Fig.2.6 ).Theaffectedtissuestakeonafirmtexture.Presumablytheinjurydenaturesnotonlystructuralproteinsbutalsoenzymes,therebyblockingtheproteolysisofthedeadcells;asaresult,eosinophilic,anucleatecellsmaypersistfordaysorweeks.Leukocytesarerecruitedtothesiteofnecrosis,andthedeadcellsareultimatelydigestedbytheactionoflysosomalenzymesoftheleukocytes.Thecellulardebrisisthenremovedbyphagocytosismediatedprimarilybyinfiltratingneutrophilsandmacrophages.Coagulativenecrosisischaracteristicofinfarcts(areasofnecrosiscausedbyischemia)inallsolidorgansexceptthebrain.
Liquefactive necrosis isseeninfocalbacterialand,occasionally,fungalinfectionsbecausemicrobesstimulaterapidaccumulationofinflammatorycells,andtheenzymesofleukocytesdigest(“liquefy”)thetissue.Forobscurereasons,hypoxicdeathofcellswithinthecentralnervoussystemoftenevokesliquefactivenecrosis(
Fig.2.7 ).Whateverthepathogenesis,thedeadcellsarecompletelydigested,transformingthetissueintoaviscousliquidthatiseventuallyremovedbyphagocytes.Iftheprocessisinitiatedbyacuteinflammation,asinabacterial infection,thematerialisfrequentlycreamyyellowandiscalledpus (Chapter3).
Althoughgangrenous necrosis isnotadistinctivepatternofcelldeath,thetermisstillcommonlyusedinclinicalpractice.Itusuallyreferstotheconditionofalimb(generallythelowerleg)thathaslostitsbloodsupplyandhasundergonecoagulativenecrosisinvolvingmultipletissuelayers.Whenbacterialinfectionissuperimposed,themorphologicappearancechangestoliquefactivenecrosisbecauseofthedestructivecontentsofthebacteriaandtheattractedleukocytes(resultinginso-called“wetgangrene”).
Caseous necrosis ismostoftenencounteredinfocioftuberculousinfection.Caseousmeans“cheeselike,”referringtothefriableyellow-whiteappearanceoftheareaofnecrosisongrossexamination(
Fig.2.8 ).Onmicroscopicexamination,thenecroticfocusappearsasacollectionoffragmentedorlysedcellswithanamorphousgranularpinkappearanceinH&E-stainedtissuesections.Unlikecoagulativenecrosis,thetissue
Fig. 2.7 Liquefactivenecrosis.Aninfarctinthebrainshowsdissolutionofthetissue.
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Fig. 2.8 Caseousnecrosis.Tuberculosisofthelung,withalargeareaofcaseousnecrosiscontainingyellow-white(cheesy)debris.
architectureiscompletelyobliteratedandcellularoutlinescannotbediscerned.Caseousnecrosisisoftensurroundedbyacollectionofmacrophagesandotherinflammatorycells;thisappearanceischaracteristicofanodularinflammatorylesioncalledagranuloma(Chapter3).