Investigating the impact of extracellular vesicle addition during IVM on the fertilization rate of equine oocytes following ICSI

Abstract

The efficacy of in vitro embryo production (IVEP) in equines is relatively limited compared to other species due to the lack of a reliable superovulation technique, limited availability of cumulus oocyte complexes (COCs), low in vitro oocyte maturation (IVM) and fertilization rates. Extracellular vesicles (EVs), which are nanoparticles involved in intercellular signaling in the ovarian environment, have shown potential as supplements to improve oocyte development during IVM. This study tested the hypothesis that EVs from small (< 20 mm) ovarian follicles could enhance fertilization rates in mares. Follicular fluid was collected postmortem, and EVs were isolated and characterized. The IVM process was conducted with or without EVs (200 µg EV protein/ml). EV internalization during IVM was examined using fluorescent labeling and confocal microscopy. Following intracytoplasmic sperm injection (ICSI), presumptive zygotes were cultured in a time-lapse system. Confocal microscopy confirmed EV internalization by COCs. Nanoparticle tracking analysis showed that obtained EVs were submicron-sized, and flow cytometry identified surface markers CD81 and CD63 on a subpopulation of EVs. Transmission electron microscopy revealed the characteristic disk shape of EV isolates. After culture, 196 oocytes (36.84 %) exhibited a first polar body and were subjected to ICSI. The EV-treated group showed a significantly higher fertilization rate (34.7 % vs. 20.2 %; P < 0.05), reduced degeneration, and increased cleavage efficiency (P < 0.1). Despite early embryonic arrest in both groups, these results suggest that follicular fluid-derived EVs could play a supportive role in equine IVF procedures.

Keywords: Cumulus-oocyte complex; Extracellular vesicles; Follicular fluid; ICSI; Mare.