In the process of making biological sections, in order to avoid tissue autolysis or damage by bacteria, and preserve its physical structure, once the tissue block is taken out of the living body, it should be immediately and properly disposed of. The treatment is fixation, usually by soaking the tissue in chemicals to preserve its morphology and chemical characteristics as much as possible.  The chemicals used to immobilize tissues are called fixatives. So how are they classified? 

According to the composition of the fixing solution, it can be divided into single fixation, double fixation and multiple fixation.

First, single fixation is only fixed with glutaraldehyde or starvation tetroxide, generally used for single-cell samples or permeable samples.

Second, double fixation refers to the first use of glutaraldehyde or paraformaldehyde for pre-fixation, and then use osmium tetroxide for post-fixation, this method has good effect and is widely used.

Third, multiple fixation refers to the combined use of three kinds of fixatives to achieve the purpose of complementing each other. 

The main purpose of this process is to truly preserve every detail of the ultrastructure of the cell at the molecular level, so that the slice structure is clearer and the morphology is more complete.