[Objectives] The present study was designed to evaluate whether oocyte maturation period affected intracellular glutathione (GSH) concentration and to examine effects of maturation period on fertilization and early developmental ability of oocytes after ICSI. [Materials and methods] The sperm-rich fraction was collected from proven-fertile boars by the gloved hand method. As the successful fertilization rate was low after ICSI in the pig, spermatozoa were preserved at 24°C for 18 h before ICSI. Oocytes were injected with sperm heads that were isolated from spermatozoa by piezo treatment. GSH concentration of oocytes was measured after maturation-culture for 0, 12, 24, 36, 38, 40, 42, 44, 46, 48, and 50 h. For the evaluation of fertilization, sperm-head-alone was injected into oocytes after maturation for 36, 40, 44 and 48h. Some oocytes were fixed 18 h after ICSI. The others were cultured for 168 h and were observed at every 24-h interval under an inverted microscope. [Results] GSH concentration was generally low in the oocytes that were maturation-cultured shorter than 36 h (4.1 to 5.5 pM), and then it increased gradually until 44 h (13.2 pM). The concentration decreased rapidly longer than 44 h. Oocytes that were maturation-cultured for 44 h showed the highest rate of development to the blastocyst stage (35%), and the rate was significantly higher than those of the others. These results suggest that the proportion of fertilization after ICSI depends on GSH-concentration of oocytes, leading to the higher blastocyst formation after ICSI. Key words: ICSI, Fertilization, Oocyte, Blastocyst, GSH.
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