Tissue preservation, or fixation, is an initial step in biological and medical fields, allowing for detailed examination of cells and tissues. Formalin is the most widely used chemical agent for preserving tissue specimens. This chemical treatment stabilizes tissue, making it suitable for subsequent analytical procedures and long-term study.
The Impermanence of Unfixed Tissue
Biological tissues, once removed from a living organism, begin to degrade rapidly due to natural processes. Two primary mechanisms contribute to this deterioration: autolysis and putrefaction.
Autolysis refers to the self-digestion of cells by their own internal enzymes, which break down cellular components. This enzymatic activity quickly destroys delicate cellular structures. Putrefaction involves the decomposition of tissue by microorganisms, such as bacteria and fungi. These microorganisms proliferate, breaking down organic compounds within the tissue. The combined effects of autolysis and putrefaction lead to significant alterations in tissue morphology, compromising the sample’s suitability for scientific investigation.
Formalin’s Chemical Action
Formalin is an aqueous solution of formaldehyde. Its main mechanism as a fixative involves reacting with and cross-linking proteins within the tissue. Formaldehyde molecules are small and reactive, allowing them to penetrate tissue rapidly.
Upon entering the tissue, formaldehyde reacts with functional groups on protein amino acid residues. The most significant reaction is the formation of methylene bridges. Formaldehyde molecules link two different protein molecules with a covalent bond, forming a stable, insoluble three-dimensional network of cross-linked proteins.
This extensive cross-linking stabilizes proteins, preventing their movement and degradation by enzymes or microbes. This stable protein meshwork maintains the structural integrity of cellular components. The process allows for even penetration and fixation throughout the tissue, preserving its delicate architecture.
Stabilizing Cellular Structures
The extensive cross-linking of proteins by formalin stabilizes cellular and tissue structures. This action preserves the intricate architecture of cellular organelles, such as mitochondria and nuclei, by immobilizing their protein components. Cell membranes also become stabilized as their associated proteins are cross-linked, maintaining cell boundaries and shapes.
Formalin also impacts the extracellular matrix, the complex network of proteins and carbohydrates that surrounds cells. Proteins within this matrix, such as collagen and elastin, are cross-linked, contributing to tissue rigidity. This stabilization inhibits endogenous enzymes, halting autolysis. The altered chemical environment and physical barrier prevent bacterial and fungal proliferation, stopping putrefaction.
Fixation also makes tissue firmer and more resilient. This increased rigidity is beneficial for subsequent processing steps, such as sectioning, where thin slices are cut for microscopic examination. The stabilization ensures the tissue remains intact and its morphology is preserved.
Essential for Medical Science
Formalin fixation plays an important role in histology, the microscopic study of tissues, and pathology, the diagnosis of diseases. By preserving tissue samples, formalin enables their long-term storage, providing a permanent record for future analysis. This stability is important for accurate diagnosis, allowing pathologists to examine specimens for signs of disease, such as cancer or inflammation.
Fixed tissues can undergo further processing, including embedding in paraffin wax, sectioning into thin slices, and staining with various dyes. These steps enhance the visibility of cellular components and abnormalities under a microscope. The ability to preserve and analyze tissue over extended periods is important for medical research, contributing to understanding disease mechanisms and developing new treatments.