Reflections on the Principles of Fixation in Histochemistry and Cytochemistry: With a Special Focus on Immunohistochemistry and In Situ Hybridization.

Abstract

In the field of histochemistry and cytochemistry (histocytochemistry), fixation is a critical process for preserving biological structures and enabling accurate analysis. Fixation methods, broadly categorized into precipitating and cross-linking techniques, stabilize biomolecules such as proteins, sugars (carbohydrates) and nucleic acids, although lipids often require specific handling due to the loss during a routine procedure. Traditional staining methods have evolved into advanced techniques like immunohistochemistry (IHC) and in situ hybridization (ISH), which allow for precise analysis of the expression of specific molecules. IHC employs antibodies to visualize specific antigens, with fixation playing a vital role in maintaining antigen integrity. However, excessive fixation can mask epitopes, requiring antigen retrieval techniques to restore antigenicity. Microwave-induced retrieval, for instance, enhances staining efficacy while introducing further fixation by promoting molecular interactions. ISH, which targets nucleic acids with specific base sequences, is also sensitive to fixation conditions. Formaldehyde-based fixatives react variably with purines and pyrimidines, affecting hybridization efficiency with a probe. Positive controls like 28S rRNA help to standardize ISH across facilities, ensuring reproducible and reliable results. Variability in fixation protocols among institutions brings fatal defects in achieving consistent results. Shared standards or the use of robust controls can alleviate these issues, enhancing the accuracy and reliability of histocytochemical analyses for both research and clinical applications.