What is Fixed Tissue?
In
Histology, fixed tissue refers to biological tissues that have been treated with chemical agents to preserve their structure and composition. This process, known as
fixation, is crucial for preparing tissues for microscopic examination, preventing decay, and maintaining the tissue's integrity.
Why is Fixation Necessary?
Fixation is essential to prevent the
autolysis (self-digestion) and
putrefaction (decomposition by bacteria) of tissues. It stabilizes the biological material by cross-linking proteins and inactivating enzymes, ensuring that the tissue's cellular and subcellular structures remain intact for detailed study.
Formaldehyde: Commonly used in the form of formalin, it is effective in cross-linking proteins and preserving tissue architecture.
Glutaraldehyde: Often used for electron microscopy due to its ability to preserve fine cellular details.
Alcohols: Such as ethanol and methanol, which dehydrate tissues but can cause shrinkage.
Acetic Acid: Used to fix and preserve nucleic acids.
How is Fixation Performed?
Fixation is typically performed by either
immersion or
perfusion. In immersion fixation, tissues are submerged in a fixative solution, allowing the fixative to penetrate the tissue over time. Perfusion fixation involves delivering the fixative through the blood vessels, which is more efficient for large or dense tissues, ensuring rapid and uniform fixation.
Fixative Concentration: The strength of the fixative can affect the extent of tissue preservation.
Fixation Time: Adequate time is necessary for the fixative to penetrate and stabilize the tissue.
Temperature: Fixation is often performed at room temperature, but some protocols require refrigeration or warming.
Tissue Size and Thickness: Smaller and thinner tissues fix more rapidly and uniformly.
What are the Challenges of Fixation?
Despite its importance, fixation can present challenges. Over-fixation can lead to excessive cross-linking, making tissues hard and brittle, while under-fixation can result in incomplete preservation. Additionally, some fixatives may distort cellular components or cause artifacts, complicating the interpretation of histological findings.
Conclusion
Fixation is a cornerstone of histological practice, enabling detailed examination of tissue architecture and cellular components. Understanding the principles and techniques of tissue fixation helps ensure high-quality histological preparations, providing valuable insights into the structure and function of biological tissues.
