User:J dotter/Fixation (histology)

Types
There are generally three types of fixation processes depending on the initial specimen:

Heat fixation: After a smear has dried at room temperature, the slide is gripped by tongs or a clothespin and passed through the flame of a Bunsen burner several times to heat-kill and adhere the organism to the slide. Routinely used with bacteria and archaea. Heat fixation generally preserves overall morphology but not internal structures. Heat denatures the proteolytic enzyme and prevents autolysis. Heat fixation cannot be used in the capsular stain method as heat fixation will shrink or destroy the capsule (glycocalyx) and cannot be seen in stains.

Immersion: The sample of tissue is immersed in fixative solution. The fixative solution must have a volume at least 20 times greater than the volume of the tissue. In order for fixation to be successful, the fixative must diffuse throughout the entire tissue, so tissue size and density, as well as type of fixative must be considered. This is a common technique for cellular applications. Using a larger sample means it must be immersed longer for the fixative to reach the deeper tissue. [4]

Perfusion: Perfusion is the passage of fluid through the blood vessels or natural channels of an organ or organism. In tissue fixation via perfusion, the fixative is pumped into the circulatory system, usually through a needle inserted into the left ventricle. This can be done via ultrasound guidance, or by opening the chest cavity of the subject. [1] The fixative is injected into the heart with the injection volume matching the typical cardiac output. Using the innate circulatory system, the fixative is distributed throughout the entire body, and the tissue doesn't die until it is fixed. When this method is used, a drainage port must also be added somewhere in the circulatory system to account for the addition of the volume of the fixative and buffer. The fixative is pumped into the circulatory system until it has replaced all of the blood. Using perfusion has the advantage of preserving morphology [2], but the disadvantages are that the subject dies and the cost of the volume of fixative needed for larger organisms is high. It is possible to decrease the necessary volume of fluid to perform a perfusion fixation by pinching off arteries that feed tissues not of interest to the research involved. Perfusion fixation is commonly used to image brain, lung, and kidney tissues in rodents, although it is also used in performing autopsies in humans [4].