ONSET OF TRANSCRIPTION IN BOVINE OOCYTES AND PREIMPLANTATION EMBRYOS

The transition from the maternal to embryonic control of early embryon ic development [MET] in mammals is not fully understood. The objective of this study was to determine the amount of transcriptional activity in immature oocytes containing germinal vesicle (GV), mature metaphas e II arrested oocytes (MII), 2-, 4- and 8-cell bovine embryos by label ing with S-35-UTP followed by isolation of total RNA and autoradiograp hy. Expression of counts per minute (CPM) per cell showed that incorpo ration of 35S-UTP in GV oocytes was significantly higher than the back ground (P < 0.01) and decreased sharply by the time the oocytes reache d MII arrest. Incorporation significantly increased during the 2-cell stage and remained at the same level during the 4- and 8-cell stages. Uptake remained constant throughout different development stages (P > 0.05) with the highest variability observed during the 2-cell stage. W hen CPM were expressed per oocyte or embryo incorporation remained hig h at the GV stage, decreased to the background levels at the time of M II and increased again at the 2-cell stage. It remained at the same le vel during the 4-cell stage but increased significantly for the second time during the 8-cell stage. Uptake remained at the same level until the 8-cell stage when a significant increase was observed. The negati ve controls showed a significantly lower amount of incorporation compa red to the positive control [P < 0.05). Similar results were observed by autoradiography. Our observations suggest that MET starts as early as the 2-cell stage in bovine embryos, Mol. Reprod. Dev. 51:36-41, 199 8. (C) 1998 Wiley-Liss, Inc.