cornea physiology

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lmporLance of Corneal hvsloloavO8asls of undersLandlna corneal phvsloloav Lo address Lhe phvsloloalc lmporLanceof corneal eplLhellal endoLhellal barrler and meLabollc pump funcLlonCornea| Lp|the||umO uncLlonsO1o form a 8arrler beLween Lhe envlronmenL and Lhe sLroma of Lhe corneaO 8arrlerformed as Lhe cells move superflclallv Lo Lhe surface of Lhe cornea dlfferenLlaLlna unLll Lhe superflclal cells form Lwo lavers of Lhe flaLLened cells enclrcled bv Lhe LlahL [uncLlon whlch serves as a hlah reslsLance seml permeable membraneO 8arrler funcLlons Lo prevenL Lhe movemenL of Lhe fluld from Lears Lo Lhe sLroma O1o form a smooLh refracLlve surface on Lhe corneaOroLecLs Lhe cornea and Lhe lnLraocular sLrucLures from lnfecLlon bv paLhoaensO smooLhes Lhe surface of Lhe cornea lncreases lLs ablllLv Lo become weL bv aqueous Lherebv forms smooLh opLlcal surface requlred for clear vlslon439/,alnLenance of Lhe LplLhellumO ,alnLalned bv a balance amona (x?Z PvpoLhesls)rocess of ce|| m|grat|on ,laraLlon of new basal cells lnLo cornea from llmbus ,laraLe cenLrlpeLallv aL abouL 120 m/wk orlalnaLe from sLem cells ln llmbal eplLhellumM|tos|s ccurs onlv ln basal cell laver uauahLer cells move upward from basal laver dlfferenLlaLlna lnLo wlna cells and flnallv lnLo superflclal cells5hedd|ng of superf|c|a| ce||s Corneal eplLhellum SLraLlfled Squamous eplLhellum from whlch Lermlnallv dlfferenLlaLed superflclal cells conLlnuouslv shed LplLhellum Lurn over compleLelv everv weekWCUND nLALINGOLplLhellum normallv responds Lo woundlna bv lnlLlaLlon of Lhe process of mlaraLlonacross Lhe basemenL membrane Lo cover Lhe abraded area and reesLabllsh LhebarrlerOcell mlaraLlon occursabouL 3 hrs afLer woundlna cells Lravellna aL a consLanL raLe of 6080um/hr Llll wound closesOuurlna laa phase cellular oraanlzaLlon occurs hemldesmosomes dlsappearfrom basal cells and blochemlcal processes prepare Lhe cell for heallnaO1he leadlna edae of Lhe mlaraLlna cells ls onlv one lavered buL Lhe wound ls acLuallv covered bv mulLllavered sheeL made of basal and squamous cellsOuurlna corneal heallna from con[uncLlva aobleL cells also mlaraLe onLo Lhe cornea followlna wound closure bv blochemlcal LransducLlon loss of aobleL cells occursOLarae number of reoraanlzaLlon and svnLheLlc chanaes lnvolved ln corneal eplLhellal cell mlaraLlonO,laraLlon needs enerav as alvcoaen ls depleLed from Lhe cell aL Lhe wound edae durlna heallna Orocess also requlres calclum slnce Lhe mlaraLlon ls lnhlblLed bv calmodulln lnhlblLorsContdO cell mlaraLlon ls sLlmulaLed bv cA, lncreased ln mlaraLlna cellsO lncrease ln proLeln svnLheslsdurlna wound heallnaO a speclflc lncrease ln 110ku cvLoplasmlc proLeln vlnculln promoLes Lhe lnLeracLlon of acLln wlLh adheslon proLelns(Lalln and lnLearln) Oollowlna Lhe heallna eplLhellum ls reesLabllshed bv Lhe formaLlon of hemldesmosomes OWhen a corneal abraslon ls llmlLed Lo Lhe eplLhellum and Lhe basemenL membrane ls noL damaaed a normal eplLhellum wlLh adheslon complex ls formed soon afLer heallnaOln wounds wlLh basemenL membrane damaae adheslon complex ls formed afLer 12monLhsOSlmple eplLhellal abraslon heals qulcklv wlLhouL anv compllcaLlonsOln laLLlce dvsLrophv amvlold deposlLlon beLween eplLhellum and bowmans laverdlsrupLlna eplLhellal aLLachmenL complex OPuman cornea aeneraLes 23 33 mv LranseplLhellal poLenLlalO1hls hlah volLaae ls conslsLenL wlLhlow lonlc conducLance of Lhe aplcal eplLhellal cell membrane and hlah reslsLance of Lhe LlahL [uncLlons of paracellular paLhwavsO1he Lranscellular and Lhe paracellular poLenLlal are dependenL on Lhe exlsLence of Lhe aplcal LlahL [uncLlonO1he poLenLlal ls due Lo lonlc aradlenL seL up bv Lhe eplLhellal LransporL of sodlum and Lhe chlorlde lonsO30 of Lhe shorL clrculL currenL across Lhe corneal eplLhellum carrled bv chlorlde lons movlna Lhrouah aplcal membrane channel lnLo Lhe LearO Sodlum lons are pumped Lowards Lhe sLroma from Lhe cell bv Lhe na k A1ase pump ln Lhe basolaLeral membrane of Lhe cells malnLalns Lhe lnward sodlum aradlenLO Sodlum chlorlde coLransporLer presenL aL Lhe basolaLeral membrane faclllLaLe Lhe lnflux of sodlum down Lhe concenLraLlon aradlenL carrvlna wlLh lL chlorlde lons O Chlorlde lons Lhen dlffuse Lhrouah Lhe aplcal membraneO Corneal eplLhellum also conLalns Lhe na P exchanaer and a lacLaLe P co LransporLer helps Lo malnLaln Lhe lnLracellular pP bv exLruslon of Lhe lacLaLe and P lonsO Cvcllc A, also lncreases Lhe conducLance of aplcal chlorlde channels sLlmulaLlna chlorlde LransporLO 1hese svsLems are worklna homeosLaLlcallv ln Lhe corneal eplLhellum Lo malnLaln Lhe eplLhellal barrler.439/Metabo||c athwaysOCorneal eplLhellum uses prlmarllv Lhe alucose and Lhe alvcoaen for Lhe enerav producLlonOClucose reaches Lhe cell bv Lhe dlffuslon from Lhe aqueous humorOCorneal eplLhellal cells sLore hlah level of alvcoaenOClvcoaen ls rapldlv depleLed under sLress as ln hard conLacL lens wear or LraumaO Clucose ls prlmarllv meLabollsed bv Lhe anaeroblc alvcolvsls (Lmbden ,everhof paLhwav) and abouL 33 of Lhe alucose enLers Lhe P, shunLO P, shunL converLs hexoses Lo penLoses requlred for Lhe nuclelc acld svnLhesls and produces nAuP a hlah enerav reduclna aaenL requlred for Lhe faLLv acld svnLhesls boLh of whlch are requlred bv Lhe Llssues wlLh hlah mlLoLlc lndex ea corneal eplLhellumContdO Clucose mav also enLer Lhe sorblLol paLhwavproduclna sorblLol and frucLoseO ln Lhe presence of excess alucose excess sorblLol ls produced ln Lhe lens and perlpheral nerves causlna osmoLlc cell damaaeContdOCorneal eplLhellum recelves oxvaen dlrecLlv from Lhe aLmosphere under open eve condlLlon wlLh Lhe parLlal oxvaen pressure ln Lear fllm belna 133mm PaOWhen Lhe llds are closed oxvaen pressure drops Lo 33 mm PaapparenLlv adequaLe Lo malnLaln Lhe eplLhellumOSllahL eplLhellal oedema occurs durlna sleepO1he cornea consumes abouL 33 uL 2 /cm2/hrO Dnder aeroblc condlLlon pvruvaLe from Lhe alvcolvsls can enLer Lhe kreb's cvcle(1CA Cvcle)O Dnder hvpoxlc condlLlon pvruvaLe ls converLed Lo lacLaLe whlch ls LransporLed from cells Lo malnLaln lnLracellular pP aL 73 74O LacLaLe cannoL dlffuse across Lhe aplcal barrler and bullds up ln Lhe lnLracellular space of Lhe eplLhellum and sLromaWCllnlcallv corneal sLroma appears homoaenous and LransparenL buL Lhere ls dlfference beLween anLerlor and posLerlor corneal sLromaW1he anLerlor sLroma has less waLer (304am P20/am drv welahL) compared Lo Lhe posLerlor sLroma (383am P20/am drv welahL) Lhe dlfference belna aLmospherlc drvlna of corneal eplLhellum OAnLerlor sLroma has less alucose Lhan Lhe posLerlorO ,ore of Lhe dermaLan sulphaLe ls presenL anLerlorlvhas less waLer sorpLlve properLv and areaLer waLer reLenLlve properLvO Whlle keraLan sulphaLe ls locaLed posLerlorlv has areaLer waLer sorpLlve properLv wlLh llLLle reLenLlve properLvO So cllnlcallv mosL oedema occurs ln posLerlor sLromaO A areaLer dearee of corneal swelllna occurs when endoLhellal barrler ls removed compared Lo eplLhellal barrlerOnce Lhe barrler or meLabollc pump funcLlon ls losL corneal sLroma swells because of Lhe hvperLonlclLv of Lhe sLromal collaaen salLs and proLeoalvcans compared Lo Lear and aqueous humourO corneal sLroma swells on endoLhellal dlsrupLlon as l of 13 mm of Pa ls unopposed and aqueous waLer wlll dlffuse lnLo Lhe sLromaO Loss of elLher barrler and upLake of waLer bv sLromaloss of proLeoalvcans( uermaLan and keraLan sulphaLe)O 1he relaLlve reslsLance Lo Lhe nonedemaLous eplLhellumsLroma endoLhellum ls 2000110 Lherebv resLrlcLlna Lhe elecLrolvLes Lo Lhe sLromal comparLmenLO SLroma has Lhe properLv Lo lmblbe waLer reflecLs Lhe waLer lmblblna properLv of Lhe proLeoalvcansO SLromal edema occurs ln Lhe around subsLance leads Lo lncreased spaclna of Lhe collaaen flbrlls439/ContdO ln cornea of normal Lhlckness swelllna pressure ls 60mmPa and ls lnverselv relaLed Lo Lhe corneal LhlcknessO 1he swelllna pressure of Lhe sLroma relaLes Lo Lhe elecLrosLaLlc repulslon of Lhe neaaLlvelv charaed alvcosamlnoalvcans (keraLan sulphaLe dermaLan sulphaLe chondrolLln sulphaLe)O 1he acLual concenLraLlon of Lhe na+and k+ls hlaher ln Lhe sLroma Lhan ln Lhe aqueousContdO 1he lonlc acLlvlLv ls low ln Lhe sLroma Lhan aqueous reflecLlna caLlonlc blndlna slLes on Lhe sLromal CACLhus favorlna Lhe movemenL of waLer from sLroma Lo Lhe aqueousO 1he lonlc acLlvlLv whlch deLermlnes Lhe osmoLlc and dlffuslonal aradlenLs for na ls less ln Lhe sLroma Lhan Lhe aqueous reflecLlna caLlonlc blndlna bv Lhe anlonlc slLesO 1hls blndlna decreases Lhe effecLlve osmolarlLv of Lhe sLroma Lhus favours waLer movemenL from Lhe sLroma Lo Lhe aqueousContdO WaLer evaporaLes from Lhe corneal surface aL a raLe of 23uL/cm2/hourO LvaporaLlon accounLs for 3 Lhlnnlna of Lhe cornea durlna Lhe dav Llme compared wlLh Lhe Lhlckness measured aL mornlna afLer nlahL sleepO 1he normal cornea malnLalnsa consLanL Lhlckness ln Lhe presence of l of 30mm Pa because Lhe sLromal swelllna pressure ls also ln Lhe same ranaeConLd neaaLlvelv charaed sLromal CAC Lend Lo repel each oLher produclna swelllna pressure(S) 1he relaLlon of swelllna pressure of sLroma Lo l ls l(lmblblLlon pressure of corneal sLroma) l S As sLromal pressure decreases preclplLlouslv bv lncrease ln corneal Lhlckness mlld corneal edema comblned wlLh lncrease ln l leads Lo hlah lmblblLlons pressure and subsequenL mlcrobullous formaLlon and eplLhellal edemaO ln eves wlLh lncreased l or wlLh abnormal endoLhellal funcLlonLhere ls eplLhellal edema and lncreased sLromalLhlcknessO Arcus sen|||sconslsLs of cholesLerol cholesLerol esLers phosphollplds and LrlalvcerldesLlplds found exLracellularlv ln sLroma ln hour alass paLLernLlmbal vessels lncreaslnalv permeable wlLh aaeLuLpass lnLo cornea and carrv llplds lnLo Lhe eveODe||enareas of locallzed Lhlnnlna of cornea due Lo reverslble P20 loss from sLroma.InvoIves :OesvnLhesls and crossllnklna of Lhe collaaenOAlLeraLlon ln Lhe proLeoalvcan svnLheslsOCradual wound remodelllna and resLoraLlon of Lhe Lenslle sLrenaLhWlLhln 26 hours of peneLraLlna ln[urv ,n cel