Nucleic acid synthesis in preimplantation rabbit embryos. III. A "dark period" immediately following fertilization, and the early predominance low molecular weight RNA synthesis.

This report stems from efforts to characterize the products of transcription during the initial stages of cleavage in the rabbit embryo, since previous studies have suggested that RNA synthesis is required during this period for the continuation of normal development. Early cleaving embryos were exposed to radiolabeled precursors of nucleic acids in vitro and RNA extracted and analyzed by polyacrylamide gel electrophoresis and BD-cellulose chromatography. No unequivocal evidence of RNA synthesis could be obtained during the first three cleavage divisions; all apparent "embryonic" RNA synthesis could be ascribed to contaminating follicle cells. After the fourth cleavage, however, there is readily detectable incorporation of radioisotope into RNA by embryos which have been denuded of follicle cells. Two-thirds of the cytoplasmic RNA labeled during a 4-hour period is 25-30 X 10(3) daltons in size, and some of this low molecular weight RNA can be aminoacylated and shown to be transfer RNA. A low level of ribosomal RNA synthesis is detectable at this stage. The contribution, if any, of the rabbit embryonic genome to development prior to the 16-cell stage remains to be defined.