Transcription and cell cycle-dependent development of intranuclear bodies and granules in two-cell bovine embryos

Bovine two-cell embryos were produced by maturation and fertilization in vitro and isolated at 27, 30 or 33 h after insemination. Embryos were incubated with [3H]uridine for 10 h. Other embryos were incubated with [3H]uridine for 1 h starting at 0-3 h, 3-6 h, 6-9 h or 9-12 h after cleavage (hpc) to the two-cell stage. Subsequently, all embryos were washed, fixed, dehydrated, embedded in Epon and sectioned for light microscope autoradiography and transmission electron microscopy. Thus, at the time of fixation the embryos incubated in [3H]uridine for 1 h represented the periods 1-4, 4-7, 7-10 and 10-13 hpc. Among embryos subjected to [3H]-uridine incubation for 10 h, a majority of those in interphase displayed auto-radiographic labelling over the nuclei, whereas none of the embryos incubated for 1 h did so. At 1-4 hpc, two-cell embryos presented electron-dense nucleolus precursor bodies and large clusters of electron-dense granules of various sizes in their nuclei. At 4-7 hpc, ring-shaped or horseshoe-shaped bodies of the same electron density as the nucleolus precursor bodies were found in the periphery of the granule clusters. At 7-10 hpc, several incomplete ring-like bodies of the same electron density as the nucleus precursor bodies were found deeply embedded in the granule clusters. The interior of these bodies contained granules lining a central vacuole. At 10-13 hpc, all two-cell embryos were in mitosis. It is concluded that bovine embryos produced in vitro display a certain rate of transcription during the second cell cycle without the presence of a well-defined transcriptional peak, and that this activity is paralleled by cell cycle-dependent interaction of intranuclear bodies and granules.