The development of human and mouse zygotes to blastocysts in culture does not require the presence of nucleic acid precursors in the medium, although mouse embryos can incorporate exogenous radiolabeled nucleosides into their RNA and DNA (2). The ability of embryos to grow in the absence of nucleosides indicates that de novo pathways of nucleic acid synthesis are active at this stage. Loutradis et al. (150) observed that hypoxanthine, present in Ham's F-10, induced a block in mouse embryo development at the 2-cell stage in vitro. Hypoxanthine is thought to inhibit the purine salvage pathway (151). Ham's F-10 without hypoxanthine is available commercially. Subsequent studies have revealed that both adenosine and inosine are also detrimental to the development of mouse embryos after the first cleavage division (152). Without further research into the role of nucleotides in embryo development, their omission from embryo culture media formulations would seem advisable.
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