MORPHOLOGYTherearenoconsistentordistinctivepatternsoforganortissuedistributionofamyloiddepositsinanyofthecategoriescited,butafewgeneralizationscanbemade.InAAamyloidosissecondarytochronicinflammatorydisorders,kidneys,liver,spleen,lymphnodes,adrenalglands,thyroidglands,andmanyothertissuesaretypicallyaffected.AlthoughALamyloidosisassociatedwithplasmacellproliferationscannotreliablybedistinguishedfromtheAAformbyitsorgandistribution,itmoreofteninvolvestheheart,gastrointestinaltract,respiratorytract,peripheralnerves,skin,andtongue.Thelocalizationofamyloiddepositsinthehereditarysyndromesisvaried.InfamilialMediterraneanfever,theamyloidosisisoftheAAtypeandaccordinglymaybewidespread,involvingthekidneys,bloodvessels,spleen,respiratorytract,and(rarely)liver.
Amyloidmaybeappreciatedmacroscopicallywhenitaccumulatesinlargeamounts.Theorganisfrequentlyenlarged,andthetissueappearsgrayandhasawaxy,firmconsistency.Histologically,theamyloiddepositionisalwaysextracellularandbeginsbetweencells,oftencloselyadjacenttobasementmembranes(
Fig.5.43A ).Astheamyloidaccumulates,itencroachesonthecells,intimesurroundinganddestroyingthem.Intheformassociatedwithplasmacellproliferation,perivascularandvasculardepositsarecommon.
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The diagnosis of amyloidosis is based on histopathology.Withthelightmicroscopeandhematoxylinandeosinstains,amyloidappearsasanamorphous,eosinophilic,hyaline,extracellularsubstance.Todifferentiateamyloidfromotherhyalinematerials(e.g.,collagen,fibrin),avarietyofhistochemicalstainsareused.ThemostwidelyusedistheCongoredstain,whichunderordinarylightgivesapinkorredcolortotissuedeposits,butfarmorestrikingandspecificisthegreenbirefringenceofthestainedamyloidwhenobservedbypolarizingmicroscopy(
Fig.
5.43B ).Thisstainingreactionissharedbyallformsofamyloidandisimpartedbythecrossedβ-pleatedsheetconfigurationofamyloidfibrils.Confirmationcanbeobtainedbyelectronmicroscopy,whichrevealsamorphousnonorientedthinfibrils.Subtypingofamyloidismostreliablydonebymassspectroscopy,asimmunohistochemicalstainsarenotentirelysensitiveorspecific.
Thepatternoforganinvolvementindifferentformsofamyloidosisisvariable.