells, strengthening cell structure and insuring tissue preservation. Examples of additive fixatives are the aldehydes.
As can be said thus far, there is no one substance that will successfully fix all components of a cell or tissue equally. It is necessary to combine fixatives in such a way so as to minimize the shrinking or swelling action of the various ingredients and to stabilize the various cell structures. Any fixation schedule should be regarded as a starting point from which the best method of fixation for one’s specimen will be developed.