Fixation

Question 1

What are the methods of fixation?

Answer 1

• Heat
• Freezing
• Chemically

These methods are used to preserve biological tissues for examination.

Question 2

What is the purpose of fixation?

Answer 2

• Stabilize proteins
• Protect tissue from subsequent damage
• Harden the tissue
• Preserve life-like state of tissue

Fixation is crucial for maintaining the integrity of biological samples.

Question 3

How does fixation work?

Answer 3

• Prevents autolysis (enzyme degradation)
• Prevents putrefaction (bacterial decomposition)
• Stabilizes proteins
• Increases visibility and contrast (RI = 1.54)

The refractive index (RI) indicates how light interacts with the tissue.

Question 4

What are the qualities of an ideal fixative?

Answer 4

• Rapid penetration
• Kills tissue quickly
• Hardens tissue slightly for easier cutting
• Non-toxic

An ideal fixative minimizes damage to the tissue while preserving its structure.

Question 5

What are major fixatives used in tissue preservation?

Answer 5

1. Aldehyde
2. Alcohols
3. Mercurials
4. Oxidizing agents
5. Picrate’s

Each fixative has unique properties and applications in histology.

Question 6

What is the fixative of choice for most applications?

Answer 6

NBF (Neutral Buffered Formalin)

NBF is widely used due to its effectiveness and compatibility with various staining techniques.

Question 7

What factors affect fixation?

Answer 7

• Temperature
• Size
• Volume (20:1)
• Time
• Fixation of choice
• Penetration
• Storage
• pH
• Osmolality

These factors influence the quality and effectiveness of the fixation process.

Question 8

What are sources of error in fixation?

Answer 8

• Insufficient fixation
• Improper ratio of fixative to tissue
• Poor penetration (wrong fixative used)
• Tissue not suspended (not exposed from all sides)
• Insufficient time in fixative (mushy)

Errors during fixation can lead to compromised tissue integrity and artifacts.

Question 9

What are common artifacts that can occur during fixation?

Answer 9

• Protein precipitation
• Autolysis / desquamation
• Putrefaction
• Diffusion / substance loss
• Shrinkage
• Distortion
• Excess hardening
• Formalin pigment (brown)
• Mercuric pigment (brown)

Artifacts can significantly impact the interpretation of histological samples.

Question 10

What is the difference between coagulant and non-coagulant fixatives?

Answer 10

Coagulant: harsh denaturation of proteins, better penetration, forms a meshwork due to shrinkage
Non-Coagulant: gentler denaturation, preferred, additive fixation

The choice between coagulant and non-coagulant fixatives can affect tissue morphology.

Question 11

What are additive vs non-additive fixatives?

Answer 11

Additive: chemically links to the tissue; common feature of all primary fixing agents except ethanol
Non-Additive: does not combine with tissue; coagulates proteins by dehydrating them

Understanding the mechanism of each type of fixative is crucial for selecting the appropriate one for specific applications.

Question 12

Fill in the blank: The only fixative that acts as a non-additive fixative is _______.

Answer 12

Ethanol

Ethanol’s unique properties make it different from other fixatives.