Article

478

Cell polarityPolarity is an essential feature of development in allorganisms, from the 1-cell embryo to the establishment oftissue polarity through the whole organism. It is acharacteristic of most cells in metazoan organisms and isassociated with asymmetries in cell shape, protein content,organelle distribution and, ultimately, cell function (for review,see Nelson, 2003). Epithelial cells from simple transportingepithelia, as well as neurons, have been frequently used asmodels for somatic cell polarity (Le Gall et al., 1995; Nelson,2003). Epithelial cells show polarized organization of theiractin and microtubule cytoskeleton to mediate vectorialmembrane trafficking and direct specific proteins to their

basolateral or apical surfaces (Figure 1). Another example isthe polarized way in which neurons are organized tocommunicate and accomplish vectorial movement of specificneurotransmitters across long distances in the body, or conductnerve impulses through reversible signals of membranepolarity (Jockusch et al., 2004). Overall, establishment andmaintenance of cellular polarity is a conserved feature ofvertebrate and invertebrate organisms (Wodarz, 2002; forreview see Muller and Bossinger, 2003; Nelson, 2003).Evolutionarily conserved mechanisms were adopted bydifferent cell types to facilitate specific cytoskeletonreorganization and oriented localization of cortical proteinsresponsible for spatial allocation of resources. Identification ofthe mammalian homologues of the invertebrate polarity genes

Cell polarity during folliculogenesis andoogenesis

Carlos E Plancha received his Medical Degree in 1985 at the Lisbon Medical School, Portugal,and completed his internship in 1987 at the Hospital de Santa Maria in Lisbon. He submittedhis PhD thesis on female gamete biology at the Lisbon Medical School in 1996, where he isnow Assistant Professor. In 2001 he began collaborating with a Reproductive Medicine Clinicin Lisbon – CEMEARE – where he now works as an embryologist coordinating the IVFlaboratory. Since 2002 he is also the Director of the Biology of Reproduction Unit in theInstitute of Molecular Medicine, Lisbon. His main scientific areas of research are oogenesisand folliculogenesis. He teaches and supervises numerous pre- and post-graduate activitiesand has coordinated nationally and internationally funded research projects in his areas ofexpertise. Dr Plancha has also been involved in the organization of national and internationalmeetings. He has published several articles in peer-reviewed scientific journals and bookchapters. He maintains long-term collaborations with David F Albertini, with whom he isgraduate supervisor of Alexandra Sanfins and Patrícia Rodrigues.

Carlos E Plancha1,3, Alexandra Sanfins1, Patrícia Rodrigues1,2, David Albertini21Unidade de Biologia da Reprodução, Instituto de Medicina Molecular, Faculdade de Medicina de Lisboa, Av. Prof.Egas Moniz, 1649–028 Lisboa, Portugal; 2Department of Molecular and Integrative Physiology, University of KansasMedical Centre, 3901 Rainbow Boulevard, Kansas City, KS 66160–7401, USA3Correspondence: e-mail: carlos.plancha@mail.telepac.pt

Dr Carlos E Plancha

AbstractPolarity is an important aspect of oogenesis and early development for many animal groups, but only recently it has becomerelevant to the study of mammals. Mammalian oocyte development occurs through tight coordination and interaction betweenall ovarian structures. In fact, bi-directional communication between the oocyte and its companion granulosa cells (GC) in theovarian follicle seems essential for GC proliferation, differentiation, and production of a functional female gamete. The trans-zonal projections (TZP), which are specialized extensions from granulosa cells that terminate on the oolema after crossing thezona pellucida, are major structural components necessary for oocyte–GC interaction. Granulosa cell polarity seems to be anecessary requisite for appropriate function of TZP, and the role of FSH as modulator of a polarized phenotype on GC isdiscussed. This article also discusses oocyte polarity with special reference to the partial loss of polarity that occurs during in-vitro oocyte maturation and possible implications in the modulation of oocyte competencies. Cytoskeletal markers that mayaccount for oocyte quality were defined and found to be distinct in in-vivo and in-vitro matured oocytes. Implications of partialloss of oocyte polarity during in-vitro maturation, reflected by distinct distribution of these markers, are further discussed. It isalso proposed that expression of both somatic and germ cell polarity in the ovarian follicle will ultimately determine acquisitionof meiotic, fertilization and developmental competences by the oocyte.

Keywords: in-vitro maturation, in-vivo maturation, oocyte, polarity

RBMOnline - Vol 10. No 4. 2005 478–484 Reproductive BioMedicine Online; www.rbmonline.com/Article/1603 on web 21 February 2005

479

Article - Cell polarity during folliculogenesis and oogenesis - CE Plancha et al.

has indeed reinforced the concept that important proteindomains involved in maintenance of cellular polarity areevolutionarily conserved (Muller and Bossinger, 2003).Recently, it was shown that a set of genes responsible forestablishment of spatial cues is conserved throughout theanimal kingdom: the par genes are responsible for polarizedorganization in the Caenorhabditis elegans zygote, in thefemale germline cyst of Drosophila, and in mammalianepithelial cells (Wodarz, 2002). Although the determinants ofegg polarity have been extensively discussed in Drosophila, C.elegans, and Xenopus (Edwards, 2001; Pellettieri andSeydoux, 2002; Wodarz, 2002), no attempt has been made tosearch systematically for mammalian homologues in femalegerm cells in order to draw comparison with mammalsregarding germ cells (Edwards, 2001). Thus, the subject of eggpolarity in mammals remains as a theme of interestingdiscussion (Gardner, 1999; Plusa et al., 2002). The modelsystem that has interested the authors for several years is thedeveloping mammalian ovarian follicle. This review willspecifically focus on the somatic granulosa cell and the germcell compartments of the follicle, as further examples of howcell polarity determines appropriate cell function.

Granulosa cell polarityThis section will discuss the polarity of the main somatic cellconstituents of the ovarian follicle, the granulosa cells, asopposed to discussion on the more controversial issue ofovarian follicle polarity. The ovarian follicle consists of an

association between a germ cell, the oocyte, and at least twosomatic cell types, granulosa and theca cells. Four distinctextracellular matrix (ECM) territories, zona pellucida, antrum,basal lamina and thecal matrix, are also present at specificspatial and time points, modulating follicle cellularinteractions and establishing territorial frontiers. Theseprovide the coordinates for the different polarized phenotypesfound in ovarian follicles. However, the role of suchcomponents in the establishment, maintenance and modulationof granulosa cell (GC) polarity is still poorly understood.Recently, it has been suggested by Irving-Rodgers and co-workers (2004) that GC polarity is induced by contact with thefollicular basal lamina, enabling directional secretion anduptake of molecules, and other polarized functions.

Previously, it was proposed that the follicular epithelium issimilar to other known epithelia (for review, see Rodgers et al.,1999), although being particularly complex, as it expands froma single to a multi-layered epithelium as the follicle grows. Inaddition, there is the transition from pre- to antral follicle, andthe increase in the epithelium compartment through fluidfilling. Irving-Rodgers and co-workers (2004) proposed theexistence of a novel basal lamina matrix, the focal intra-epithelial matrix, which is developmentally regulated and maybe involved in the modulation of GC polarity. During folliculardevelopment, two sets of specialized GC within the ovarianfollicle become apparent: the mural granulosa cells, whichoverlay the basal lamina; and the cumulus oophorus cells,which locate around the zona pellucida with some cells

Figure 1. Schematic representation of epithelial cells showing polarized cytoskeleton organization and bi-directional membranetrafficking. On the left, organization of the actin and microtubule cytoskeleton with the centrosome as the main microtubuleorganizing centre; on the right, membrane trafficking to/from the apical and basal membranes, allowing transport of moleculesto/from the basal or apical surfaces. PAR = protease-activated receptor.

480

Article - Cell polarity during folliculogenesis and oogenesis - CE Plancha et al.

making contact with the oolema, the corona radiata cells.The mural cells exhibit a morpho-functional organizationwith cell–cell and cell–basal lamina junctions, thus closer tothe classical examples of epithelial cells with a clearapical–basal axis differentiation (Figure 1). In contrast,besides homologous gap junctions and other cellinteractions between themselves, the cumulus cells thatconstitute the corona radiata also display heterologous gapjunctions and other bi-directional interactions with the germcell (Albertini and Anderson, 1974), through a complexmorphological and functional circuitry at the zona pellucidalevel (Motta et al., 1994; Albertini et al., 2001).Cytoplasmic projections from cumulus cells that transversethe zona pellucida matrix and terminate at the oocyte surfaceare responsible for the complex bi-directional signalling atthe oocyte–cumulus cell interface (Albertini et al., 2001).These specialized structures, referred as trans-zonalprojections (TZP) (Motta et al., 1994; Albertini et al., 2001),exist in at least two forms: microtubule-containing TZP(MT-TZP), which appear to be involved in paracrinecommunication, and actin-containing TZP (ACT-TZP),which appear to mediate gap junctional communication andcumulus cell–oocyte adhesion (Albertini et al., 2001;Navarro-Costa et al., 2005). The cellular and molecularcharacterization of these processes has uncovered theirdynamic nature throughout folliculogenesis (Albertini et al.,2001; Albertini and Barrett, 2003; Combelles et al., 2004).These are present and very numerous in pre-antral follicles,and there seems to be some degree of retraction after antrumformation in human and mouse oocytes (for review, seeMotta et al., 1994; Albertini et al., 2001). At peri-ovulatorystages in the hamster, most ACT-TZP seem to retract(Plancha and Albertini, 1994). Recently, it has beendemonstrated that FSH is an important modulator of MT-TZP organization. FSH-like priming reduces cumulus MT-TZP density in FSH-β–/– mice, which otherwise exhibitenhanced density of TZP with associated decreases in oocytedevelopment (Combelles et al., 2004). Important features ofoocyte development that include chromatin remodelling andmeiotic competence acquisition have been shown to beinduced under conditions where FSH directly modifiesoocyte–cumulus interaction via TZP (Plancha and Albertini,1994; Combelles et al., 2004). In short, evidence that FSHmodulates the polarized phenotype of GCs, particularlythrough mediation of TZP dynamics, is gaining furtherimportance. Cellular interactions structurally dependent onTZP may be a specialized mean by which paracrine factorsare exchanged between the germ line and the soma duringfollicular development. It remains to be demonstrated ifgonadotrophins are the main modulators for this signallingcircuit, and how local regulators come into place. A betterunderstanding of the complex hormonal and paracrineenvironment that directs the interplay between GC and theoocyte would surely be crucial for answering this question inthe near future.

Oocyte polarityEstablishment and maintenance of polarity is of centralimportance in the process of oogenesis, particularly duringoocyte maturation. Mitochondria distribution in the oocyteis frequently taken as evidence for the existence of polarityin this cell, or at least of an asymmetric localization that

could have functional significance (Calarco et al., 1972; VanBlerkom and Runner, 1984; Calarco, 1995). In fact, thelocalization of the Balbiani body, a concentration of materialthat contains RNA and basic proteins as well as numerousmitochondria, cisternae of endoplasmic reticulum and Golgi,is first localized at one side of the germinal vesicle (GV)during oocyte growth, and although its role in themammalian oocyte is not yet clear, it raises questions aboutthe putative functional role of organelle polarization(Zamboni, 1970; Smedt et al., 2000). It has been shown thatthere is a polarization of mitochondria during oocytematuration. Van Blerkom and Runner (1984) have shownthat mitochondria translocate to the perinuclear regionduring first metaphase spindle formation and disperseduring first polar body extrusion. This translocation ismediated by microtubules in pig oocytes (Sun et al., 2001).Interestingly, in-vitro matured (IVM) pig oocytes exhibitincomplete movement of mitochondria to the innercytoplasm, hence affecting cytoplasmic maturation (Sun etal., 2001).

During meiosis, the oocyte undergoes asymmetriccytokinesis necessary for proper accumulation of maternalfactors in the ooplasm and restriction of the material lostduring polar body extrusion. The cortical localization of themeiotic spindle with formation of microvillus-free areas andthickening of the actin layer associated with the oocytecortex supports the existence of a polarized phenotype(Longo and Chen, 1985). Therefore, during oocytematuration, some link between the spatial regulation of cellcycle cues and the first asymmetric cytokinesis, may serveto preserve a polarized distribution of factors within theoocyte. This polarization may extend after fertilization toensure proper second asymmetric polar body extrusion.

Considering that nuclear positioning may defineasymmetrically localized functions within a given cell, it isinteresting to note the polarized position of the GV inmammalian oocytes (Raven, 1961). In most mammalianspecies, the GV is positioned eccentrically (for review, seeLefèvre, 2003); specifically, this has been characterized inthe human (Combelles et al., 2002), mouse (Mattson andAlbertini, 1990), and hamster (Plancha and Albertini, 1992).However, exceptions to this tendency have been reported inthe mouse, where the GV is apparently located centrally andthe meiotic spindle, according to some, seems to assemblefrom a central position and only later migrates to the oocytecortex (Maro and Verlhac, 2002). Although the mechanismsestablished to maintain the anchoring of the GV to theoocyte cortex remain unknown, it was recently postulatedthat central GV positioning reported in mouse or rat oocytesmay be due to an artefact of culture (Albertini and Barrett,2004).

Overall, proper coordination between karyokinesis andcytokinesis is essential for fertilization and earlyembryogenesis. Coordination between the cortical actincytoskeleton and the meiotic spindle is essential inmaintaining proper asymmetric divisions during the finalstages of oogenesis. For example, formin-2 is a maternal-effect gene, expressed in oocytes, necessary for themaintenance of the correct positioning of the meiotic spindleto the oocyte cortex (Leader et al., 2002). Fmn-2–/– females

481

Article - Cell polarity during folliculogenesis and oogenesis - CE Plancha et al.

are sub-fertile and fertilization of Fmn-2–/– oocytes resultsin failure of polar body extrusion, polyploid embryoformation and recurrent pregnancy loss. It is tempting tospeculate that proteins that interact with the corticalcytoskeleton may be additionally involved in maintenanceof proper spindle position necessary for correct asymmetricdivisions. On the other hand, single chromosomes were alsofound to be able to induce cortical and plasma membranemodifications, normally seen in proximity of intact MI orMII spindle (Van Blerkom and Bell, 1986). Therefore,polarity of cortical cytoplasm could be determined by thepresence of a tight set of chromosomes beneath the oolema(Van Blerkom and Bell, 1986). Whether GV and spindleposition are due to asymmetric surface signalling comingfrom the granulosa cells through TZP is still argued(Albertini and Barrett, 2004) and should be considered withcaution, since no experimental evidence has been found tosuggest that disruption or maintenance of TZP relates to GVand spindle positioning.

Partial loss of polarity mayaccount for competence decreasein IVM oocytesSince the seminal experiments of Pincus and Enzmann(1935) and Edwards (1965), it has been known that oocytesreleased from their follicular environment spontaneouslyresume meiosis in vitro. However, in-vitro maturation stillrepresents a challenge in assisted reproductive technologies(Hardy et al., 2000), as IVM oocytes exhibit decreaseddevelopmental competence relative to their in-vivo matured(IVO) counterparts (Eppig and O’Brien, 1998; Schroeder etal., 1988; Mermillod et al., 1999; Moor and Dai, 2001;Trounson et al., 2001). Besides a variety of arguments thatmight explain the low quality of IVM oocytes, and sincecytoskeletal orientation is a key factor for polaritydetermination, new cellular markers of oocyte quality haverecently been proposed, after careful comparative analysesof cytoskeleton organization between IVM and IVO mouseoocytes (Sanfins et al., 2003, 2004). These experimentshave allowed the uncovering of previously undetectedmorphological differences between IVM and IVO mouseoocytes that seem to be of functional significance.However, great care in extrapolating this interpretation toother species, namely to the human, must be exercised,particularly taking into account the known differences interms of centrosome dynamics and inheritance (Schatten etal., 1986).

Centrosome positioning has been considered a criticalfactor in the establishment of cell polarity, since it directsmicrotubule organization (for review, see Bornens, 2002).Even though the relevance of distribution and quantity ofmicrotubule organizing centres (MTOC) in mammalianoocytes is still unclear, it has been reported that inDrosophila, maternally derived MTOC are involved inreorganizing oocyte microtubules used to reorientmolecular gradients for both anterior/posterior anddorsal/ventral axes (Pellettieri and Seydoux, 2002; Wodarz,2002). Furthermore, centrosome migration to the mid-bodyvicinity is required for proper cytokinesis in humanfibroblasts (Piel et al., 2001).

Regarding the distribution of microtubules andcentrosomes, IVM mouse oocytes exhibit barrel shape andlarger meiotic spindles with scattered γ-tubulin focidistributed throughout the spindle proper. These oocytesalso exhibit a lower number of cytoplasmic MTOC andpresent increased polar body size. In contrast, IVO mouseoocytes display pointed shape and more compact spindles;with γ-tubulin distribution restricted to the spindle poles.These oocytes show increased cytoplasmic MTOC andreduced polar body size (Figure 2). Furthermore, thedistinct organization of these cytoskeleton markers has beenshown to be related to distinct cell cycle progression duringspindle morphogenesis. While IVM mouse oocytes exhibitprotracted and asynchronous meiotic progression, IVOmouse oocytes display synchronous and efficient meioticprogression relying on proper location of cell cycle factorsduring maturation (Sanfins et al., 2003).

Interestingly, in contrast to IVO oocytes, in IVM oocytesthe spindle anchoring seems to be partially lost.Considering that the spindles produced in IVM do notunderlay the oolema as do IVO spindles, this could reflecta partial loss of polarity of IVM oocytes as described above,namely by perturbing spindle positioning and orientation atthe oocyte cortex. Consequently, this abnormal positioningof the spindle compromises successful cytokinesis,resulting in mouse oocytes with increased size of polarbodies. Therefore, oocyte volume, and more importantlyMTOC distribution, is partially lost for the first polar bodyduring IVM. This partial loss of polarity of IVM oocytesmay be aggravated by an additional observation: IVMoocytes recruit massive amounts of centrosomal material(either γ-tubulin or pericentrin) for M-I and M-II spindlemorphogenesis, originating larger spindles and depletingthis material from the oocyte cytoplasm (Sanfins et al.,2003, 2004). Consequently, not only is there an initial lossof MTOC during first polar body formation, but due to theincrement in the amount of MTOC material that forms theM-II spindle, it is reasonable to suspect that even more willbe lost to the second polar body after fertilization. Sincecytoplasmic MTOC are believed to support early cleavages(Maro et al., 1985), the loss of MTOC material from IVMmouse oocytes may indicate a restriction in the amount ofmaternal factors necessary to support early embryogenesis.In clear contrast, retention of cortical MTOC in IVO mouseoocytes allows formation of compact spindles, small polarbodies and proper preservation of accumulated maternalfactors in the cytoplasm that will be used duringembryogenesis. A working model that illustrates all thesefeatures is shown in Figure 3.

In conclusion, the defective alignment of themicrotubule–centrosome complex in murine IVM oocytesresults in partial loss of spatial asymmetries, whichultimately may contribute to lower oocyte quality. Theimplications of these findings in terms of fertilization anddevelopmental competence are currently being tested inmouse oocytes. Extension of these studies to oocytes fromother mammalian species such as humans will allow abetter understanding of the competencies in both nuclearand cytoplasmic compartments during the process of oocytematuration. This may ultimately lead to improved quality inassisted reproductive technologies.

482

Article - Cell polarity during folliculogenesis and oogenesis - CE Plancha et al.

Figure 2. In-vivo matured (IVO) (A) and in-vitro matured (IVM) (B) mouse oocytes denoting microtubules (tubulin, green),centrosomes (pericentrin, red) and DNA (Hoechst, blue). Note the pointed-shape spindle of IVO oocytes, contrasting with thebarrel-shape spindle of IVM oocytes. In addition, IVO oocytes display a small polar body that does not contain microtubuleorganizing centres (MTOC); in contrast, IVM oocytes exhibit a large polar body containing MTOC.

Figure 3. Working model showing the fundamental differences between in-vivo matured (IVO) and in-vitro matured (IVM)oocytes. Note that IVO oocytes present a small polar body and retain increased amounts of cortical microtubule organizing centres(MTOC) that keep them spatially segregated from the cortically positioned compact spindle.

a b

483

Article - Cell polarity during folliculogenesis and oogenesis - CE Plancha et al.

Acknowledgements

This work was supported by Fundação para a Ciência e aTecnologia POCTI/ESP/43628/2000 (CEP),#SFRH/BD/2757/2000 (AS), #SFRH/BD/6439/2001 (PR),March of Dimes Birth Defects Foundation 01–248 (DA)

References

Albertini DF, Anderson E 1974 The appearance and structure ofintercellular connections during the ontogeny of the rabbitovarian follicle with particular reference to gap junctions. Journalof Cell Science 63, 234–250.

Albertini DF, Barrett SB 2004 The developmental origins ofmammalian oocyte polarity. Seminars in Cell and DevelopmentalBiology 15, 599–606.

Albertini DF, Barrett SL 2003 Oocyte–somatic cell communication.Reproduction Supplement 61, 49–54.

Albertini DF, Combelles CMH, Benecchi E et al. 2001 Cellular basisfor paracrine regulation of ovarian follicle development.Reproduction 121, 647–653.

Bornens M 2002 Centrosome composition and microtubuleanchoring mechanisms. Current Opinion in Cell Biology 14,25–34.

Calarco PG 1995 Polarization of mitochondria in the unfertilizedmouse oocyte. Developmental Genetics 16, 36–43.

Calarco PG, Donahue RP, Szollosi D 1972 Germinal vesiclebreakdown in the mouse oocyte. Journal of Cell Science 10,369–385.

Combelles CMH, Carabatsos MJ, Kumar TR et al. 2004 Hormonalcontrol of somatic cell oocyte interactions during ovarian follicledevelopment. Molecular Reproduction and Development 69,347–355.

Combelles CMH, Cekleniak NA, Racowsky C et al. 2002Assessment of nuclear and cytoplasmic maturation in in-vitromatured human oocytes. Human Reproduction 17, 1006–1016.

Edwards RG 2001 Ovarian differentiation and human embryoquality. 1. Molecular and morphogenetic homologies betweenoocytes and embryos in Drosophila, C. elegans, Xenopus andmammals. Reproductive BioMedicine Online 3, 138.

Edwards RG 1965 Maturation in vitro of mouse, sheep, cow, pig,rhesus monkey and human ovarian oocytes. Nature 208,349–351.

Eppig JJ, O’Brien M 1998 Comparison of preimplantationdevelopmental competence after mouse oocyte growth anddevelopment in vitro and in vivo. Theriogenology 49, 415–422.

Gardner RL 1999 Scrambled or bisected mouse eggs and the basis ofpatterning in mammals [review]. Bioessays 21, 271–274.

Hardy K, Wright CS, Franks S et al. 2000 In vitro maturation ofoocytes. British Medical Bulletin 56, 588–602.

Irving-Rodgers HF, Harland ML, Rodgers RJ 2004 A novel basallamina matrix of the stratified epithelium of the ovarian follicle.Matrix Biology 23, 207–217.

Jockusch BM, Rothkegel M, Schwarz G 2004 Linking the synapse tothe cytoskeleton: a breath-taking role for microfilaments.Neuroreport 15, 1535–1538.

Leader B, Lim H, Carabatsos MJ et al. 2002 Formin-2, polyploidy,hypofertility and positioning of the meiotic spindle in mouseoocytes. Nature Cell Biology 12, 921–928.

Le Gall AH, Yeaman C, Muesch A, Rodriguez-Boulan E 1995Epithelial cell polarity: new perspectives. Seminars inNephrology 15:272–84.

Lefèvre B 2003 Nuclear phosphoinositide signalling and theresumption of oocyte meiosis. In: Cocco L, Martelli AM (eds)Nuclear Lipid Metabolism and Signalling. Research Signpost,Italy, pp. 63–77.

Longo FJ, Chen D-Y 1985 Development of cortical polarity inmouse eggs: involvement of the meiotic apparatus.Developmental Biology 107, 382–394.

Maro B, Verlhac MH 2002 Polar body formation: new rules for

asymmetric divisions. Nature Cell Biology 4, E281–283.Maro B, Howlett SK, Webb M 1985 Non-spindle microtubule

organizing centers in metaphase II–arrested mouse oocytes.Journal of Cell Biology 101, 1665–1672.

Mattson BA, Albertini DF 1990 Oogenesis: chromatin andmicrotubule dynamics during meiotic prophase. MolecularReproduction and Development 25, 374–383.

Mermillod P, Oussaid B, Cognié Y 1999 Aspects of follicular andoocyte maturation that affect the developmental potential ofembryos. Journal of Reproduction and Fertility Supplement 54,449–460.

Moor RM, Dai Y 2001 Maturation of pig oocytes in vivo and invitro. Reproduction Supplement 58, 91–104.

Motta PM, Makabe S, Naguro T et al. 1994 Oocyte follicle cellsassociation during development of human ovarian follicles. Astudy by high resolution scanning and transmission electronmicroscopy. Archives of Histolology and Cytology 57, 369–394.

Muller HA, Bossinger O 2003 Molecular networks controllingepithelial cell polarity in development. Mechanisms ofDevelopment 120, 1231–1256.

Navarro-Costa P, Correia SC, Gouveia-Oliveira et al. 2005 Effects ofmouse ovarian tissue cryopreservation on granulosa cell–oocyteinteraction. Human Reproduction (in press).

Nelson WJ 2003 Adaptation of core mechanisms to generate cellpolarity. Nature 422, 766–774.

Pellettieri J, Seydoux G 2002 Anterior-posterior polarity in C.elegans and Drosophila – parallels and differences. Science 298,1946–1950.

Piel M, Nordberg J, Euteneuer U, Bornens M 2001 Centrosome-dependent exit of cytokinesis in animal cells. Science 291,1550–1553.

Pincus G, Enzmann EV 1935 The comparative behavior ofmammalian eggs in vivo and in vitro. Journal of ExperimentalMedicine 62, 665–675.

Plancha CE, Albertini DF 1994 Hormonal regulation of meioticmaturation in the hamster oocyte involves a cytoskeleton-mediated process. Biology of Reproduction 51, 852–864.

Plancha CE, Albertini DF 1992 Protein synthesis requirementsduring resumption of meiosis in the hamster oocyte: early nuclearand microtubule configurations. Molecular Reproduction andDevelopment 33, 324–332.

Plusa B, Grabarek JB, Piotrowska K et al. 2002 Site of the previousmeiotic division defines cleavage orientation in mouse embryos[comment]. Nature Cell Biology 4, 811–815.

Raven P 1961 Polarity and symmetry. In: International Series ofMonographs on Pure and Applied Biology – Oogenesis.Pergamon Press, New York, vol. 10, pp. 163–180.

Rodgers RJ, Lavranos TC, van Wezel IL et al. 1999 Development ofthe ovarian follicular epithelium. Molecular and CellularEndocrinology 151, 171–179.

Schatten H, Schatten G, Mazia D et al. 1986 Behavior ofcentrosomes during fertilization and cell division in mouseoocytes and in sea urchin eggs. Proceedings of the NationalAcademy of Sciences of the USA 83, 105–109.

Schroeder AC, Downs SM, Eppig JJ 1988. Factors affecting thedevelopmental capacity of mouse oocytes undergoing maturationin vitro. Annals of the New York Academy of Sciences 541,197–204.

Sanfins A, Plancha CE, Overstrom EW et al. 2004 Meiotic spindlemorphogenesis in in vivo and in vitro matured mouse oocytes:insights into the relationship between nuclear and cytoplasmicquality. Human Reproduction 19, 2899–2998.

Sanfins A, Lee GY, Plancha CE, Overstrom EW et al. 2003Distinctions in meiotic spindle structure and assembly during invitro and in vivo maturation of mouse oocytes. Biology ofReproduction 69, 2059–2067.

Smedt VD, Szollosi D, Kloc M 2000 The Balbiani body: asymmetryin the mammalian oocyte. Genesis 26, 208–212

Sun QY, Wu GM, Lai L et al. 2001 Translocation of activemitochondria during pig oocyte maturation, fertilization and earlyembryo development in vitro. Reproduction 122, 155–163.

Article - Cell polarity during folliculogenesis and oogenesis - CE Plancha et al.

Trounson A, Anderiesz C, Jones G 2001 Maturation of humanoocytes in vitro and their developmental competence.Reproduction 121, 51–75.

Van Blerkom J, Bell H 1986 Regulation of development in the fullygrown mouse oocyte: chromosome-mediated temporal and spatialdifferentiation of the cytoplasm and plasma membrane. Journalof Embryology and Experimental Morphology 93, 213–238

Van Blerkom J, Runner MN 1984 Mitochondria reorganizationduring resumption of arrested meiosis in the mouse oocyte.American Journal of Anatomy 171, 335–355.

Wodarz A 2002 Establishing cell polarity in development. NatureCell Biology 4, E39–44.

Zamboni L 1970 Ultrastructure of mammalian oocytes and ova.Biology of Reproduction Supplement 2, 44–63.

Paper based on contribution presented at the InternationalSerono Symposium ‘From the Oocyte to the Embryo: aPathway to Life’ in Stresa, Milan, Italy, September 24–25,2004.

Received 1 November 2004; refereed 26 November 2004;accepted 3 February 2005.

484