Incorporation of Labeled Nucleic Acid and Protein Precursors by Diapausing and Nondiapausing Blastocysts1, 2

Embryo culture and radioautographic techniques were used to detect incorporation of precursors of DNA, RNA and proteins by blastocysts of mammals with and without a delayed implantation. These experiments reveal the absence of labeled thymine incorporation by diapausing blastocysts from mink, fur seal, and lactating rat, although thymine is incorporated by nondiapausing rabbit and rat blastocysts. There is little or no incorporation of labeled uracil by blastocysts from those animals which have a relatively long and obligate diapause; but significant levels of uracil incorporation are present in the blastocysts from lactating rats, which have a shorter and facultative delay. Conversely, nondiapausing blastocysts of rat and rabbit have high levels of nucleic acid synthesis, as judged by incorporation of the labeled precursors. Incorporation of amino acids was shown to be present in mink, armadillo, and fur seal blastocysts and, contrary to earlier findings of others, also in diapausing rodent blastocysts. It is postulated that a uterine fluid component which may regulate (inhibit?) protein synthesis is washed out or diluted subsequent to removal of the blastocysts from the uterus, resulting in protein synthesis in vitro. This hypothesis is supported by the observation that diapausing rat blastocysts require twice as long as the nondiapausing ones to show incorporation of labeled methionine in vitro. The results reported here also support the hypothesis that “masked” mRNA may exist in the preimplantation mammal embryo.