Practical medical biology Tissue Preparation

When a tissue is removed from a living condition, several changes are initiated in its cells.
1- Bacteria begin to multiply and destroy them.
2- Autolysis (self-digestion), a lysis or dissolution of the cells by contained enzymes, sets in. The enzymes appear to reverse their action; instead of synthesizing amino acids into cell proteins, they begin to split proteins into amino acids. The above changes lead to so-called post-mortem conditions.
3- Shrinkage due to dehydration (due to heat or fluids).

Fixation act to:
1- prevent these post-mortem affects
2- convert the cell parts into materials that will remain insoluble during subsequent treatment,
3- Protect the cells from distortion and shrinkage when subjected to fluids such as alcohols and hot paraffin.
4- Improve the staining potential of tissue parts and to change their refractive indices toward improved visibility.
Formalin is a commonly used fixative. It is a buffered isotonic solution of 37% formaldehyde. Formaldehyde preserves the general structure of the cell and extracellular components by reacting with the amino groups of proteins (most often cross-linked lysine residues). Because formaldehyde does not significantly alter their three-dimensional structure, proteins maintain their ability to react with specific antibodies. This property is important in immunocytochemical staining methods. The solution of formaldehyde buffered with phosphates (pH 7) acts relatively slowly but penetrates the tissue well. However, because it does not react with lipids, it is a poor fixative of cell membranes.
# Glutaraldehyde is recommended for fixation of tissues for electron microscopy
# The volume of fixative is important. There should be a 10:1 ratio of fixative to tissue