- Fixative
- A medium such as a solution or spray that preserves specimens of tissues or cells. Most biopsies and specimens removed at surgery are fixed in a solution such as formalin (dilute formaldehyde) before further processing takes place. Other common ingredients used in fixatives are alcohol, mercuric chloride, potassium dichromate and sodium sulfate. "Fixative" is derived from the Latin "figere" (to fix, fasten, make stable). Related English words include "fixture" (that which remains stable and in place) and "fixity" (state of being stable, steady, permanent).
* * *1. Serving to fix, bind, or make firm or stable. 2. A substance used for the preservation of gross and histologic specimens of tissue, or individual cells, usually by denaturing and precipitating or cross-linking the protein constituents. SEE ALSO: fluid, solution.- acetone f. acetone used at low temperatures to fix enzymes, particularly phosphatases; it removes fat and glycogen.- AFA f. a combination of alcohol, formalin, and acetic acid used for the fixation of nematodes, trematodes, and cestodes.- Bouin f. a solution of glacial acetic acid, formalin, and picric acid, useful for soft and delicate tissues (as those of embryos) and small pieces of tissues; it preserves glycogen and nuclei and permits brilliant staining, but penetrates slowly, distorts kidney tissue and mitochondria, and does not permit Feulgen stain for DNA.- Carnoy f. ethanol, chloroform, and acetic acid (6:3:1) or ethanol and acetic acid (3:1), an extremely rapid f. used for glycogen preservation and as a nuclear f..- Champy f. a mixture of potassium bichromate, chromic acid, and osmic acid, considered an excellent cytologic f. with advantages and disadvantages similar to those of Flemming's f.; it differs from Flemming f. in substituting bichromate for acetic acid.- Flemming f. a mixture of chromic acid, osmic acid, and acetic acid that makes an excellent cytoplasmic and chromosomal f., especially when acetic acid is omitted; disadvantages are that it penetrates poorly, requires lengthy washing, and deterioriates rapidly.- formaldehyde f. a widely used fixing agent for pathologic histology; the commercial solution is 37–40% formaldehyde and is known as 100% formalin or formol; a common impurity is formic acid, which must be neutralized or the f. made in buffer solution; tissues fixed may have a pigment artifact precipitated.- formol-calcium f. a f. for preservation of lipids.- formol-Müller f. Müller f. containing 2% commercial formalin.- formol-saline f. a general f. for histologic and histochemical preparations.- glutaraldehyde f. a f. used in phosphate or cacodylate buffer for electron microscopy, and as a chromatin and enzyme f.; may be used preceding osmic acid as a second f. to add membrane preservation for electron microscopy.- Golgi osmiobichromate f. an osmic-bichromate mixture used to demonstrate nerve cells and their processes.- Helly f. a combination of potassium dichromate, mercuric chloride, formaldehyde, and distilled water, used as a microanatomic f. for cytoplasmic granules and nuclear staining; has the same disadvantages as Zenker f..- Kaiserling f. a method of preserving histologic and pathologic specimens without altering the color, by immersing them in an aqueous solution of potassium nitrate, potassium acetate, and formalin.- Luft potassium permanganate f. a f. useful in electron microscopy for cytologic preservation of lipoprotein complexes in membranes and myelin, because of its oxidative properties.- Marchi f. a mixture of Müller f. with osmium tetroxide, with potassium chlorate substituted for the potassium dichromate of Müller f. for better results; used to demonstrate degenerating myelin. SEE ALSO: Marchi stain.- Müller f. a hardening f. composed of potassium dichromate, sodium sulfate, and distilled water, similar to Regaud f..- neutral buffered formalin f. a general histologic f. less likely to leave formalin deposits in tissue than formol-saline f..- Newcomer f. a f. containing isopropanol, propionic acid, and dioxane, recommended as a substitute for Carnoy f. in preservation of chromatin; also useful for fixing polysaccharides; small pieces of tissue must be used, although excessive shrinkage may still occur.- Orth f. formalin added to Müller f., used for bringing out chromaffin, studying early degenerative processes and necrosis, and for demonstrating rickettsiae and bacteria.- osmic acid f. a f. used alone in buffer or as a postfixative after a glutaraldehyde f. in electron microscopy; an excellent membrane f. but a poor preservative of chromatin.- Park-Williams f. a f. for spirochetes, comprised of a 2% solution of osmic acid to the fumes of which the bacteria are exposed for a few seconds.- PVA f. schaudinn f. using either a mercuric chloride, zinc sulfate, or copper sulfate base; contains polyvinyl alcohol plastic powder that is used as an adhesive for fecal specimens in the preparation of permanent smears for subsequent staining.- Regaud f. a f. containing formaldehyde and sodium dichromate, used to preserve mitochondria but not fat; requires afterchroming and extensive washing.- SAF f. sodium acetate-acetic acid-formalin mixture used to fix fecal specimens for subsequent concentration and staining of smears.- Schaudinn f. a solution of mercuric chloride, sodium chloride, alcohol, and glacial acetic acid, used on wet smears for cytologic fixation.- single vial fixatives proprietary and commercially available solutions used for stool fixation; from the single vial, a concentration, permanent stain, and some immunoassay procedures can be performed.- Thoma f. nitric acid in 95% alcohol, used for decalcifying bone in the preparation of histologic specimens.- Zenker f. a rapid f. consisting of mercuric chloride, potassium dichromate, sodium sulfate, glacial acetic acid, and water, useful for trichrome stains; must be washed to remove potassium dichromate and treated with iodine solution to remove mercuric chloride; tissues tend to become brittle if left in the f. for more than 24 hours.
* * *
fix·a·tive 'fik-sət-iv n a substance used to fix living tissue* * *
n.a chemical agent, e.g. alcohol or osmium tetroxide, used for the preservation and hardening of tissues for microscopical study. See fixation.* * *
fix·a·tive (fikґsə-tiv) a fluid, often a mixture of several reactive chemicals, into which histological or cytological specimens are placed so that, by processes such as denaturation and cross-linking of proteins, autolysis is prevented, the specimen is hardened to withstand further processing, and the specimen is preserved in a close facsimile of the living state in regard to both cellular morphology and the location of subcellular constituents.
Medical dictionary. 2011.
