What is Tissue Fixation?
Tissue fixation is a critical step in
histological processing that involves preserving biological tissue from decay, thereby maintaining the structure and composition of the cells and tissues. This process is essential for the accurate examination and diagnosis of tissue samples under a microscope.
Why is Fixation Important?
Fixation is important because it halts cellular metabolism, prevents
autolysis, and kills pathogenic organisms. It also stabilizes the tissue to prevent changes in volume and shape during subsequent processing steps. Without proper fixation, tissues may degrade, leading to inaccurate results and potential diagnostic errors.
Types of Fixatives
There are several types of fixatives used in histology, each with specific properties suitable for different applications: Formaldehyde: A commonly used fixative that penetrates tissues well and preserves cellular structure. It is often used in the form of formalin.
Glutaraldehyde: Another aldehyde-based fixative, it provides excellent preservation of cellular details and is often used for electron microscopy.
Alcohol-based Fixatives: These include ethanol and methanol, which are used to precipitate proteins and are suitable for some histochemical techniques.
Acetic Acid: Often combined with other fixatives, acetic acid helps to preserve nuclear details.
Zenker's and Helly's Fluid: These are compound fixatives used for their excellent preservation of cellular details and ability to provide sharp staining results.
How Does Fixation Work?
Fixation works through several mechanisms, including cross-linking proteins, denaturing proteins, and altering the water content of tissues.
Cross-linking fixatives, such as formaldehyde and glutaraldehyde, form covalent bonds between proteins, stabilizing the tissue structure. Dehydrating fixatives like alcohols remove water, leading to protein precipitation and stabilization.
Factors Affecting Fixation Quality
The quality of fixation can be influenced by various factors: Fixative Concentration: Too high or too low concentrations can lead to suboptimal fixation.
pH: The pH of the fixative solution can affect the preservation of tissue components.
Temperature: Higher temperatures can accelerate fixation but may also cause artifacts.
Time: Insufficient fixation time can lead to incomplete preservation, while over-fixation can cause hardening of tissues.
Tissue Size and Thickness: Thicker tissues require longer fixation times to ensure adequate penetration.
Common Fixation Artifacts
Artifacts are unwanted changes in tissue appearance caused by improper fixation. Common artifacts include: Shrinkage: Caused by dehydration or over-fixation, leading to tissue distortion.
Crystallization: Occurs when fixatives precipitate out of the solution, leading to crystal formation within the tissue.
Formalin Pigment: A black pigment that can form in tissues fixed in acidic formalin solutions, often seen in
hematoxylin and eosin (H&E) stained sections.
Applications of Tissue Fixation
Tissue fixation is used in various applications, including: Routine Histology: For the preparation and examination of tissue samples in clinical and research settings.
Immunohistochemistry: Fixation preserves antigenic sites for the binding of antibodies.
Electron Microscopy: Specialized fixatives like glutaraldehyde are used to preserve ultrastructural details.
Histochemical Staining: Fixation preserves enzymatic activity for specific histochemical reactions.
Conclusion
Tissue fixation is a fundamental step in histology that ensures the preservation of tissue structure and composition for accurate microscopic analysis. By understanding the different types of fixatives, their mechanisms of action, and the factors affecting fixation quality, histologists can optimize the preparation of tissue samples for various diagnostic and research applications.
