Background: The maternal contribution of transcripts and proteins supplied to the zygote is crucial for the progression from a gametic to an embryonic control of preimplantation development. Here we compared the transcriptional profiles of two types of mouse MII oocytes, one which is developmentally competent (MII SN oocyte), the other that ceases development at the 2-cell stage (MII NSN oocyte), with the aim of identifying genes and gene expression networks whose misregulated expression would contribute to a reduced developmental competence.

(1) A number of factors that act upstream and downstream of Oct-4 emerge as candidate players in the acquisition of the oocyte's developmental competence; (2) we define molecular markers that identify a specific group of ovarian oocytes (SN) that have a potential to acquire developmental competence; (3) the presence of SN and NSN oocytes in human ovaries extends the interest of these results to the field of human reproduction.

Based on their chromatin organization, antral oocytes can be classified into two classes, namely surrounded nucleolus (SN, chromatin forms a ring around the nucleolus), and not surrounded nucleolus (NSN, chromatin has a diffuse pattern). Oocytes of both classes are capable of meiotic resumption, but while SN oocytes, following fertilization, develop to term, NSN oocytes never develop beyond the two-cell stage. A recent study has shown that the position of the germinal vesicle (GV) can be used as a morphological marker predictive of oocyte meiotic competence, i.e. oocytes with a central GV have a higher meiotic competence than oocytes with an eccentric GV. In the present study, we have associated both markers with the aim of identifying, with more accuracy, the oocytes' developmental competence. Following their isolation, antral oocytes were classified on the basis of both SN and NSN chromatin configuration and their GV position, matured to metaphase II and fertilized in vitro. We demonstrated that the position of the GV is a good marker to predict the oocytes' developmental competence, but only when associated with the observation of the chromatin organization.

BackgroundOct4 is a key factor of an expanded transcriptional network (Oct4-TN) that governs pluripotency and self-renewal in embryonic stem cells (ESCs) and in the inner cell mass from which ESCs are derived. A pending question is whether the establishment of the Oct4-TN initiates during oogenesis or after fertilisation. To this regard, recent evidence has shown that Oct4 controls a poorly known Oct4-TN central to the acquisition of the mouse egg developmental competence. The aim of this study was to investigate the identity and extension of this maternal Oct4-TN, as much as whether its presence is circumscribed to the egg or maintained beyond fertilisation.ResultsBy comparing the genome-wide transcriptional profile of developmentally competent eggs that express the OCT4 protein to that of developmentally incompetent eggs in which OCT4 is down-regulated, we unveiled a maternal Oct4-TN of 182 genes. Eighty of these transcripts escape post-fertilisation degradation and represent the maternal Oct4-TN inheritance that is passed on to the 2-cell embryo. Most of these 80 genes are expressed in cancer cells and 37 are notable companions of the Oct4 transcriptome in ESCs.ConclusionsThese results provide, for the first time, a developmental link between eggs, early preimplantation embryos and ESCs, indicating that the molecular signature that characterises the ESCs identity is rooted in oogenesis. Also, they contribute a useful resource to further study the mechanisms of Oct4 function and regulation during the maternal-to-embryo transition and to explore the link between the regulation of pluripotency and the acquisition of de-differentiation in cancer cells.

Purpose The final stages of antral mouse oocytes maturation are characterised by the transition from transcriptionally active NSN to inactive SN oocytes. Here, we studied the profile of histone acetylation changes occurring during the NSN-to-SN transition. Methods During the NSN-to-SN transition, oocytes were classified based on their chromatin organisation and the immunocytochemical profile of histones H2B, H3 and H4 acetylation was analysed. Results We described four patterns of immunostaining common to the three acetylated histones, corresponding to stages of progressive localisation: From a diffused distribution in NSN oocytes to the association with constitutive heterochromatin and then to the nucleolar surface region in SN oocytes. Conclusions The maintenance of a favourable transcriptional epigenetic context, in the heterochromatin of transcriptionally silent SN oocytes, might be related to the early stages of development when transcripts from these heterochromatic regions are functional to preimplantation progression.Keywords Mouse . Antral oocytes . Chromatin organisation . Histone acetylation Capsule In late antral mouse oocytes, chromatin condenses around the nucleolus, becomes transcriptionally silent, but maintains transcriptionally permissive histone acetylation profiles, likely functional to development.