What are Fixed Tissues?
In
histology, fixed tissues refer to biological tissues that have been preserved through a process called
fixation. Fixation is crucial for maintaining the integrity of tissue samples, preventing decay, and stabilizing cellular structures for microscopic examination.
Why is Fixation Important?
Fixation is essential to halt
autolysis and
putrefaction. Autolysis is the self-digestion of cells by their own enzymes, while putrefaction is the decomposition caused by microbial activity. Fixation preserves the tissue in a life-like state, allowing for accurate analysis and diagnosis.
Common Fixatives
Several chemicals are commonly used as fixatives, each with specific properties suited to different types of tissues and analyses: Formaldehyde: Often used in the form of
formalin, it crosslinks proteins and is suitable for a wide range of tissues.
Glutaraldehyde: Primarily used in
electron microscopy for its ability to preserve fine cellular details.
Alcohol-based fixatives: Such as ethanol or methanol, which precipitate proteins and are often used for cytological samples.
Bouin's solution: A combination of picric acid, formaldehyde, and acetic acid, used for preserving delicate structures.
Steps in the Fixation Process
The fixation process typically involves several steps: Sample Collection: Tissue samples are collected and immediately placed in the fixative to prevent degradation.
Immersion: The tissue is immersed in the fixative for a specific period, allowing the chemicals to penetrate and preserve the tissue.
Rinsing: The fixed tissue is often rinsed to remove excess fixative before further processing.
How Long Does Fixation Take?
The duration of fixation depends on the type of tissue and the fixative used. For example, formaldehyde fixation typically requires 24-48 hours, while glutaraldehyde fixation for electron microscopy may take only a few hours.
What Happens After Fixation?
After fixation, the tissue is further processed for
embedding, sectioning, and
staining. Embedding involves encasing the tissue in a medium like paraffin wax to facilitate thin slicing. Sectioning produces ultra-thin slices for examination under a microscope. Staining highlights specific structures and components within the tissue.
Challenges in Fixation
Despite its importance, fixation presents certain challenges. Over-fixation can lead to hardening and shrinkage of tissues, while under-fixation can result in poor preservation and artifacts. Selecting the appropriate fixative and optimizing fixation conditions are crucial for obtaining reliable results. Conclusion
Fixation is a fundamental step in histology, ensuring the preservation and stabilization of tissue samples for detailed microscopic analysis. Understanding the principles, techniques, and challenges of fixation is essential for accurate histological studies and
pathological diagnoses.
