Comparison of overall rates of RNA synthesis in implanting and delayed implanting mouse blastocysts in vitro

Implanting and delayed implanting mouse embryos were incubatedin vitro with ³H]uridine for 2–24 hr. The size and specific activity of the ³H]UTP pools were determined by means of a double isotope technique using copolymer synthesis with the ³H]UTP in the embryos, exogenous ¹⁴C]ATP, andE. coli RNA polymerase. Using the rate of incorporation of ³H]uridine into acid-insoluble material and the specific activity of the ³H]UTP pools, it was possible to calculate the overall rate of incorporation of uridine into RNA by the embryos. In implanting embryos it was constant for 24 hr. In contrast, the initial rate of uridine incorporation by the delayed implanting embryos was only 31% of that in implanting embryos (i.e., per cell); this increased steadily during the incubation period, reaching 81% of the rate in implanting embryos after 24 hr. This activation of RNA synthesis by delayed implanting embryosin vitro occurred in the absence of any uterine stimulatory factors. Further, it was shown that although 10% mouse serum would support trophoblastic outgrowthin vitro, it did not influence uptake, distribution of label into nucleotides, or rate of uridine incorporation into RNA in either implanting or delayed implanting embryos. Therefore, it is suggested that if depression and activation of metabolic activity in blastocysts are part of the mechanims of delayed implantation, and if trophoblast outgrowthin vitro is analogous to the process of implantationin vivo, then these two aspects of embryo activation are under different controls.