Fixation and Dehydration (Histology) 2024 PDF

‫المعهد العالي للعلوم والتقنية‬ ‫زليتن‬ ‫قسم التقنيات الطبية‬ ‫شعبة المختبرات الطبية‬ ‫‪HISTOTECHNOLOGY‬‬ ‫‪Fixation and dehydration‬‬ ‫أعداد‪ :‬أ‪.‬حمزة الطاهر أبوراوي‬ ‫الفصل الدراسي‪ :‬خريف‬ ‫‪2024‬م‬  Obta...

‫المعهد العالي للعلوم والتقنية‬ ‫زليتن‬ ‫قسم التقنيات الطبية‬ ‫شعبة المختبرات الطبية‬ ‫‪HISTOTECHNOLOGY‬‬ ‫‪Fixation and dehydration‬‬ ‫أعداد‪ :‬أ‪.‬حمزة الطاهر أبوراوي‬ ‫الفصل الدراسي‪ :‬خريف‬ ‫‪2024‬م‬  Obtaining a fresh specimen Fresh tissue specimens will come from various.sources It should be noted that they can very easily be damaged during removal from the patient or.experimental animal It is important that they are handled carefully and appropriately fixed as soon as possible.after dissection Ideally, fixation should take place at the site of removal, perhaps in the operating theater, or,  Fixation he specimen is placed in a liquid fixing agent (fixative).such as formaldehyde solution (formalin) This will slowly penetrate the tissue causing chemical and physical changes that will harden and preserve the tissue and protect it against subsequent processing.steps There are a limited number of reagents that can be used for fixation as they must possess particular.properties that make them suitable for this purpose For example, tissue components must retain some chemical reactivity so that specific staining techniques.can be applied subsequently Formalin, usually as a phosphate-buffered solution, is the most popular fixative for preserving tissues that will be.processed to prepare paraffin sections Ideally, specimens should remain in fixative for long enough for the fixative to penetrate into every part of the tissue and then for an additional period to allow the chemical reactions of fixation to reach equilibrium (fixation.time) Generally, this will mean that the specimen should fix for.between 6 and 24 hours Most laboratories will use a fixative step as the first station.on their processor Following fixation, the specimens may require further dissection to select appropriate areas for.examination Specimens that are to be processed will be placed in suitably labeled cassettes (small perforated baskets) to segregate them from other.specimens The duration of the processing schedule used to process the specimens will depend on the type and dimensions of the largest and smallest specimens, the particular processor employed, the solvents chosen, the solvent temperatures,.and other factors  Dehydration Because melted paraffin wax is hydrophobic (immiscible with water), most of the water in a specimen must be removed before it can be.infiltrated with wax This process is commonly carried out by immersing specimens in a series of ethanol (alcohol) solutions of increasing concentration.until pure, water-free alcohol is reached Ethanol is miscible with water in all proportions so that the water in the specimen is progressively.replaced by the alcohol A typical dehydration sequence for specimens not :more than 4mm thick would be ethanol 15 min 70% -1 ethanol 15 min 90% -2 ethanol 15 min 100% -3 ethanol 15 min 100% -4 ethanol 30 min 100% -5 ethanol 45 min 100% -6 At this point, all but a tiny residue of tightly bound (molecular) water should have been removed from the.specimen