Please try the following: If you typed the page address in the Address bar, make sure that it is spelled correctly. home page, and then look for links to the information you want. Click the Back button to try another link. HTTP 404 - File not found Technical Information (for support personnel)The purpose of fixation is to preserve tissues permanently in as life-like a state as possible. Fixation should be carried out as soon as possible after removal of the tissues (in the case of surgical pathology) or soon after death (with autopsy) to prevent autolysis. There is no perfect fixative, though formaldehyde comes the closest. Therefore, a variety of fixatives are available for use, depending on the type of tissue present and features to be demonstrated. There are five major groups of fixatives, classified according to mechanism Aldehydes include formaldehyde (formalin) and glutaraldehyde. Tissue is fixed by cross-linkages formed in the proteins, particularly between lysine residues.

This cross-linkage does not harm the structure of proteins greatly, so that antigenicity is not lost. Therefore, formaldehyde is good for immunohistochemical techniques. Formalin penetrates tissue well, but is relatively slow. The standard solution is 10% neutral buffered formalin. A buffer prevents acidity that would promote autolysis and cause precipitation of formol-heme pigment in the tissues. Glutaraldehyde causes deformation of alpha-helix structure in proteins so is not good for immunohistochemical staining. However, it fixes very quickly so is good for electron microscopy. It penetrates very poorly, but gives best overall cytoplasmic and nuclear detail. The standard solution is a 2% buffered glutaraldehyde Mercurials fix tissue by an unknown mechanism. chloride and include such well-known fixatives as B-5 and Zenker's. These fixatives penetrate relatively poorly and cause some tissue hardness, but are fast and give excellent nuclear detail. Their best application is for fixation of hematopoietic and reticuloendothelial tissues.

Since they contain mercury, they must be disposed of carefully. Alcohols, including methyl alcohol (methanol) and ethyl alcohol (ethanol), are protein denaturants and are not used routinely for tissues because they cause too much brittleness and hardness. However, they are very good for cytologic smears because they act quickly and give good nuclear detail. Spray cans of alcohol fixatives are marketed to physicians doing PAP smears, but cheap hairsprays do just as well. Oxidizing agents include permanganate fixatives (potassium permanganate), dichromate fixatives (potassium dichromate), and osmium tetroxide. They cross-link proteins, but cause extensive denaturation. Some of them have specialized applications, but are used very infrequently. Picrates include fixatives with picric acid. Foremost among these is Bouin's solution. It has an unknown mechanism of action. It does almost as well as mercurials with nuclear detail but does not cause as much hardness. Picric acid is an explosion hazard in dry form.

As a solution, it stains everything it touches yellow, including skin. There are a number of factors that will affect the fixation process: Fixation is best carried out close to neutral pH, in the range of 6-8. Hypoxia of tissues lowers the pH, so there must be buffering capacity in the fixative to prevent excessive acidity. buy vacuum cleaner shanghaiAcidity favors formation ofbosch cylinder vacuum cleaners formalin-heme pigment that appears as black, polarizable deposits in tissue. vacuum cleaners asbestosCommon buffers include phosphate, bicarbonate, cacodylate, and veronal. Commercial formalin is buffered with phosphate at a pH of 7. Penetration of tissues depends upon the diffusability of each individual fixative, which is a constant.

Formalin and alcohol penetrate the best, and glutaraldehyde the worst. Mercurials and others are somewhere in between. way to get around this problem is sectioning the tissues thinly (2 to 3 mm). Penetration into a thin section will occur more rapidly than for a thick section. The volume of fixative is important. There should be a 10:1 ratio of fixative to tissue. Obviously, we often get away with less than this, but may not get ideal fixation. One way to partially solve the problem is to change the fixative at intervals to avoid exhaustion of the fixative. Agitation of the specimen in the fixative will also enhance fixation. Increasing the temperature, as with all chemical reactions, will increase the speed of fixation, as long as you don't cook the tissue. Hot formalin will fix tissues faster, and this is often the first step on an automated tissue processor. Concentration of fixative should be adjusted down to the lowest level possible, because you will expend less money for the fixative.

Formalin is best at 10%; glutaraldehyde is generally made up at 0.25% to 4%. concentration may adversely affect the tissues and produce artefact similar to excessive heat. Also very important is time interval from of removal of tissues to fixation. The faster you can get the tissue and fix it, the better. Artefact will be introduced by drying, so if tissue is left out, please keep it moist with saline. The longer you wait, the more cellular organelles will be lost and the more nuclear shrinkage and artefactual clumping will occur. There are common usages for fixatives in the pathology laboratory based upon the nature of the fixatives, the type of tissue, and the histologic details to be demonstrated. Formalin is used for all routine surgical pathology and autopsy tissues when an H and E slide is to be produced. Formalin is the most forgiving of all fixatives when conditions are not ideal, and there is no tissue that it will harm significantly. Most clinicians and nurses can understand what formalin is and does and it smells bad enough that they are careful handling it.

Zenker's fixatives are recommended for reticuloendothelial tissues including lymph nodes, spleen, thymus, and bone marrow. Zenker's fixes nuclei very well and gives good detail. However, the mercury deposits must be removed (dezenkerized) before staining or black deposits will result in the sections. Bouin's solution is sometimes recommended for fixation of testis, GI tract, and endocrine tissue. It does not do a bad job on hematopoietic tissues either, and doesn't require dezenkerizing before staining. Glutaraldehyde is recommended for fixation of tissues for electron microscopy. The glutaraldehyde must be cold and buffered and not more than 3 months old. The tissue must be as fresh as possible and preferably sectioned within the glutaraldehyde at a thickness no more than 1 mm to enhance fixation. Alcohols, specifically ethanol, are used primarily for cytologic smears. Ethanol (95%) is fast and cheap. Since smears are only a cell or so thick, there is no great problem from shrinkage, and since smears are not sectioned,