NUCLEUS STRUCTURE AND TRANSCRIPTIONAL ACTIVITY IN RELATION TO OOCYTE DIAMETER IN CATTLE

Bovine abattoir ovaries were sliced, and recovered oocytes were washed and incubated in medium enriched with H-3-uridine for 30 min. Uridine incorporation was stopped by washing at 4 degrees C in PBS supplement ed with cold uridine. The oocytes were grouped according to their insi de diameter - <100, 100-<110, 110-<120, and greater than or equal to 1 20 mu m - and processed for autoradiography and transmission electron microscopy. Oocytes <110 mu m in diameter typically presented fibrillo granular nucleoli and were actively transcribing; in contrast, most oo cytes >110 mu m displayed electron-dense fibrillar nucleoli and lacked transcriptional activity, as measured by the present means. Based on morphological and transcriptional information, a dynamic model of nucl eolus inactivation is proposed. The degree of chromatin condensation v aried among oocytes. Fibrillogranular nucleoli were most frequently ac companied by lightly condensed chromatin. The dense fibrillar nucleoli were usually encapsulated by heavily condensed chromatin. The oocyte nuclei underwent a peripheral translocation as the oocyte diameter inc reased from <100 to 110 mu m. In conclusion, RNA synthesis appeared to cease as the oocyte diamter exceeded 110 mu m, and concomitantly the nucleoli restructured from fibrillogranular to dense fibrillar. (C) 19 96 Wiley-Liss, Inc.