Nuclear and cytoplasmic synchrony is important during oocyte maturation within antral follicles. CNP binds with its receptor, supporting high cyclic-adenosine-monophosphate (cAMP) levels thus holding the oocyte in meiotic arrest. COCs are removed in vitro, leading to a decrease of cAMP causing spontaneous nuclear maturation and an asynchrony with the oocytes’ cytoplasmic maturation. Adding CNP to pre-IVM media has the capacity to maintain cAMP levels in the oocytes, emulating in vivo conditions and improving IVM outcomes.
COCs (N=908) were matured either directly in IVM media for 18h or incubated with 25, 50,100,150 and 200nM CNP for 4 or 24h prior to IVM. After IVM, the COCs were denuded and assessed for germinal vesicle breakdown (GVBD) and some were inseminated to develop embryos.
The 25, 50, 100, 150 and 200nM CNP maintained meiotic arrest after 4h (75%, 87.5%, 89.7%, 97.3%, 100%) and 24h pre-incubation (37.8%, 55.3%, 54%, 87.8%). Maturation rates did not differ (P<0.05) between groups except that 24h pre-incubation with 25nM CNP gave significantly (P>0.05) lower rates (53.1%, P<0.05) than conventional IVM (75.3%). Two-cell per MII in the conventional IVM was 54.8% while at 4h was 43.3%, 55.9%, 43.7%, 37%, 37.8%, respectively; and 24h was 23.5%, 46.7%, 56.7%, 52.6% and 58.8%, respectively. Blastocyst rate per two-cell in the conventional IVM was 67.5% while at 4h was 76.9%, 52.6%, 71.4%, 70% and 71.4%; and at 24h was 50%, 50%, 38.1%, 40% and 50%. The two-cell and blastocyst rates did not differ between groups (P<0.05).
These results did not align with previous studies, but subtle variations in protocols, even within the same species, would suggest caution is required before clinical translation.
This study suggested that when conventional IVM works well, small differences in cytoplasmic and nuclear synchrony may not have a significant impact overall, using unstimulated animals.