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1、AbsorptionofToxicantsthroughtheSkin(abstract)Humanskinisnotverypermeableandthereforeisrelativelygoodbarrierforseparatingorganismsfromtheirenvironment.However,somechemicalscanbeabsorbedbyskininsufficientquantitiestoproducesystemiceffects.Tobeabsorbedthroughtheskin,atoxicantmustpassthroughtheepidermis
2、ortheappendages(sweatandsebaceousglandsandhairfollicles).Chemicalsthatareabsorbedthroughtheskinhavetopassthroughseveralcelllayers(atotalofseven)beforeenteringthesmallbloodandlymphcapillariesinthedermis.ThemostimportantconsiderationsregardingthedermalabsorptionofXenobioticsrelatetothestratumcorneum.T
3、hefirstphaseofpercutaneousabsorptionisdiffusionofXenobioticsthroughtherate-limitingbarrier,thestratumcomeumthatconsistsofabout75%to80%moderatelylipophilicmaterials.Itisclearthatalltoxicantsmoveacrossitbypassivediffusion.Moreover,humanstratumcomeumdisplayssignificantdifferencesinstructureandchemistry
4、fromoneregionofthebodytoanother,andthesedifferencesaffectthepermeabilityoftheskintochemicalsthatdependsonboththediffusivityandthethicknessofthestratumcorneum.Thesecondphaseofpercutaneousabsorptionconsistsofdiffusionofthetoxicantsthroughthelowerlayersoftheepidermis(stratumgranulosum,spinosumandgermin
5、etivum)andthedermisthatcontainaporous,nonselectiveaqueousdiffusionmedium.Thenthetoxicantsenterthesystemiccirculationandlymphaticcapillaries.Andthisrateofdiffusiondependsonbloodflow,interstitialfluidmovementandperhapsinteractionswithdermalconstituents.Asisreported,someagentscouldinjurethestratumcomeu
6、mandincreaseitspcneability,whichmayenhancetheabsorptionofsometoxicants.Andthestratumcorneumthatispartiallyhydratedismorepermeablethanthatwhichisdry.Havingbeenemployedinstudyingthedermalabsorptionoftoxicants,considerablespeciesvariationshavebeenobservedincutaneouspermeability.Speciesdifferencesinperc
7、utaneousabsorptionareaccountforthedifferentialtoxicityofinsecticidesininsectsandhumans.AccordingtoDugard,theappendages(sweatducts,piIosebaceousducts)andthestratumcorneumthatarehighlyspecies-dependentareimportantindermalabsorptionofchemicals.Besides,thecutaneousbloodflowisanotherpotentialrate-limitingstepinthedermalabsorptionofchemicals.
