maturation and development of mammalian oocytes

In vitro embryo production (IVEP) is one of the tool to gain rapid genetic improvement in livestock besides its use in conservation of endangered species. IVEP provides necessary starting material for the basic research in animal development, embryo genomics and stem cells. The success of IVEP depends upon several major factors such as quality of oocyte and sperms, type of culture medium, culture conditions etc. In vitro produced embryos differ substantially from in vivo derived embryos in the mRNA expression patterns of genes important for development. As female germplasm (oocyte) is more valuable when compared to male germplasm (sperm) because of scanty availability and relatively fixed number, it is essential to maintain quality of oocyte in IVEP. Oocyte maturation is an inevitable step towards successful IVEP as the process of maturation is still not clearly revealed, it is essential to identify temporal expression pattern of marker genes involved during maturation process for production of developmentally competent embryos. Mammalian models can provide better insight into the oocyte maturation mechanism. This review focuses on the various marker genes which are essential for successful maturation of mammalian oocyte and are key regulators in deciding the fate of oocytes for production of developmentally competent embryos.


Introduction
embryo is quite challenging even when less is known about the molecular transformation for In recent time much attention has been given such an important and basic aspect of reproductive to implement biotechnological tools in reproduction biotechnology.For the production of good quality and their application in successful embryo embryos and blastocyst the oocyte must have high production is obvious.Currently number of studies developmental potential (Pfeffer et al., 2007), reported successful production of embryos in which is specifically characterized by the temporal domestic animals (Bousquet et al.,1999, Luvoni, expression of some essential oocyte marker 2000, Hinrichs,2010, Paramio,2010).In vitro genes viz; BMP15, GDF9, c-kit, c-Mos, MATER, production of mammalian embryos has become ZAR1, Estradiol, Leptin, NALP9, IGFBP1, p180 widely accepted and is commercially applicable etc. during the period of growth, differentiation in live stock industry due to its advantages and and maturation of oocyte.Pre-implantation flexibility.However, the efficiency of producing development studies indicates, series of viable and developmentally competent embryos morphological changes/events take place starting and the development of such embryos after from germ cell development, their growth, embryo transfer are seeming to be inferior to that differentiation, cumulus cells expansion, and which occurs in vivo conditions.oocyte maturation.Physiologically these are very To perform such techniques for the complex events which lead to several alterations production of developmentally competent in cellular morphology.To understand the administration of chorionic gonadotropin (CG).molecular pattern of gene expression clearly, it is The ability of the oocyte to complete preimplannecessary to know the up regulation or down tation development to the blastocyst stage, regulation of these marker transcripts in time successful initiation and sustaining pregnancy point scale during the growth, differentiation, and ultimately the birth of a healthy offspring is cumulus cells expansion, oocyte maturation up to generally referred as developmental competence balstocyst formation and it is also essential to (Krisher, 2004;Sirard et al., 1992).know role of protein and their respective functional During follicle growth, the oocyte obtains amino acids involved in these events so their the complement of cytoplasmic organelles and accumulates mRNAs and proteins which enable molecular characterization appears mandatory.
it to be fertilized and to progress through the first The molecular events controlling mammalian cleavage until embryonic genes start getting oocyte maturation and early embryogenesis viz; expressed.Activation of zygotic genome is the first reports on molecular analysis of these important event of gene expression after fertilization which events involved in oocyte growth and maturation.
occurs within very short time intervals.This

Oocyte Maturation
maternal zygotic transition (MZT) varies in its timing between different species.Such an oocyte The mammalian oocyte maturation is defined embryonic transitions are sustained by gene as the reinitiating and completion of the first meiotic transcripts and polypeptides produced in oocyte division, subsequent progression to metaphase II, during oogenesis.nuclear and cytoplasmic processing, which is The activation of embryonic transcription ultimately essential for fertilization and early also known as zygotic gene activation (ZGA) embryo development.Completion of the first represents the transition from maternal to embryonic meiotic division takes place when oocytes undergo control of development.During this transition the extensive growth through cellular interaction embryo begins to synthesize its own mRNA and with the granulosa and theca cells.Asymmetric then protein rather than relying on that which was cytokinesis occurs in oocytes and first polar body inherited from mother in the egg.In absence of containing haploid chromosome compliment is the appropriate activation and subsequent extruded.Second meiotic division follows maintenance of embryonic gene expression an immediately but oocytes remain arrested in embryo will simply fail to develop further for metaphase II until it comes in contact with sperm.
early cleavage divisions.The maternal contribution In antral follicles, the initiation of maturation in to the successful development of the zygote fully-grown oocytes (cumulus-oocyte-complex) begins during oogenesis with the synthesis and is dependent on the mid-cyclic onset of the accumulation of mRNAs and proteins in addition luteinizing hormone (LH) surge or the external to sustaining the embryo during the early opening a new era in understanding oocyte cleavage this material is also likely to contribute biology, MPF was the first of the cyclin-cyclin to development after the onset of embryonic dependent kinase (cdk) complexes to be transcription.
characterized.After this a number of studies Developmental competence is gradually demonstrate several other molecules involved during growth differentiation and maturation of acquired during the period of oocyte growth and maturation via appropriate follicular differentiation mammalian oocyte, some important of those are under the control of gonadotropins (Gosden, described here.2002;Sutton et al., 2003;Sirard et al., 2007).
Transforming growth factor (TGF) : The TGF Oocytes matured in vitro are known to be inferior super family of growth and differentiation factors in their developmental competence compared has been shown to play important roles in with in vivo matured counterparts ( adult.These family members are synthesized as oocytes has been proposed to be due to failure of pre-propeptides which are processed to form the timely onset of embryonic genome activation mature, disulfide-linked dimers.Several TGF-b resulting from incomplete cytoplasmic maturation family members have been shown to get of these oocytes (Schramm et al., 2003).This expressed in the ovary, including Mullerian suggests that IVM conditions may not provide inhibiting substance, inhibinA, activin bA, the ideal conditions for oocytes and this in turn, activin bB, growth differentiation factor 9 (GDFmay be related to the failure of the cumulus cells 9), bone morphogenetic protein (BMP)-6, and to provide the correct signaling.Therefore, it BMP-15, and several of these factors have been would be helpful to define marker genes that shown in vivo and/or in vitro to play important predict the developmental competence of oocytes roles in regulating reproductive function ( only in both female and male gonads but also in The molecular mechanisms controlling meiosis, nongonadic tissues including mouse hypothalamus, and the regulation of the cell cycle in general, was human pituitary, uterus, and bone marrow and well known in 1971 with the characterization of ewe pituitary (Fitzpatrick et al., 1998).Within the M phase promoting factor (MPF, also referred to ovary GDF-9, BMP-6, and BMP-15 are expressed as maturation promoting factor) in mature specifically in the oocyte (Lyons et al., 1989; Xenopus oocytes (Masui and Markert, 1971;McGrath et al., 1995;Dube et al., 1998).In Smith and Ecker,1971).MPF was originally particular, GDF-9 and BMP-15 mRNA are characterized as an activity present in mature frog expressed specifically in the oocyte of the type 3a oocytes, which could rapidly induce nuclear preantral follicle (small primary follicle with onematuration when injected even in minute layer of granulosa cells), and expression persists in quantities into immature oocytes.In addition to oocytes throughout all stages of folliculogenesis and in cumulus cell-oocyte complexes after IGFs system: IGFs have been implicated in et al. et al.
ovarian physiology because they have significant ovulation (McGrath , 1995; Dube , trophic and steroidogenic effects on granulosa 1998).It has been shown that a knockout of the GDF-9 gene leads to infertility due to a block at cells in vitro (Adashi et al., 1985).Earlier it has the type 3b (primary) follicle stage, absence of been shown that granulosa cells produce Insulin thecal layer formation, and defects in oocyte like growth factor binding proteins (IGFBPs) in meiotic competence (Elvin et al., 1999).However, vitro, and clinical studies have raised the possibility the potential role of GDF-9 at later stages of that production of IGFBPs by granulosa cells folliculogenesis is unknown.
(GC) might be instrumental in the regulation of follicular survival or maturation.The concentrations Maternal-effect genes: Maternal antigen that of IGFBP1 in follicular fluids with mature embryos require (MATER) and zygote arrest 1 oocytes are found significantly higher than those (Zar1) are known maternal effect genes required in follicular fluids with intermediates and for early development prior to zygotic genome immature oocytes in human whereas IGFBP-1 activation (Tong et al., 2000;Wu et al., 2003).demonstrates a rather obscure expression pattern However, fundamental knowledge regarding the in the primate ovary (Jose et al., 2002).identification of additional oocyte-expressed Other Oocyte Markers: proteins and their role in early embryonic i) c-kit: Presence of c-kit mRNA and soluble c-kit development is limited.Identification and protein first time reported in human oocytes and characterization of such factors will help resolve follicular fluid (Tanikawa et al., 1998).Ovarian the physiological mechanism by which the c-kit, found primarily on oocytes, has autophooocyte drives oocyte maturation and early sphorylation activity, and c-kit and KL are embryonic development, these putative factors required for maintenance of oocyte growth in may also function as novel objective molecular vitro.markers of oocyte and embryo competence in ii) Leptin: It is the product of the obese gene (ob), farm animals, the maternal contribution to and is secreted in plasma from mature adipocytes.successful development occurs during oogenesis It has been recently reported that leptin is with synthesis and accumulation of mRNA and synthesized in granulosa and cumulus cells proteins.
within the follicle of the ovary, and is present in MATER was discovered as an antigen mature human oocytes, suggesting possible roles associated with ovarian autoimmunity in of leptin in several aspects of pre-and postthymectomized mice (Tong et al., 1999).Mater ovulatory follicular development (Matsuoka et human homolog was found to be expressed only al.,1999). in oocytes (Tong et al., 2002).Contrasting with iii) MSY2: It is a germ-cell-specific member of the the results in mice, human ZAR1 transcript was Y-box family of DNA-/RNA-binding proteins, is detected both in ovary and testis (Tong et al., proposed to function as a coactivator of transcription 1999, 2000; Wu et al., 2003).Expression of in the nucleus and to stabilize and store maternal MATER, ZAR1, GDF9, and BMP-15 was and paternal mRNAs in the cytoplasm.MSY2 analyzed by RT-PCR during oocyte IVM and preexpression is highly restricted and essentially implantation embryo development in bovine.But confined to the oocytes (Yang et al., 2005).it remained to be elucidated whether these iv) VASA: It is a DEAD box mRNA helicase maternal-effect genes were conserved in other involved in oocyte differentiation and germline cyst development (Styhler et al., 1998).mammalian species.In addition to the oocyte itself, ZAR1 transcript was detected in the ovary v) Stat-3: an oocyte marker which stains the cytoplasm of all oocytes, has critical role in early and in the testis.In human, expression is higher in mammalian development (Antczak and Van the testis whereas the opposite is true in mouse Blerkom, 1997  development into healthy and viable embryos 7. Dube J.L., Wang P., Elvin J., Lyons K.M., (Juengel et al., 2005).developmental competence than simple 10.Fair T., Hyttel P., Greve T. (1995).Bovine oocyte morphological assessments.
diameter in relation to maturational competence and transcriptional activity.Mol.Reprod.Dev., characteristics of the oocyte such as appearance of the rat oocyte in vitro: inhibition and of cumulus cells and cytoplasm, oocyte size and induction of maturation in the presence or time of polar body extrusion are related to the absence of the cumulus oophorus.Dev.Biol., 75: fertilization ability of the oocytes and subsequent 247-254.

Zygotic Genome Activation: Develop- comprehension of developmental aspect of mental Competence reproductive biotechnology. There are scanty
Leibfriedregulating mammalian ovarian functions such as Rutledge et al., 1987; van de Leemput et al., follicular development and maturation, production 1999; Rizos et al., 2002).In addition, significant of steroids, and regulation of gonadotropin receptors differences have been reported in global gene (Knight et al., 2006, Juengel et al., 2005).TGF-b expression profile between in vitro and in vivo super family is comprised of secreted peptide matured human oocytes (Jones et al., 2008; Wells growth factors which are critical for regulating a and Patrizio, 2008) as well as for specific variety of developmental events, including cell candidate transcripts in bovine oocytes proliferation, differentiation, matrix secretion, (Lonergan et al., 2003).The poor developmental and apoptosis during embryogenesis and in the competence of in vitro maturation (IVM) primate ). (Wu et.al. 2003).