Fixation Flashcards
What is fixation?
Preserving tissue in a life-like condition to prevent autolysis and bacterial and fungal growth.
It represents what is actually in patients body and ensures an accurate diagnosis.
What are some things a fixative should and should not do?
Be reasonably priced
Be safe to use
Have longshelf life
Penetrate tissue quickly and evenly
Be compatible with automated tissue procesing
Shouldn’t add extra material
Shouldn’t swell or shrink tissue
Describe some types of fixatives
Formaldehyde
Alcohol
Glutaraldehyde
AZF
Describe key feautres of formaldehyde
40% of formaldehyde solution in water is formalin.
It binds proteins together forming crosslinks.
Most commonly used fixative for tissue.
Describe buffered formalin.
Has a low molecular weight so can enter tissues well.
Fixes slowly and preserves most components, other than lipids and enzymes.
It is cost effective, readily available and easy to prepare.
Kills pathogens.
Has a smell, showing whether a sample has formalin or not.
Toxic - class 1 carcinogen.
Describe alcohol fixative
Non-additive so doesn’t create any bonds.
Denatures proteins by removing & replacing water with alcohol.
Shrinks tissues and makes them brittle.
Used for cytology specimens, where only individual cells are viewed, not tissue structure.
Describe gluteraldehyde
Very toxic and acts fast.
Hard to remove from tissues, rendering them useless for anything else.
Inactivates enzymes.
Gives good morphology and can be used in electron microscopy.
Describe AZF
Acetic Acid Zinc Formalin.
Gentle fixative so used for bone marrow trephines.
Hammersmith Protocol is an approach on how to handle and fix tissues when exra care is needed, for example for bone marrow trephines.
Describe AZF
Acetic Acid Zinc Formalin.
Gentle fixative so used for bone marrow trephines.
Hammersmith Protocol is an approach on how to handle and fix tissues when exra care is needed, for example for bone marrow trephines.
Describe factors affecting fixation
pH - 7
Temperature - >20 C
Concentration - balance between not too weak and not too strong
Duration - at least 24 hours
Surface area
Pentrtion rate
Volume - x10-x20 volume of specimen
What is decalcification?
Removing mineralised calcium from tissues to allow sectioning.
Give examples of types of samples for decalcification
Bone
Teeth
Calcium laid down during pathological processes - balance is essential as it shouldn’t remove calcium which is indicative of a pathological state, while removing unnecessary calcium.
Describe some types of decalcifying agents
Strong acids, including: nitric acid, hydrochloric acid and formic acid.
Shouldn’t be too strong to cause maceration of tisues.
Gentle decalcyfiers: EDTA, Gooding and Stewart’s.
What is end point testing used for?
Determines whether decalcification is finished and if it can be removed from decalcifying solution.
How can X-ray be used for end oint testing?
Look for any white pockets on film. If any white pockets an be seen it means calcium is still present.
