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The development of pig reproductive biotechnology has made it possible to use this species not only as farm animals but also as important models in biomedical research. Systems based on pig embryos from in vitro fertilization (IVF) are used much more frequently, although the quality of these embryos differs from those produced by in vivo fertilization. The most frequent problems in the IVF procedure are disturbances in both nuclear and cytoplasmic oocyte maturation, and the mechanism of polyspermy specific for this species. Although there has been progress in increasing the number of oocytes produced by in vitro maturation processes, the quality of embryos from these oocytes and their developmental competence is still unsatisfactory. It has been suggested that the most important factor in this process is the establishment of optimal conditions for the development and maturation of oocytes. In this review the authors attempt to explain the molecular basis of oocyte maturation, meiosis resumption, and meiotic spindle assembly. The article is also an opportunity to use the knowledge gained to date in order to increase the efficiency of animal breeding, to evaluate the genetic value of oocytes, as well as to improve assisted reproductive techniques (ART).
The fertilization potential of mammalian oocytes may be regulated at the molecular level by the expression of species-specific sperm-egg interaction molecules, whose activities and/or cellular distribution determine the recognition and fusion of gemetes. Although there exist studies on the expression of integrins (ITGs) and zona pellucida glycoproteins (ZPs) in developmentally fully competent oocytes, the mRNA levels encoding these proteins in immature and developmentally incompetent porcine oocytes have to be elucidated. Therefore, our aim was to determine the expression of ITGB2, ZP3, and ZP3α mRNAs in porcine oocytes before in vitro maturation (IVM), in oocytes stained with BCB test but colorless, and in BCB positive oocytes after IVM. Porcine cumulus-oocyte complexes (COCs) were collected from 32 crossbred Landrace gilts, and then separated into three groups: (i) oocytes analyzed immediately after collection (n = 50), (ii) oocytes stained with brilliant cresyl blue (BCB+) and remained colorless (BCB–) (n = 50). After collection and/or staining and cultivation, all oocytes were denuded and analyzed regarding ITGB2, ZP3, and ZP3α by QT-PCR. We found a higher expression of ITGB2, ZP3, and ZP3α in oocytes immediately after collection and in BCB+ oocytes compared to BCB– oocytes (P < 0.001, respectively). No differences in the ITGB2 and ZP3 mRNA levels were observed between oocytes after collection and BCB+ oocytes. In addition, BCB– oocytes revealed lower transcript expression of all the genes under study. It is presumed that the similar mRNA levels of ITGB2 and ZP3 in oocytes after collection and in BCB+/ IVM oocytes may be related to (i) a toxic effect of BCB staining and/or to (ii) the degradation of accumulated maternal templates in porcine oocytes. Additionally, lower ITGB2, ZP3, and ZP3α transcript levels point to the down-regulation of the mRNA synthesis of stored maternal transcripts in developmentally incompetent porcine oocytes.
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