RNA Synthesis in Rabbit Embryos

The information reviewed above has generally shown that 4 S (tRNA), rRNA, and heterogeneous RNA synthesis is detectable as early as the 4-cell stage in mouse embryos. It is further clear that the rate of incorporation of uridine into RNA increases steeply after the 8-cell stage and continues to rise throughout the development of the preimplan- 

There is no net increase in RNA content, per embryo, however, until 72 hours postcoitum at which time there is a marked acceleration of uridine incorporation accompanying the transformation of the morula into a blastocyst as the embryo leaves the oviduct and enters the uterus (Manes, 1969). The qualitative aspects of RNA synthesis in preimplantation rabbit embryos have been investigated by the electrophoretic separation of tritium-labeled embryonic RNA on polyacrylamide gels (Manes, 1971). Labeling of RNA species having migratory properties identical with those of tRNA are detectable from the 2-cell stage onward. 

The small differences in transcriptional patterns displayed by mouse and rabbit embryos may be related to morphological ifferences in the embryos. The rabbit ovum, for example, is about twice the diameter of the mouse ovum, cleaves more rapidly, reaches a morula stage of about 100-130 cells before cavitation begins and retains its zona pellucida throughout the preimplantation period. It is possible that further slight variations in transcriptional patterns will be uncovered as other orders of mammals are studied. 

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