Changes in the protein content of blastocysts from normal and delayed implanting mice

Total protein content of preimplantation mouse embryos was measured at 06.00 and 16.00 hours on day 5 of pregnancy (day 1, the day of vaginal plug formation). During this ten hour period, just prior to implantation, the amount of protein increased from 23 to 35 ng per blastocyst; a net increase of 52%. In contrast, protein content was unchanged in delayed implanting embryos recovered from ovariectomized animals during a comparable period on days 5 or 10 (day 5, 24 ng at 06.00 hours and 24 ng at 16.00 hours; day 10, 29 ng at 06.00 hours and 30 ng at 16.00 hours). These results demonstrate that the different uptake and incorporation of amino acids between normal and delayed implanting embryos (Weitlauf and Greenwald, '65, '68; Weitlauf, '73) reflect a difference in the actual net synthesis of protein by the two types of blastocysts.