Oocyte large-scale chromatin configuration re-modelling: state of the art and perspectives

During meiotic arrest, and particularly during the oocyte growth phase leading to the formation of the fully-grown and differentiated oocyte, the chromatin enclosed within the Germinal Vesicle (GV) is subjected to several levels of regulation controlling both its structure and function. These events include mechanisms acting both locally, on specific loci, and on a large scale to remodel wide portions of the oocyte genome. Morphologically, the chromosomes lose their individuality as well as their characteristic appearance and form a loose chromatin mass, which in turn undergoes profound and dynamic rearrangements within the GV before the meiotic resumption. These ‘large- scale chromatin configuration changes’ are temporally correlated with the process of transcriptional silencing in the oocyte nucleus as well as with epigenetic modifications such as histone tail modifications and changes in the global level of DNA methylation. Moreover, chromatin configuration rearrangements are tightly associated with the acquisition of meiotic and developmental competence. The molecular mechanisms governing changes in large-scale chromatin configuration still remain largely unknown. Most likely, strategies set in place for the control and coordination of these events are part of a complex physiological process that ultimately confers the oocyte with meiotic and developmental competence. Here, we summarize some studies intended to explain the mechanism(s) regulating this complex process, including recent findings indicating that ovarian granulosa cells and their coupling with the oocytes through gap junctions are implicated in such a process. During meiotic arrest, and particularly during the oocyte growth phase leading to the formation of the fully-grown and differentiated oocyte, the chromatin enclosed within the Germinal Vesicle (GV) is subjected to several levels of regulation controlling both its structure and function. These events include mechanisms acting both locally, on specific loci, and on a large scale to remodel wide portions of the oocyte genome. Morphologically, the chromosomes lose their individuality as well as their characteristic appearance and form a loose chromatin mass, which in turn undergoes profound and dynamic rearrangements within the GV before the meiotic resumption. These ‘large-scale chromatin configuration changes’ are temporally correlated with the process of transcriptional silencing in the oocyte nucleus as well as with epigenetic modifications such as histone tail modifications and changes in the global level of DNA methylation. Moreover, chromatin configuration rearrangements are tightly associated with the acquisition of meiotic and developmental competence. The molecular mechanisms governing changes in large-scale chromatin configuration still remain largely unknown. Most likely, strategies set in place for the control and coordination of these events are part of a complex physiological process that ultimately confers the oocyte with meiotic and developmental competence. Here, we summarize some studies intended to explain the mechanism(s) regulating this complex process, including recent findings indicating that ovarian granulosa cells and their coupling with the oocytes through gap junctions are implicated in such a process.