Abstract
In vitro maturation (IVM) is an assisted reproductive technology (ART) that matures immature oocytes in culture, avoiding invasive gonadotropin stimulation. Clinically, it provides a safer, patient-friendly ART for individuals at risk of ovarian hyperstimulation syndrome, with oestrogen-sensitive cancers or gonadotropin resistance. In livestock, juvenile in vitro embryo transfer (JIVET) uses IVM to mature oocytes from juvenile animals, such as lambs, accelerating genetic gain and shortening generational intervals.
Despite the appeal, IVM remains inefficient, limiting its clinical and agricultural applications. Progesterone (P4), a crucial reproductive hormone, is also thought to influence oocyte quality and developmental competence. P4 supplementation during IVM has caused conflicting effects on oocyte maturation across diverse species. Interestingly, lamb oocytes of low quality were recently discovered to contain half the P4 as higher-quality adult oocytes. The present study investigated whether supplementing IVM media with increasing P4 concentrations enhanced ovine oocyte maturation and embryo development rates.
Prepubertal lamb ovaries were collected from the abattoir, and aspirated oocytes were randomly assigned to IVM culture drops supplemented with increasing P4 concentrations (0 (control), 2, 20 and 50 µg/ml). Following IVM, oocytes underwent in vitro fertilisation (IVF), and resulting embryos were cultured in vitro until day 8. Oocytes (post-IVM) and zygotes (post-IVF) from the control and 20 µg/ml P4 treatment groups were fixed and processed for electron microscopy to examine the effect on oocyte ultrastructure, organelle areas and organelle distribution across an equatorial section.
P4 supplementation appeared to increase cleavage and blastocyst rates in the 2 and 20 µg/ml P4 groups when compared to controls. However, these differences were not statistically 2 significant due to variability between experimental replicates in the control group. The highest P4 concentration appeared to decrease embryo development rates relative to controls. Interestingly, P4 supplementation improved lamb cleavage rates to levels comparable to those of adult ewe oocytes. Organelle quantification via scanning electron microscopy revealed that P4-treated oocytes had a significant reduction in total oocyte area and vesicle area when compared to controls. No significant differences were observed between nuclear maturation rates and the overall oocyte ultrastructure of control and P4-treated oocytes following IVM.
The reduction in vesicle area caused by P4 treatment indicates an acceleration in the metabolism of the lipid contained in these organelles. Although P4 did not significantly improve embryo development, the apparent increase in development rates and reduced vesicle volume indicate that P4 may influence oocyte quality and developmental competence; however, the exact mechanism remains unclear. Overall, our findings highlight the potential of P4 as a supplement to improve human and livestock ART.