Cytoplasmic changes.Necroticcellsshowincreasedeosinophilia(i.e.,theyarestainedredbythedyeeosin—theEinthehematoxylinandeosin[H&E]stain),attributablepartlytoincreasedbindingofeosintodenaturedcytoplasmicproteinsandpartlytolossofbasophilicribonucleicacid(RNA)inthecytoplasm(basophiliastemsfrombindingofthebluedyehematoxylin—theHin“H&E”).Comparedwithviablecells,thecellmayhaveaglassy,homogeneousappearance,mostlybecauseofthelossoflighterstainingglycogenparticles.Myelinfiguresaremoreprominentinnecroticcellsthanincellswithreversibleinjury.Whenenzymeshavedigestedcytoplasmicorganelles,thecytoplasmbecomesvacuolatedandappears“moth-eaten.”Byelectronmicroscopy,necroticcellsarecharacterizedbydiscontinuitiesinplasmaandorganellemembranes,markeddilationofmitochondriaassociatedwiththeappearanceoflargeamorphousintramitrochondrialdensities,disruptionoflysosomes,andintracytoplasmicmyelinfigures.
Nuclear changes.Nuclearchangesassumeoneofthreepatterns,allresultingfromabreakdownofDNAandchromatin.Pyknosis ischaracterizedbynuclearshrinkageandincreasedbasophilia;theDNAcondensesintoadarkshrunkenmass.Thepyknoticnucleuscanundergofragmentation;thischangeiscalledkaryorrhexis.Ultimately,thenucleusmayundergokaryolysis,inwhichthebasophiliafadesbecauseofdigestionofDNAbydeoxyribonuclease(DNase)activity.In1to2days,thenucleusinadeadcellmaycompletelydisappear.
Fates of necrotic cells.Necroticcellsmaypersistforsometimeormaybedigestedbyenzymesanddisappear.Deadcellsmaybereplacedbymyelinfigures,whichareeitherphagocytosedbyothercellsorfurtherdegradedintofattyacids.Thesefattyacidsbindcalciumsalts,whichmayresultinthedeadcellsultimatelybecomingcalcified.
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Fig. 2.6 Coagulativenecrosis.(A)Awedge-shapedkidneyinfarct(yellow) withpreservationoftheoutlines.(B)Microscopicviewoftheedgeoftheinfarct,withnormalkidney(N) andnecroticcellsintheinfarct(I).Thenecroticcellsshowpreservedoutlineswithlossofnuclei,andaninflammatoryinfiltrateis present(difficulttodiscernatthismagnification).
Morphologic Patterns of Tissue Necrosis
In severe pathologic conditions, large areas of a tissue or even entire orgrans may undergo necrosis. This may happen in association with marked ischemia, infections, and certain inflammatory reactions. There are several morphologically distinct patterns of tissue necrosis that may provide etiologic clues. Although the terms that describe these patterns do not reflect underlying mechanisms, such terms are commonly used and their implications are understood by pathologists and clinicians.
grossappearances;theexceptionisfibrinoidnecrosis,whichis detectedonlybyhistologicexamination.