哺乳類初期胚発生に及ぼす環境因子の解析(<シンポジウム>妊娠の成立機構 : 胚発生から着床まで)

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Human in vitro fertilization and embryo transfer (IVF-ET) became an indispensable modality for treating infertile patients. The principle of this method is simple : that is, recovery of gametes from the gonads of men and women and transfer of the embryos into the uterus. This method can be expected, therefore, to be applied to many patients with a variety of causes of infertility. Unfortunately, the success rates are not satisfactory in the majority of clinics in the 14 years since the first report of a test tube in 1978. In view of improving the success rate, one major issue is the protocol used for ovulation induction, which may influence the quality of eggs as well as the environmental conditions in the endometrium at the time of embryo replacement. Another major issue should be the technique for embryo culture because, in general, mammalian embryos, including humans', are known to exhibit developmental retardation in vitro. In a significant number of embryos, cleavage is arrested at the first or second cell cycle when cultured under the conventional culture conditions. This phenomenon in rodents is known as "block to development in vitro" or "two-cell block in vitro". Recently, the mouse two-cell block was found to be attenuated by the addition of superoxide dismutase (SOD) to the culture medium. SOD is the enzyme that catalyzes the dismutation reaction of superoxide anion radicals : 2O_2^- + 2H^+ → H_2O_2 + O_2. This suggests that developmental retardation in vitro may be related to the potential oxygen toxicity that embryos encounter in vitro. Following to this finding, a variety of culture conditions have been found to attenuate blocking phenome-non and to increase blastulation rate in the mouse embryos. By the addition of chemicals to the culture medium such as L-Cysteine, L-Ascorbic acid, EDTA, DTPA or thiredoxine, blastulation rates could be increased overcoming blocking phenomenon. From these findings, it seemed possible to hypothesize that developmental retardation is caused by the oxidative stress that embryos encounter in vitro. Oxidative stress is defined as an increased intracellular concentration of the active oxygen species in a stead-state condition. To make the hypothesis validated, intracellular generation of active oxygen species was measured by using DCHF-DA, a fluorescence dye procursor. The results showed that the fluorescent emissions of embryos were lowest in embryos cultured under 5% O_2 and highest under 40% O_2. L-Cysteine and thioredoxin, both of which have been shown to promote the embryo development, decreased the fluorescence emissions of embryos. All of these results would give direct evidence to the hypothesis that oxygen toxicity is involved in the developmental blockage. To elucidate the molecular mechanism of the two-cell block in the mouse, M-phase promoting factor (MPF) activity was measured in embryos grown in vivo or cultured in vitro. In the second cycle, MPF was not activated in embryos cultured under the standard culture conditions but activated in embryos cultured in the presence of SOD or thioredoxin. These facts suggests that one of major cause of the block to development may be due to impairment the cell cycle regulatory components, and that the impairment may be elicited by the oxidative stress, which embryos encounter while treated in vitro.
社団法人日本産科婦人科学会の論文
1992-08-01

哺乳類初期胚発生に及ぼす環境因子の解析(<シンポジウム>妊娠の成立機構 : 胚発生から着床まで)

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