The morphogenesis of the ciliary body of the avian eye: II. Differential enlargement causes an epithelium to form radial folds

[摘要] Themorphogeneticmechanismresponsiblefortheradialfoldingoftheanteriorretinaofthechickeyeasitformstheciliarybodyhasbeeninvestigatedintwoways.First,eyegrowthandcelldivisionhavebeenassayedtofindouttheoriginsoftheextrafoldmaterial,andsecond,possiblemechanismshavebeentested,andinsomecasesexcluded,byartificiallyincreasingthesizeoftheembryoniceyeinvitrounderarangeofrestrictiveconditions.Growthstudiesshowthat,whileonaveragetheeyeballincreaseslinearlyinareabyafactorofabout14overtheperiod4–8daysorstages24–33,thereisaslowingdowningrowthatstage28whichisfollowedbyarapidcatchupasthesurfaceareaincreasesbyabouttwo-thirdsinthe12hrbetweenstages29and30,justastheciliarybodyforms.Thymidineincorporationstudiesshowthatcelldivisionisroughlyuniformovertheeyeatthisstage.Thesuddenincreaseintheoverallsizeoftheeyeisnot,however,matchedbygrowthintheregionofthepupilattheretinaltip;thisringoftissuegrowsslowlyanditsdiameterremainsvirtuallyconstantovertheperiodofciliarybodymorphogenesis.Theseobservationssuggestasimplemorphogeneticmechanism.Tissueneartheretinaltip,unlikesuchtissueintherestoftheeye,isunabletoswelluniformlyunderintraocularpressureasitisconstrainedbytherigidpupillaryring;theresultingcomplextensionscauseradialfolding.Thisprocessisfacilitatedbylateralcelldetachmentintheneuralretinalepithelium(NRE)andnucleatedbyexistingradialcapillariessuperficialtotheretina.Itisapredictionofthemechanismthatanydifferentialgrowthoftheeyeballshouldcausestage29eyestofold.Suchgrowthhasbeeninducedinvitrobyimmersingearlystage29eyesin50%ethanolandwater,asolutionwhichcausesan8%orsoincreaseineyediameterinafewminutes,equivalenttoaboutseveralhoursgrowthinvivo.Pupillaryexpansion,however,lagssome15minbehindthatoftherestoftheeye.Inmostcases,immersedeyesgeneratelargenumbersofradialfoldsoverabouthalftheircircumferenceinabout10minafterimmersion,thedegreeoffoldingexpectedforsimilargrowthinvivo.Thisresultarguesagainstmorphogeneticmechanismsbasedonlocalisedgrowthoronotherslowdevelopmentalevents.SuchradialfoldformationinvitroaftertreatmentofeyesinvivowithcolchicineandcytochalasinBhasalsobeenobserved,resultswhichargueagainstanymorphogeneticmechanismbasedonmicrotubulesormicrofilaments.Somefoldsalsoformwithethanolswellingafterthecorneaandperipheralmesenchymehavebeenremovedfromtheeye,aresultwhichexcludesanymajordirectiveroleforthispartofthetissue.Theobservationsthereforesupportthesimpledifferentialgrowthstimulusforciliarybodymorphogenesis.

[学科分类] 生物科学（综合）