Fixation (histology)

Lore & Background

There are generally three types of fixation processes. Heat fixation is used for single-cell organisms like bacteria and archaea, where a smear is dried and passed through a flame to heat-kill and adhere the organism. Immersion fixation involves submerging tissue in fixative solution, which must have a volume at least 10 times greater than the tissue. Perfusion fixation pumps fixative through the circulatory system, often via the left ventricle, preserving morphology but requiring the subject's death and potentially high volumes of fixative.

Reader's Guide

Fixation (histology) is significant because it enables the study of tissue structure by preventing decay and stabilizing macromolecules. Fixatives denature proteins by coagulation, forming additive compounds, or cross-linking—a process that combines parts of two different macromolecules for effective stabilization. Even careful fixation alters samples and introduces artifacts, such as the bacterial mesosome, which was misinterpreted as an organelle in the 1970s but later shown to be an artifact of chemical fixation. Standardization of fixation procedures helps researchers anticipate artifacts and choose techniques that minimize them in areas of interest. The legacy of fixation lies in its foundational role in histology, pathology, and cell biology, allowing reproducible analysis of tissues and cells across diverse applications.

Did You Know?

• Fixatives denature proteins by coagulation, forming additive compounds, or by a combination of coagulation and additive processes.
• The bacterial mesosome was thought to be an organelle in gram-positive bacteria in the 1970s but was later shown to be an artifact of chemical fixation.
• Perfusion fixation uses the circulatory system to distribute fixative throughout the entire body; the tissue dies upon contact with the fixative, not after fixation is complete.
• Heat fixation cannot be used in the capsular stain method because heat will shrink or destroy the capsule (glycocalyx).