It is a process by which the cells or tissues are fixed in chemical and partly
physical state so that they can withstand subsequent treatment with various
reagents, with minimal distortion of morphology and no decomposition

 AIMS OF FIXATION:-

(a) To preserve the tissues as close to their living state as possible

 (b) To prevent autolysis and bacterial attack 

(c) To prevent tissues from changing their shape and size during processing 

(d) To harden the tissues 

(e) To allow clear staining of sections subsequently 

(f) To improve the optical differentiation of cells & tissue

 PRINCIPLE OF FIXATION:-

Fixation results in denaturation and coagulation of protein in the tissues. The
fixatives have a property of forming cross links between proteins, thereby
forming a gel, keeping everything in their in vivo relation to each other.

PROPERTIES OF FIXATIVES AND FACTORS AFFECTING FIXATION:-

1. Coagulation and precipitation of proteins in tissues. 

2. Penetration rate differs with different fixatives depending on the molecular weight of the fixative 

3. pH of fixatives – Satisfactory fixation occurs between pH 6 and 8. Outside this range, alteration in structure of cell may take place. 

4. Temperature – Room temperature is alright for fixation. At high temperature there may be distortion of tissues.

 5. Volume changes – Cell volume changes because of the membrane permeability and inhibition of respiration. 

6. An ideal fixative should be cheap, nontoxic and non-inflammable. The tissues may be kept in the fixative for a long time

TYPE OF FIXATION:-

Immersion fixation
Perfusion fixation
Vapour fixation
Coating/Spray fixation
Freeze drying
Microwave fixation/Stabilization
The most commonly used technique is simple immersion of tissues/smears in an excess of fixative. For all practical purposes immersion fixatives are most useful. These may be divided into routine and special

SIMPLE FIXATIVES :-