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Functions of Fixatives
1. Kill the tissue (prevent autolysis)
2. Maintain relationship b/w cells and extracellular substance
3. Increase differences in Refractive Index to increase the contrast b/w different tissue elements
4. Enhance staining (w/ exceptions)
5. Rendering cell constituents insoluble (especially proteins)
6. Make tissue firmer iot make gross dissection easier
Actions of Fixatives
1. Inactivate enzymes
2. Kill bacteria/molds
3. Make tissue more receptive to dyes
4. Modify tissue constituents for the maximum retention of form throughout processing
Effects of overheating tissue
1. Pyknotic, over-stained nuclei
2. Loss of enzyme activity
3. Loss of antigenicity
4. False localization of nucleic acids
5. Lysis of RBCs
Physical methods of fixation
When is desiccation used for fixation?
Air drying of touch preps for Wright staining
Methods of Fixation
Classification of Chemical Fixatives
How does an Additive Fixative work?
They chemically link to - and change - the tissue
Common Additive Fixatives
Common Non-additive Fixatives
How do Non-additive Fixatives work?
They dissociate bound water molecules from tissue protein groups
Which fixatives cause shrinkage and hardening with over-exposure?
How do coagulant fixatives work?
They establish a network in tissue allowing solutions to readily penetrate the interior of the tissue
How do non-coagulant fixatives work?
The create a gel, making penetration by solutions difficult
Common Coagulant Fixatives
Common Non-coagulant Fixatives
Which fixative is sometimes considered a coagulant and sometimes a non-coagulant?
nucleic acids - coagulant
cytoplasm - non-coagulant
Factors affecting fixation
Temperature - increase => increased rate of fixation and autolysis
Size of specimen - larger specimen => longer to penetrate and fix
Volume Ratio - should be 15 - 20 x more fixative than tissue volume
Time - depends on tissue size/type and fixative
Overall histotechnology review