Whole rat embryos require methionine for neural tube closure when cultured on cow serum.

Headfold-stage rat embryos, when cultured on cow serum without supplemental methionine, failed to close their neural tubes, lacked eyes and branchial arches, were abnormally shaped and were reduced in protein content compared to methionine-supplemented embryos. Methionine was essential during the first 18 h of culture, a period in which neural tube closure was initiated in supplemented cultures. All cow serum samples tested were found to require methionine addition, and the methionine was not replaced by other amino acids or vitamins, including folate. Methionine was not toxic to cultured rat embryos at concentrations up to at least 500 micrograms/ml. Analyses of serum free amino acids revealed lower levels of free methionine in cow serum compared to rat serum, and cow serum proteins contained less methionine relative to other amino acids than did rat serum proteins. Dialysis of cow serum reduced but did not eliminate the requirement for methionine. This suggested either that the free amino acids of cow serum were imbalanced or that a dialyzable component in serum interfered with the availability/utilization of methionine. Dietary supplementation of cows with rumen-protected DL-methionine increased the serum methionine level, and serum drawn from supplemented cows supported normal rat embryo development without additional methionine.