Differential transcriptional activity associated with chromatin configuration in fully grown mouse germinal vesicle oocytes

It was previously shown that fully grown ovarian germinal vesicle (CV) oocy tes of adult mice exhibit several nuclear configurations that differ essent ially by the presence or absence of a ring of condensed chromatin around th e nucleolus. These configurations have been termed, respectively, SN (surro unded nucleolus) and NSN (nonsurrounded nucleolus). Work from our and other laboratories has revealed ultrastructural and functional differences betwe en these two configurations. The aims of the present study were 1) to analy ze the equilibrium between the SN and the NSN population as a function of t he age of the mice and the time after hCG-induced ovulation and 2) to study the polymerase I (pol I)- and polymerase II (pol II)-dependent transcripti on in both types of oocytes through the detection of bromouridine incorpora ted into nascent RNA. We show 1) that ovarian GV oocytes exhibiting the SN- type configuration can be found as soon as 17 days after birth in the C57/C BA mouse strain and 2) that the SN:NSN ratio of ovarian GV oocytes is very low just after hCG-induced ovulation and then increases progressively with the time after ovulation. Furthermore, we demonstrate that the SN configuration correlates strictly w ith the arrest of both pol I- and pol II-dependent transcription in mice at any age. Finally, we show that ribosomal genes are located at the outer pe riphery of the nucleolus in the NSN configuration and that pol I-dependent perinucleolar transcription sites correspond to specific ultrastructural fe atures of the nucleolus. Altogether, these results provide clear-cut criteria delineating transcript ionally active CV oocytes from those that are inactive, and confirm that th e SN-type configuration is mostly present in preovulatory oocytes.