What is Formalin?
Formalin is a solution of formaldehyde in water, typically with a concentration of 10%. It is widely used in
histology for the preservation and fixation of biological tissues. The solution may also contain small amounts of methanol to prevent polymerization.
Why is Formalin Used in Histology?
Formalin is used in histology primarily because it effectively preserves the tissue's cellular structure and morphology. It cross-links proteins, which helps in maintaining the architecture of the tissue, making it suitable for microscopic examination. Additionally, it is relatively inexpensive and has a long shelf life.
How Does Formalin Work?
Formalin works by forming cross-links between amino acids in proteins, particularly lysine residues. This process, known as
cross-linking, stabilizes the tissue and prevents it from decaying. The cross-links also render the proteins insoluble, which helps in maintaining the structural integrity of the tissue during subsequent processing and staining.
How is Formalin Prepared and Stored?
Formalin is typically prepared as a 10% solution of formaldehyde in water, often referred to as 10% neutral buffered formalin (NBF). The buffering agents, usually sodium phosphate, help maintain a neutral pH, which minimizes tissue damage and preserves antigenicity. Formalin should be stored in a cool, dark place to prevent degradation and to avoid the formation of formic acid, which can damage tissues.
What Are the Safety Concerns?
Formalin is a hazardous chemical and should be handled with care. It is a potent
carcinogen and can cause respiratory irritation, skin burns, and eye damage. Proper
safety protocols must be followed, including the use of personal protective equipment (PPE) such as gloves, lab coats, and goggles. Work with formalin should be conducted in a well-ventilated area, preferably under a fume hood.
What Are the Alternatives to Formalin?
Although formalin is widely used, there are alternative
fixatives available that may be suitable for specific applications. Some common alternatives include alcohol-based fixatives, Bouin's solution, and glutaraldehyde. Each alternative has its own advantages and disadvantages, and the choice of fixative may depend on the specific requirements of the study, such as the preservation of nucleic acids, antigens, or fine cellular structures.
How is Formalin Used in Tissue Processing?
In tissue processing, formalin-fixed tissues are typically dehydrated through a series of alcohol baths, cleared with a solvent such as xylene, and embedded in paraffin wax. This process allows for the preparation of thin sections that can be mounted on slides and stained for microscopic examination. Proper fixation is crucial for obtaining high-quality histological sections and accurate diagnostic results.
Limitations of Formalin Fixation
While formalin is highly effective for preserving tissue morphology, it does have limitations. Formalin fixation can mask some antigens, making immunohistochemical staining more challenging. Additionally, prolonged fixation can lead to over-hardening of tissues, making them difficult to section. It is important to optimize fixation times and consider the specific requirements of the study to achieve the best results.
