Chemical fixation employs agents which permeat tissues and cells and combine covalently with their major biochemical constituents (lipids, proteins and carbohydrates) and fix them into place.

Coagulative Fixatives (ethanol, methanol, Hcl, chromic acid)-

used primarily for fixing specimens for light microscopy and

precipitate (coagulate) and denature proteins not normally

visible at the light microscopy level. Not suitable for EM.

Noncoagulative Fixatives (formaldehyde, glutaraldehyde, acrolein)

form intra and intermolecular links and change the sol form

of the cytoplasm into an electron transparent gel.

Formaldehyde - H3C=O

penetrates tissues rapidly but may be extracted by repeated

washing

Glutaraldehyde- O=CH-CH2-CH2-CH2-CH=O

penetrates tissues slower but due to its bifunctionality

crosslinks proteins into permanent stasis. Cell membranes

are partially permeabilized but cells remain osmotically

active.

Acrolein- H2C=CH-CH=O

rapid penetration, used to fix thick specimens, frozen

specimens and cells with thick walls (plant cells)

Fidelity of Protein Preservation- tertiary protein structure is often

preserved to a degree that enzyme activity and antigenic specificity

are retained.

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