Induction of reversible meiosis arrest of bovine oocytes using a two-step procedure under defined and nondefined conditions

The objective was to study the effect of a defined culture system, on nuclear and cytoplasmic maturation of bovine oocytes, using the two-step procedure of IVM to detect possible inhibition and subsequent resumption of meiosis arrest. In the first step, called the prematuration period (PMP), COCs were cultured in T1—non-defined medium (NDM), or T2—defined medium (DM), both for 24 h. In step 2, called the resumption period (RP), COCs were cultured in: NDM (T1); DM + NDM (T3); or DM+DM (T4) for 24 h in each medium. The NDM was composed of TCM-199 supplemented with FCS and FSH. The DM was composed of alpha-MEM supplemented with PVA, insulin, IGF-1, androstenedione, nonessential amino acids, transferrin, and sodium selenium. Oocytes from T2 had a lower (P < 0.05) rate of nuclear maturation (19.8%) than T1 oocytes (83.2%). Also, T2 COCs appeared to be in the process of cytoplasmic maturation, according to the distribution of organelles assessed by transmission electron microscopy (MET). These COCs had characteristics previously described as mature: erect microvilli on the plasmembrane, presence of cortical/evenly distributed mitochondria throughout the ooplasm, and presence of 50% aligned/cluster cortical granules. Immature characteristics such as small PvS, compact cumulus cells, and presence of 50% cortical granule clusters were also observed. The T1 COCs had only characteristics of maturation (P < 0.05). In step 2 (RP), meiosis arrest induced by DM was resumed after an additional 24 h of culture in NDM (T3) with 79.2% mature COCs, whereas in T4, meiosis arrest was maintained, resulting in almost 70% immature COCs (P < 0.05). At the end of RP, T3 COCs had the mature characteristics of mitochondria spread throughout the cytoplasm (P < 0.05), cumulus expansion, and alignment of cortical granules, whereas the T4 group had both immature and mature characteristics. We inferred that DM can be used in lieu of meiosis inhibitors and furthermore, it can provide extra time to study nuclear and cytoplasmic maturation synchrony of IVM.