Meiotic arrest, resumption and TZP retraction in bovine COCs undergoing pre-IVM: lessons from a refined GV stage classification

In brief

Oocytes with subtle differences in chromatin configuration and nuclear lamina characteristics, detectable by a refined germinal vesicle (GV) classification system here described, respond differently to meiotic maturation systems leading to different in vitro maturation (IVM) outcomes.

Abstract

The nuclear, cytoplasmic and molecular maturation of the mammalian oocyte is a finely orchestrated sequence of events that relies on proper cumulus-oocyte communication. Bovine oocytes enter the IVM systems at the GV stage exhibiting four different chromatin configurations (GV0-GV3). Herein, we associate the oocyte chromatin and nuclear lamina configurations to propose a refined GV classification (GV0, GV1.1-GV1.3, GV2.1-GV2.3 and GV3.1-GV3.3). This refined GV classification system was correlated with oocyte meiosis resumption and transzonal projections (TZPs) density of cumulus-oocyte complexes (COCs) submitted to three IVM systems (control IVM and a modified IVM preceded or not by a pre-IVM step). Pre-IVM resulted in lower polar body extrusion rates at 19 h IVM, albeit ∼24% of the oocytes extruded their first polar body at 9 h IVM. Pre-IVM sustained 80% of oocytes meiotically arrested but altered GV distribution, reducing GV2 and increasing GV1.3 and GV3.3 categories. Pre-IVM reduced TZP densities predominantly in pre-matured GV3 and GVBD COCs. At 9 h of IVM, both groups matured in modified IVM showed lower TZP densities compared to immature and IVM control. Gene expression supports the TZP density differences, with ERK2 and PRKACA upregulation in pre-matured cumulus and in modified IVM groups at 9 h of IVM. GDF9 and BMP15 levels were similar between treated and control groups at all time points. Our findings indicate that despite the IVM system, the initial oocyte GV stage influences pre-IVM and IVM outcomes. The refined GV classification system is a useful tool to oocyte biologists.