Micron and MicroscopicaAda. Vol. 22. No, 3. pp. 271—272, 1991. 1)739-6260/91 $3.16) +0.0))Printed in Great Britain. PergamonPressplc

NUCLEAR MEMBRANE BIOGENESISIN “IN VITRO” FERTILIZED BOVINE

OOCYTES.

DEPIESSE V., PETRE-PARENTB.*, THINES-SEMPOUXD.* AND DESSYF.

* LaboratoiredeBiologie CellulaireetLaboratoiredePhysiologiedesAniinauxDomestiques.Université

CatholiquedeLouvain,B-l348 Louvain-la-Neuve.

A still unresolvedproblemis thenuclearenvelopereformationaftereggfertilization. Theorigin ofthenuclearmembrane,itsdifferentiationin pores,thereassemblyof thelaminaarematterof debate.

Thereforewe mademorphologicalobservationson bovineoocytesin vitro maturedandfertilized.Twenty oocytesat eighteenhoursafter contactwith spermatozoawereclassicallyfixed, embeddedandseriallysectionedfor observationby light microscopy.All the 5 ~imsemithinsectionscharacterizedby thepresenceof a nucleuswerereembeddedandultrathin sectionsfor electronmicroscopywereprepared.Immatureandin vitro maturedoocytesweretreatedin asimilarway.

We observedthat thegerminalvesiclemembraneof theimmatureeggis veryrichin pores.Afterits desintegrationat prophase,no traceof nuclearmembranewith its characteristicporeswasobservedanywherein thecytoplasm.

After fertilization, thenuclearmembranereappeareddependingon thetime after spermatozoonpenetration.At eighteenhours, it is relativelydevoidof pores(fig. 1). At that time, we observedalsonumerouspiles of annulatelamellae(fig.2). They consistof 2 to 6 circularcisternaeof 0.5 to 1.5 ~tmcarryingup to 40 poresasobservedin tangentialsections.Thesepiles appearsometimesisolatedin thecytoplasm.Manyare linkedto ahoneycombstructure(fig.2)madeof boundregularcircularprofilesof 25nmdiameterfromwheretheyradiatein all directions.Otherannulatelamellaepiles areobservedparalleltothenuclearenvelopegenerallyat asitewheresuchacisternaseemsto havebeenyet incorporated(fig.1). Invery nearplacesbuddings(fig. 1) from theexternalnuclearmembraneareoftenobserved,envelopingoneor two circularprofileswith adarkercontentsuggestiveof somehowcondensednucleoplasm(fig.3). Freeannularprofileswerealsoseengrazingthe nuclearenvelope(fig.4).

We suggestthat after spermatozoonpenetration,the nuclearmembraneis reformedfrompreexistingsmoothendoplasmicreticulum.At thesametime,in thecytoplasm,annulatelamellaeappearbydifferentiationfrom large smoothtubules. They aredispatchedto the nucleusfrom the honeycombstructureswhichareprobablybuilt from shortmicrotubules.Aspiecesof annulatelamellaeareintegratedin

the nuclearenvelope,removalof excessof undifferentiatednuclearmembraneoccursby buddingof the

271

272 V. Depiesse et a!.

perinuclearcisternaanddetachment.The enclosednucleoplasmwill maybeserveas signal for furthercellularevents.

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Fini to 4:Electronmicrographsof in vitro fertilized bovineoocytes.

1. The nuclear membrane(NM) is relatively devoid ofporeswith annulatelamellae(AL) parallelto it. A bud (B) is seen withdarker central core. 4Ø~)Ø*

2. Annulate lamellaein parallel rows radiating from a honeycomb(HC) structuremade 25 nm circular profiles. Largesmooth

tubules in direct contactwith annulatelamellae(AL). 24.400*

3. Two darkvesiclesareenclosedin a large bud (B) from the externalnuclear membrane.54•ØØØ*

4. Freeannular profile (AP) in the near vicinity ofthe nuclear membrane.69.000*