The morphogenesis of the ciliary body of the avian eye: I. Lateral cell detachment facilitates epithelial folding

摘要

Theciliarybodyoftheavianeyecomprisesabout90radialfoldsintheanteriorretinathatformoveraperiodofabout2days(stages29a€“32).Theyextendfromclosetothelenscircumferenceoutwardsforabout1mmandwithineachthereisasmallbloodvessel.Thetipoftheretinaitselfdoesnotfold,butadherestotheanteriorsurfaceofthelensandeventuallyformstheiris.TEMstudiesshowthattheareaoftheretinathatfoldsisadomaindisplayingamostunusualphenomenon:itsinnercells,theanteriorregionofthemonolayerneuralretinalepithelium(NRE)detachthemselvesfromoneanotherovermostoftheirlateralsurfacessome2daysbeforefoldingoccursandreattachabout3daysafterwards.Gapjunctionsbetweenthecellsofthepigmentedretinalepithelium(PRE)andtheneutralretinalepitheliummaintaintheintegrityofthebilayerwhilejunctionalcomplexesbetweenthebasalregionsofthecellsineachlayerreinforcethestructure.Thewell-definedbasementmembraneonthevitreous-facingsurfaceoftheNREalso,presumably,helpsmaintainitsintegrity.AsimplephysicalanalysisshowsthatlateraldetachmentwithintheNREfacilitatestissuebendinginthat,foragiveninputofenergy,itpermitstheamountoffoldingtoincreasebyafactorof20ormore.Theretinaltipitselfdoesnotfold,presumablybecausetheNREcellstheredonotdetach;indeed,theregionseemsparticularlyrigid.Thestrengthofthistipregionmaybeenhancedbythefiberadhesions(ZonulesofZinn)thatitformswiththelensjustbeforefoldingoccurs.Studyofthedevelopingbloodsystemoftheeyeshowsthatsmallradialvesselsfromtheophthalmiccerebralarterysurroundingtheeyeareregularlyspacedaroundtheretinaltipsome6hrbeforethefirstfoldsappear.Theseresultssuggestthat,beforemorphogenesisoccurs,severalearliermorphologicaleventsensurethatthetissuehasbeenmadereadytofold.Theobservationsdonot,however,giveinsightintothedynamicforcesthatareresponsibleforfolding;theseareinvestigatedinthefollowingpaper.