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 3H-uridine for 30 min. Uridine incorporation was stopped by washing at 4 degrees C in PBS supplemented with cold uridine. The oocytes were grouped according to their inside diameter- < 100, 100- < 110, 110- < 120, and > or = 120 microns-and processed for autoradiography and transmission electron microscopy. Oocytes < 110 microns in diameter typically presented fibrillogranular nucleoli and were actively transcribing; in contrast, most oocytes > 110 microns 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 nucleolus inactivation is proposed. The degree of chromatin condensation varied among oocytes. Fibrillogranular nucleoli were most frequently accompanied 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 increased from < 100 to 110 microns. In conclusion, RNA synthesis appeared to cease as the oocyte diameter exceeded 110 microns, and concomitantly the nucleoli restructured from fibrillogranular to dense fibrillar.