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2009 | 3 | 59-84
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Maturation and activation of porcine oocytes in the aspect of fertilization and somatic cell cloning ? biochemical, molecular and biophysical determinants

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The commonly used source of nuclear recipient cells in the somatic cell cloning of pigs are in vivo-matured (ovulated) or in vitro ? matured oocytes, reversibly blocked at the second metaphase (MII) stage. One of the most important factors that significantly affect the developmental competences of porcine cloned embryos is the artificial activation of oocytes reconstructed with somatic cell nuclei. The ability of an artificial stimulus to activate MII-stage oocytes and to initiate embryo development is essential for successfull cloning by somatic cell nuclear transfer. This ability is especially important for species such as the pig where relatively little is known about early embryonic development and where in vitro handling procedures have not been optimized. An optimal time frame to activate gilt or sow oocytes may depend on both the time required for completion of nuclear-cytoplasmic maturation and the time by which aging process of mature oocytes starts. Cytoplasmic maturation is likely to include changes in the properties, size, and density of cytoplasmic Ca2+ release channels necessary for the oocyte to elicit an increase in intracellular Ca2+ concentration in response to the activating stimuli and subsequent development. Activation of oocytes, which has been induced either during fertilization or by artificial agents during the cloning procedure, evokes the cytosolic calcium concentration ([Ca2+]c) oscilations or single [Ca2+]c transients. Despite the uncertainty of how the initial rises in [Ca2+]c are prompted, it is widely accepted that physiological or artificial activation stimulates the phosphoinositide pathway, with the generation of myo-inositol-1,4,5-trisphosphate (InsP3) by the enzymatic action of phospholipase C (PLC), and the subsequent release of calcium cations from endoplasmic reticulum. Further investigation in nto the role of PLC isoforms as the triggers of [Ca2+]c increases has led to the recent identification of a sperm-specific PLC (PLC-ksi) as the putative sperm-derived oocyte activating factor. It is known that InsP3-mediated calcium signaling pathway is responsible for downregulation of maturation-promoting factor (MPF), which contributes to such events during oocyte activation as resumption and termination of meiosis, extrusion of the second polar body, pronuclear formation, transition from meiotic to mitotic control of cell cycle and initiation of embryonic cleavage.

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Marcin Samiec, Dzial Biotechnologii Rozrodu Zwierzat, Instytut Zootechniki - Panstwowy Instytut Badawczy, 32-083 Balice k. Krakowa, Poland
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