Fixatives cross-linking

As a gold standard, fresh tissue prepared by snap-frozen method, cut by cryostat, and fixed in acetone, ethanol, or other non-cross-linking fixatives, has been generally accepted as reliable. 

Although formaldehyde penetrates very quickly, its protein cross-linking fixative effects are not as immediate. Modifications to this and improvements on the model used originally in studies by Baker et al.,7 Fox et al.,8 and Helander9 have demonstrated that in tissue and in these more complex models, the penetration and therefore Medawar s constant for formaldehyde is not as great as 5.5 and is probably more like 3.6.7... 

Some fixatives work by combining with tissue molecules, hence the term addition reactions. This may continue as cross-linking, whereby the original adducted (added-onto) molecule attaches to another portion of the same molecule or to an adjacent molecule. A small branched polymer is thus created. Formaldehyde is the prime example of an additive and cross-linking fixative. 

The slide preparation is fixed by using either precipitating fixatives such as ethanokacetic acid or cross-linking fixatives such as paraformaldehyde. They are further treated to remove protein with a mixture that includes proteinase K. Removal of proteins facilitates the access of probe to the sample DNA target. 

One should avoid the use of highly cross-linking fixatives, such as mercuric chloride, glutaraldehyde, modified formalin, and picric-acid-based fixatives. These extensively cross-linking fixatives render the tissue virtually impermeable to the RT-PCR reaction components and to the probe. 

In the second stage, cross-link formation triggers phase separation, and further cross-link fixes the unevenly localized and deformed shape of particle. Thus, the red-cell-like particles are prepared during polymerization and not formed during isooctane evaporation. The red-cell-like particles find practically use for controlling rheological and optical characteristics in the coating and paint industry. 

Use of inappropriate fixative. Use of certain fixatives may damage or destroy antigens or epitopes in the tissue specimen. Use of non-cross linking fixatives may allow the elution of antigens soluble in IHC reagents. Check manufacturer s specifications regarding recommended fixative. 29-33... 

Cross-linking of protein in situ avoids leaching out of proteins that may diffuse in water or alcohol. Many low-molecular-weight antigens (peptides) are extracted by non-cross-linking fixatives such as alcohol, or methanol-based solutions, but they are well preserved in tissue by formalin. 

Formaldehyde (CH2O) - a cross-linking fixative the best fixative for light microscopic immunocytochemistry. 

Fresh frozen tissue - samples with no fixation are frozen and sectioned gives distorted the morphology because no cross linking fixative and components of the tissue can wash out of the section. 

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