O'Neill C
Department of Physiology, University of Sydney, Royal North Shore Hospital of Sydney, St Leonards, NSW, Australia.
Hum Reprod. 1998 May;13(5):1312-6. doi: 10.1093/humrep/13.5.1312.
Preimplantation mammalian embryos develop with a high degree of autonomy. To date, there have been no unequivocal demonstrations of a requirement for vitamins in preimplantation embryo development. Reduced folic acid acts as an important methyl donor in many reactions including the synthesis of thymidine. Thymidine does not accumulate in cells so it might be expected that significant amounts of reduced folate would be required to support the exponential increase in DNA synthesis that occurs during early embryo development. The reduction of folate is catalysed by dihydrofolate reductase (EC 1.5.1.3) which is selectively inhibited by the anti-cancer drug methotrexate. Methotrexate caused a dose-dependent inhibition of cell division in 1-cell, 2-cell and 8-cell mouse embryos with 50% inhibition of division occurring at concentrations of 1-10 microM. At a concentration of 0.1 microM only minimal inhibition of the initial cell division occurred, but continuous culture in this concentration of methotrexate completely inhibited further cell divisions. This suggests that most of the exogenous store of reduced folates was used in the first round of cell division. The effects of methotrexate were apparently primarily due to thymidine starvation, since a 10-fold excess of thymidine over methotrexate in culture media reversed the inhibition of development. Supplementing media with folic acid had no beneficial effect on the rate at which zygotes produced by in-vitro fertilization developed to the blastocyst stage. It is concluded that the development of the early embryo has an absolute requirement for reduced folate for thymidine synthesis which is met entirely by endogenous sources.
植入前的哺乳动物胚胎以高度自主的方式发育。迄今为止，尚无明确证据表明维生素对植入前胚胎发育是必需的。还原型叶酸在包括胸苷合成在内的许多反应中作为重要的甲基供体。胸苷不会在细胞中积累，因此可以预期，在早期胚胎发育过程中，需要大量的还原型叶酸来支持DNA合成的指数增长。叶酸的还原由二氢叶酸还原酶（EC 1.5.1.3）催化，该酶被抗癌药物甲氨蝶呤选择性抑制。甲氨蝶呤对1细胞、2细胞和8细胞小鼠胚胎的细胞分裂产生剂量依赖性抑制，在1-10 microM浓度下，50%的分裂受到抑制。在0.1 microM浓度下，仅对初始细胞分裂有最小程度的抑制，但在该浓度的甲氨蝶呤中持续培养完全抑制了进一步的细胞分裂。这表明大部分外源储存的还原型叶酸在第一轮细胞分裂中被消耗。甲氨蝶呤的作用显然主要是由于胸苷饥饿，因为培养基中胸苷的量比甲氨蝶呤多10倍可逆转发育抑制。向培养基中添加叶酸对体外受精产生的受精卵发育到囊胚阶段的速率没有有益影响。结论是，早期胚胎发育绝对需要还原型叶酸来合成胸苷，而这完全由内源性来源满足。
