Chromatin configurations in the germinal vesicle of mammalian oocytes

Abstract

In all the studied mammalian species, chromatin in the germinal vesicle (GV) is initially decondensed with the nucleolus not surrounded by heterochromatin (the NSN configuration). During oocyte growth, the GV chromatin condenses into perinucleolar rings (the SN configuration) or other corresponding configurations with or without the perinucleolar rings, depending on species. During oocyte maturation, the GV chromatin is synchronized in a less condensed state before germinal vesicle breakdown (GVBD) in species that has been minutely studied. Oocytes may also take on a SN/corresponding configuration during early atresia, but they undergo GVBD at the advanced stage of atresia. As not all the species show the SN configuration while in all the species, gene transcription always stops at the late stage of oocyte growth, it is suggested that not the formation of perinucleolar rings but a thorough condensation of GV chromatin is essential for transcriptional repression. The GV chromatin configuration is highly correlated with oocyte competence; oocytes must end the NSN configuration before they gain the full meiotic competence, and they must take on the SN/corresponding configurations and stop gene transcription before they acquire the competence for early embryonic development. While factors inhibiting follicle atresia tend to synchronize oocytes in a chromatin configuration toward maturation, factors inducing follicle atresia tend to synchronize oocytes in a chromatin configuration reminiscent of early atresia. Furthermore, although condensation of GV chromatin is associated with transcriptional repression, both processes may not be associated with histone deacetylation during oocyte growth.

Prediction of Developmentally Competent Chromatin Conformation in Mouse Antral Oocytes

“…The growing oocytes possess so-called NSN type chromatin relatively evenly distributed throughout the nucleus. At the end of the growth phase, however, the NSN conformation changes to SN which is characterized by more condensed chromatin with a significant portion surrounding the nucleolus (WICKRA-MASINGHE et al 1991;DEBEY et al 1993; reviewed by TAN et al 2009). Accordingly, among the oo-cytes isolated from antral follicles, often referred to as "fully grown oocytes", there are still oocytes with NSN chromatin as well as oocytes having more developmentally advanced SN chromatin.…”

In search of the transcriptional blueprints of a competent oocyte

“…Germinal vesicle oocytes possess different states of chromatin condensation. In particular, they can be observed either with a non-surrounded nucleolus (NSN) configuration, which is associated with low oocyte competence, or with a surrounded nucleolus (SN) configuration, which is associated with high competence and a condensed chromatin, which reflects a repressed transcriptional state (Zuccotti et al, 1995;Tan et al, 2009). Compared with SN oocytes, NSN oocytes display a higher gene transcription activity and a lower rate of meiosis resumption (G2/M transition), and they are mostly arrested at the two-cell stage after in vitro fertilization.…”

Immunolabeling of “Nucleoli” in Mouse Fully-Grown Oocytes and One-Cell Embryos is Dependent on Upstream Molecular Fixatives

“…To determine the type of GV oocytes destined for fixation with 70% ethanol, a fixative that can deteriorate large-scale chromatin configuration (Tan et al, 2009), isolated oocytes were placed in 50-100 μm drops of 1 μg/ml Hoechst 33342 (Life Technologies, USA) dissolved in M2 medium containing 100 μg/ml dbcAMP. Oocytes were quickly analyzed under an Axiovert200 epifluorescence microscope (Carl Zeiss) equipped with a Plan Apochromat phase contrast ×20/0.8 numerical aperture objective, and divided into the NSN-type and the SN-type oocytes, which then were separately fixed and processed for immunolabeling.…”

“…Instead, they possess large, up to 10 microns in diameter, intranuclear entities -nucleolus-like bodies (NLBs) which substitute normal nucleoli present in younger oocytes. Based on large-scale chromatin configuration as well as transcriptional activity and meiotic competence, two main types of GV oocytes can be distinguished [16,23]. In the mouse, GV oocytes, where NLBs contact with distinct blocks of condensed chromatin (chromocenters), are called non-surrounded (NSN) oocytes, while the oocytes, which contain NLBs surrounded by a prominent layer of condensed chromatin, are called surrounded (SN) GV oocytes.…”