Histology Fixation Lecture Notes

A comprehensive set of vocabulary flashcards based on lecture notes covering the chemistry, actions, and types of histological fixatives, including detailed information on formaldehyde and acetic acid.

Fixative

A substance that transforms soluble cell components into insoluble substances to prevent their loss during processing and to prevent autolysis and putrefaction.

Denaturation

The change in the 3D structure of macromolecules; in enzymes, this change results in a loss of enzymatic activity.

Autolysis

The decomposition of tissues by their own enzymes, which is stopped by the shape-changing effects of fixation.

Putrefaction

The decomposition of tissue caused by bacterial content, often contributing to the desquamation of intestinal epithelium.

Refractive index

The ratio of the velocity of light in air to the velocity of light in a liquid or solid medium.

Antigen retrieval

A process used in IHC to reverse the masking of antigenic sites caused by fixation.

Heat fixation

A method of physical fixation typically performed in a microwave using non-ionizing radiation to oscillate polar molecules and produce instantaneous heat.

Two-stage microwave fixation

A method where large specimens are first fixed in saline for grossing, followed by heating $2\,\text{mm}$ thick blocks in saline to a temperature of $45\,^∘\text{C}$ to $68\,^∘\text{C}$.

Desiccation

A method of physical fixation involving air drying, typically used for touch preps for Wright staining.

Additive fixatives

Chemical fixatives that link physically or chemically to the tissue and change it, such as mercuric chloride, formaldehyde, and glutaraldehyde.

Non additive fixatives

Fixatives that act on tissue without chemically combining with it, primarily by dissociating water molecules; examples include alcohol, acetone, and acetic acid.

Coagulant fixatives

Reagents that create a tissue network allowing solutions to easily penetrate, such as mercuric chloride, chromic acid, and picric acid.

Non-coagulant fixatives

Reagents that create a gel in the tissue that makes solution penetration difficult, such as formaldehyde, glutaraldehyde, and osmium tetroxide.

Ischemic time

The interval between the interruption of the blood supply and the placement of the specimen into fixative.

Nuclear bubbling

An artifact caused by incomplete fixation of a specimen before dehydration begins.

Volume ratio (Fixation)

The requirement that the fixative volume be $15$ to $20$ times greater than the tissue volume.

Post fixation (Mordanting)

The treatment of a tissue section that has already been fixed with one reagent with a different fixative reagent.

Medawar constant ($k$)

The coefficient of diffusability used in the penetration formula $d = k(t)^{1/2}$, where $d$ is depth and $t$ is time.

Osmolality

The number of particles in solution, defined as one formula weight of a solute per $1000\,\text{g}$ of solution.

Hypertonic

A solution with a high amount of dissolved particles that causes a cell to shrink when placed within it.

Hypotonic

A solution with a low amount of dissolved particles that causes a cell to swell or burst; $10\%$ aqueous formalin is an example.

Michel transport solution

A solution used for kidney biopsy specimens that require storage for IHC.

Primary structure of a protein

The amino acid sequence of the protein.

Secondary structure of a protein

The shape of the polypeptide chain, such as an alpha helix or beta pleated sheet, held together by hydrogen bonds.

Tertiary structure of a protein

The 3D shape of the protein, determined by the R groups of the amino acids.

Glacial acetic acid

The concentrated form of acetic acid ($\text{CH}_3\text{COOH}$); it lyses red blood cells and is often used in compound fixatives to counteract shrinking.

Paraformaldehyde

A larger insoluble form of formaldehyde that appears as a white powder; it is used in electron microscopy after depolymerization to yield a pure solution.

Methylene bridges

Cross-links formed after the initial addition step of formaldehyde to complete the fixation of tissue.

Formalin pigment

A dark brown, microcrystalline, birefringent pigment formed when the pH of formalin drops below $6.0$ or when it is exposed to bloody tissue.

Calcium formalin

A fixative recommended for the preservation of phospholipids, containing formaldehyde, di water, and calcium chloride.

Formalin ammonium bromide

An acidic fixative recommended for central nervous system tissues and the Cajal gold special stain.

10% Neutral Buffered Formalin (NBF)

The most common routine fixative, containing sodium phosphate (monobasic and dibasic) to maintain a pH of $6.8$ to $7.2$.