Gene expression and chromatin organization during mouse oocyte growth

Mouse oocytes can be classified according to their chromatin organization a nd the presence [surrounded nucleolus (SN) oocytes] or absence [nonsurround ed nucleolus (NSN) oocytes] of a ring of Hoechst-positive chromatin around the nucleolus. Following fertilization only SN oocytes are able to develop beyond the two-cell stage. These studies indicate a correlation between SN and NSN chromatin organization and the developmental competence of the fema le gamete, which may depend on gene expression. In the present study, we ha ve used the HSP70.1 Luc transgene (murine HSP70.1 promoter + reporter gene firefly luciferase) to analyze gene expression in oocytes isolated from ova ries of 2-day- to 13-week-old females. Luciferase was assayed on oocytes af ter classification as SN or NSN type. Our data show that SN oocytes always exhibit a higher level of luciferase activity, demonstrating a higher gene expression in this category. Only after meiotic resumption, metaphase IH oo cytes derived from NSN or SN oocytes acquire the same level of transgene ex pression. We suggest that the limited availability of transcripts and corre sponding proteins, excluded from the cytoplasm until GVBD in NSN oocytes, c ould explain why these oocytes have a lower ability to sustain embryonic de velopment beyond the two-cell stage at which major zygotic transcription oc curs. With this study we have furthered our knowledge of epigenetic regulat ion of gene expression in oogenesis, (C) 1999 Academic Press.