Methionine requirements for the preimplantation bovine embryo.

The early embryo's nutritional environment plays an important role in establishing its developmental potential. However, little is known about the specific nutrient requirements of the embryo. The objective of the present study was to determine requirements of the in vitro produced bovine embryo for the essential amino acid methionine. In addition to serving as a precursor for polypeptides, methionine plays roles in regulation of translation, DNA methylation, and antioxidant balance. In the first experiment, embryos were cultured in potassium simplex optimized medium - bovine embryo modification 2 containing 0, 35, 50, 100, 200 or 400 µmol/l L-methionine for 8 days. There was no effect of methionine concentration on cleavage rate. The percent of oocytes that developed to blastocyst was lower for embryos without methionine at Day 7 and 8 than other groups but was similar for embryos cultured with 35-400 µmol/l. Neither total cell number, allocation of cells to trophectoderm or inner cell mass, or frequency of apoptosis was affected by methionine concentration. In the second experiment, embryos were cultured with 0, 7, 14, 21, 28 or 35 µmol/l methionine. There was no effect of methionine concentration on cleavage rate. The percent of oocytes that developed to blastocyst was lower for embryos without methionine at Day 7 and 8 but was not different between embryos cultured with 7-35 µmol/l methionine. However, the proportion of blastocysts that were expanded, hatching or hatched on Day 7 was reduced at lower concentrations of methionine (7 and 14). DNA methylation of blastocyst nuclei was unaffected by methionine concentration but intracellular glutathione content was higher for embryos cultured without methionine. In conclusion, the methionine requirement for preimplantation development is between 14 and 21 µmol/l. These concentrations are lower or similar to those found in the reproductive tract and suggest that methionine deficiency is not a common cause of embryonic mortality.