WHAT IS FIXATION??????? GIVE THE DETAIL OF DIFFERENT FIXATIVES USE FOR LIGHT MICROSCOPE SUBMITTED BY:SUBMITTED TO: 11-arid-959 M.UMARDr. RIAZ HUSSAIN PASHA 11-arid-975 Waqas nawaz
Fixation• The decisive step in obtaining a permanent microscopic slide• A physical or chemical process that stops as soon as possible vital cellular processes, maintaining with minimal alteration the shape, volume, spatial and molecular relation between elements
Fixation• By fixation we intend: 1. To conserve the tissue from autolysis and bacterial attack 2. To prevent the loss of cellular constituents 3. To increase optical differentiation of cellular structures 4. To increase tissue consistency, in order to facilitate their going through the other steps of the technique – especially slicing
Fixatives classification• Mixtures of fixatives: – By chemical nature: • Aqueous • Alcoholic – By purpose: • Universal / topographic • Cytologic
Fixation mechanism• Formes cross-links between proteins, thereby forming a gel – keeps structures in their in vivo relations to one another• Soluble proteins are fixed to structural proteins – insoluble → gives mechanical strength for next steps• Protein denaturation
Qualities of a good fixative• Good tissue penetration• Stabilizes the tissue, preserving the character and distribution of cellular components• Prevents fixation artefacts• Prevents structure deformation – maintaining shape and volume• Preservs cellular constituents
Qualities of a good fixative• Destroys microorganisms• Extracts inactivated autolytic enzymes• Increases tissue consistancy• Confers optical differentiation• Maintains its chemical composition• Cheap, nontoxic, nonflamable, nonirritant
Post-fixation treatments• Decalcification – for bone• Sample dissociation• Mordansation – helps both fixation and staining
FIXATIONSamples should be fixed immediately after they are removed from the body.Fixation (conservation) is used to:- terminate cell metabolism- prevent AUTOLYSIS (degradation of tissues by own enzymes)- kill pathogenic microorganisms (prevent decomposition of tissue by bacteria, fungi or viruses)- harden tissue as a result of their cross-linking or denaturing protein moleculesRequirements:- rapid penetration of fixative into the tissue- preservation of the structure- maintenance of the affinity for dyes (stainability)PHYSICAL FIXATIONHeat, microwave, freezing
CHEMICAL FIXATIONSubmerging samples in the fluid fixative (immersion-fixation).Perfusion by the fixative (perfusion-fixation in an experimental use).Some fixatives promote cross-linking of proteins (formaldehyde, glutaraldehyde),the other precipitation of proteins (picric acid, mercuric dichloride).
TYPES OF CHEMICAL FIXATIVESFORMALDEHYDE fixativesFormalin – trade name of 37 – 40% formaldehyde aqueous solution10-20% neutral formalin (formic acid, which is present in the solution, isneutralized by calcium carbonate), formalin saline (NaCl is added) bufferedformalin (phosphate buffer), Baker´s fluid (calcium chloride; for fixation of lipids)Paraformaldehyde (histochemistry, electron microscopy)Picric acid fixatives - fixation of glycogen (Bouin´s fluid)Mercuric dichloride - containing fixatives: SUSA fluid, Zenker´s fluid(black precipitates of mercury developed in tissue must be removed byiodination)Less used fixatives: methanol (smear), 96% ethanol (Nissl method) cooled acetone (histochemistry)
Formaldehyde Usually in form of paraformaldeyde powder or 37% to 16% aqueous solution•Low MW makes it one of the best penetrating of all the fixatives, thus itis widely used in fixation of resistant materials, such as seeds, spores,plant material, etc., usually in conjunction w/ another aldehyde.•Formalin contains many impurities, so formaldehyde for use in EM isnormally prepared from the dissolution, heating, and alkalination ofpowdered paraformaldehyde. Since this solution contains no inhibitors, ithas a shelf life of only a few weeks.
Glutaraldehyde•Glutaric acid dialdehyde, a 5 Carbon dialdehyde, is the most widely applied fixativein both scanning and transmission electron microscopy.Most highly cross-linking of all the aldehydes. GTA fixation is irreversible.•In TEM, buffered GTA has the reputation of providing the best ultrastructuralpreservation in the widest variety of tissue types of any known chemical fixative.
Osmium Tetroxide (OsO4)•A non-polar tetrahedral molecule with a•molecular weight of 254 and solubility water•and a variety of organic compounds.•Its principle utility is its ability to stabilize and stain lipids- preferentiallyunsaturated fatty acids•Commercially available as a coarse yellow crystalline material packaged in glassampoules sealed under inert gas. Similarly packaged aqueous solutions are alsoavailable.•An additive, non-coagulative type of fixative,but lacks the ability to crosslink many proteins.•Very poor rate of penetration
Basic factors affecting chemical fixationpH (Isoelectric point)Total ionic strength of reagentsOsmolarityTemperatureLength of fixationMethod of application of fixative