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1、ReadingMaterials131.ToothdevelopmentToothdevelopmentorodontogenesisisthecomplexprocessbywhichteethformfromembryoniccells,grow,anderuptintothemouth.Althoughmanydiversespecieshaveteeth,non-humantoothdevelopmentislargelythesameasinhumans.Forhumanteethtohaveahealthy
2、oralenvironment,enamel,dentin,cementum,andtheperiodontiummustalldevelopduringappropriatestagesoffetaldevelopment.Primary(baby)teethstarttoformbetweenthesixthandeighthweeks,andpermanentteethbegintoforminthetwentiethweek.Ifteethdonotstarttodevelopatornearthesetime
3、s,theywillnotdevelopatall.Asignificantamountofresearchhasfocusedondeterminingtheprocessesthatinitiatetoothdevelopment.Itiswidelyacceptedthatthereisafactorwithinthetissuesofthefirstbranchialarchthatisnecessaryforthedevelopmentofteeth.Invertebratesseveralspecializ
4、ationsofepithelialtissuegenerateafterthickeningspecificstructures:keratinizedstructure(hair,nails)orexoskeletonsstructure(scales,teeth).Placoidsscalesandteethofsharksareconsideredhomologousorgans.2.ToothStructureAnindividualtoothconsistsofanexposedcrownandaroot,
5、buriedinthegumandjaw.Thecrownisusuallyatleastpartlycoveredbyanouterlayerofanespeciallyhardsubstancerelatedtobonecalledenamel.Beneaththeenamel(andsometimesexposedtothesurfaceiftheenamelismissingorwornaway)isanintermediatelayerofmaterialcalleddentine,whichisalsosi
6、milartobonebutisnotnearlyashardasenamel.Itsurroundsaninnerpulpcavityfilledwithpulp(aliving,vascularandwellinnervatedtissue).Bloodvesselsandnervesreachthepulpcavitythroughachannel,therootcanal,thatpenetratestheroot.Anadditionallayerofbonymaterial,cementum,usually
7、surroundstheroot.13Asmostteethmature,therootcanalgraduallyclosesandthepulpcavityissealedoff.Theseteetharecalled"rooted."Incontrast,"rootless"teetharethoseinwhichtherootcanalremainsopenandthetoothcontinuestogrowindefinitely.Rodentincisorsandthemolarsofmanyarvicol
8、inerodentsareexamplesofrootlessorevergrowingteeth;themolarsofdogsandhumansarerooted.Teetharepresentinmostvertebrates(turtlesandmodernbirdsarenotableexceptions),andins
