Transcriptional activity in in vivo developed early cleavage stage bovine embryos

Bovine embryos developed in vivo from the first to the fourth post-fertiliz ation cell cycles were processed for ultrastructural autoradiography after incubation with H-3-uridine for 10 h. We wished to detect and localize tran scriptional activity. During the first (l-cell stage) and second (2-cell st age) cell cycles we observed electron-dense fibrillar spheres (nucleolus pr ecursor bodies) and fibrillo-granular complexes in the nuclei. During these cell cycles, autoradiographic labeling was observed in heterochromatic are as and at the periphery of the fibrillo-granular complexes. During the thir d cell cycle (4-cell stage) the electron dense fibrillar spheres exhibited vacuolization. Autoradiographic labeling was found in heterochromatic areas and in the vacuoles of the fibrillar spheres. During the fourth cell cycle (8-cell stage), the electron dense fibrillar spheres exhibited both a larg e eccentric vacuole and peripheral smaller vacuoles. Autoradiographic label ing was found in heterochromatic areas throughout the nucleus and over the substance of the vacuolated fibrillar spheres, especially where chromatin p enetrated into them and where presumptive fibrillar centers were formed. In conclusion, a low level of transcription can be detected in in vivo develo ped bovine embryos as early as the, one-cell stage. Moreover, nuclear entit ies that probably prepare for nucleolus formation during the fourth cell cy cle, display a progressive autoradiographic labeling that signals a possibl e initiation of transcription of the ribosomal RNA genes during the third c ell cycle. (C) 2001 by Elsevier Science Inc.