Determination of the dorsal-ventral axis in eggs of Xenopus laevis: Complete rescue of uv-impaired eggs by oblique orientation before first cleavage☆

摘要

EggsofXenopuslaeviswereexposedtoultraviolet(uv)radiation(2537?…)onthevegetalhemispheresoonafterfertilizationatdosessufficienttoimpairgreatlythesubsequentdevelopmentofdorsalstructures.Itwasfoundthattemporaryorientationofirradiatedeggs90?°offthenaturalverticalaxisrescuestheseeggs,allowingthemtodevelopintonormalembryos.Completerescueresultswhenobliqueorientationisinitiatedwellbeforefirstcleavage,andeggsremaininthispositionuntilthe16-cellstage.Significantrescueisseen,however,ineggswhichremainoffaxisforshorterperiodsoftimeorwheneggsareobliquelyoriented,evenafterfirstcleavage.Furthermore,aperiodofobliqueorientationpriortouvirradiationresultsininsensitivityofeggstoirradiation.Ultravioletirradiationisfoundtorandomizethepositionofthedorsalsidewithrespecttothespermentrancepoint,whereasthepositionofthedorsalsideofrescuedembryosisstronglyspecifiedbytheorientationoftheeggduringtherescueperiod,andnotbythespermentrancepoint.Othereffectsofuvirradiationonearlydevelopmentincludedecreasedpigmentationdifferencesamong4-cellstageblastomeresanddelayedgastrulation.Itisproposed(1)thatobliqueorientationpromotesinirradiatedeggsasetofinternalrearrangementsmimickingthosenormallyaccomplishedbytheunirradiatedegginaperiodpriortofirstcleavageandaspartofanearlydorsalizationprocess,and(2)thattheuv-sensitivetargetsarepartofthemorphogenicmachineryusedbytheeggforinternalrearrangementsinthisperiodandarenotelementsofasystemoftransmittedparticulatedorsaldeterminants.