What is a Fixative?
A fixative is a chemical substance used to preserve biological tissues from decay, thereby maintaining their structure and composition. The primary purpose of a fixative is to stabilize the tissues, preventing any autolysis, putrefaction, or other post-mortem changes.
Aldehyde-Based Fixatives
Aldehyde-based fixatives are the most commonly used fixatives in histology.
Formaldehyde and
glutaraldehyde are the primary representatives of this group.
Formaldehyde: Typically used in the form of 10% neutral buffered formalin (NBF), it is widely used for routine tissue fixation. It penetrates tissues quickly and preserves the general structure well.
Glutaraldehyde: More commonly used for electron microscopy due to its ability to preserve fine cellular details. It penetrates tissues more slowly compared to formaldehyde but offers superior cross-linking properties.
Alcohol-Based Fixatives
Alcohol-based fixatives, such as
ethanol and
methanol, are used to precipitate proteins and are especially useful for cytological smears.
Ethanol: Commonly used for fixing blood smears and cytological preparations. It preserves nucleic acids well but can cause tissue shrinkage.
Methanol: Often used for fixing blood films and bone marrow smears. It is effective but can be more toxic compared to ethanol.
Potassium Dichromate: Used in combination with other fixatives, it is beneficial for preserving the mitochondria and other organelles.
Osmium Tetroxide: Primarily used for electron microscopy, it preserves lipids and offers excellent membrane contrast, but it is highly toxic and volatile.
Mercurials
Mercurial fixatives, such as
B-5 fixative, contain mercury and are effective at preserving fine nuclear and cytoplasmic details. However, their use has decreased due to toxicity and environmental concerns.
B-5 Fixative: Comprises mercuric chloride and is traditionally used for lymphoid and hematopoietic tissues.
Picrates
Picrate-based fixatives are derived from
picric acid. A well-known example is Bouin's solution, which is excellent for preserving soft and delicate tissues.
Bouin's Solution: Contains picric acid, formaldehyde, and acetic acid. It offers good tissue penetration and excellent preservation of soft tissues but can cause some tissue shrinkage.
Choosing the Right Fixative
The choice of fixative depends on various factors, including the type of tissue, the intended staining technique, and the specific structures to be examined. Each fixative has its own advantages and limitations, so selecting the appropriate one is crucial for obtaining accurate and reliable results.Conclusion
Understanding the different types of fixatives and their applications is fundamental in histology. Proper fixation is the cornerstone of successful tissue analysis and plays a critical role in the accurate interpretation of histological samples.
