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Cumulus expansion, nuclear maturation and connexin 43, cyclooxygenase-2 and FSH receptor mRNA expression in equine cumulus-oocyte complexes cultured in vitro in the presence of FSH and precursors for hyaluronic acid synthesis

DOI: 10.1186/1477-7827-2-44

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Abstract:

In mammals, embryos produced in vitro, in sequential steps of oocyte maturation in vitro (IVM), conventional fertilization in vitro (IVF) or intracytoplasmic sperm injection (ICSI) and embryo culture (EC), display marked differences from their in vivo counterparts with regard to morphology, timing of development, resistance to low temperature, metabolism and gene expression [1]. Thus, their clinical applications remain sub-optimal [2-4]. Evidence has emerged to support the involvement of various locally produced factors as co-regulators of folliculogenesis and oocyte nuclear and cytoplasmic maturation in addition to extrinsic regulation by pituitary gonadotropins and metabolic hormones [5]. Optimal expansion of the cumulus mass appears to be essential for cytoplasmic maturation [6,7]. In the bovine, the induction of cumulus expansion prior to fertilization increased the incidence of oocyte penetration [8]. In the mouse, successful fertilization was correlated with the quantity and quality of the expanded cumulus mass [9]. In the mare, cumulus expansion in oocytes retrieved from excised ovaries of slaughtered mares has been related to granulosa cell apoptosis with no relation to follicle size. Expanded oocytes issuing from apoptotic follicles show increased meiotic competence, but not increased activation rate after ICSI [10].The process of cumulus expansion is accompanied by modifications of gap junctions, which contain transmembrane channels formed by hexamers of proteins belonging to the connexin family. Equine, bovine, ovine and mouse cumulus cells express connexin 43 proteins [11-15]. In equine, porcine and rat cumulus cells, initiation of meiotic resumption is associated with the reduction of connexin 43 protein level [11,16,17]. In the same way, during in vitro maturation of bovine cumulus-oocyte complexes, the connexin 43-positive gap junctions disappeared [12]. Prostaglandin E2 are involved in cumulus expansion in vitro in mice [18], rats [19], and bovine [2

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