RESUMEN
RESEARCH QUESTION: Does a three-dimensional culture system based on magnetic levitation with nanoparticles assembly maintain the follicular structure and viability with adequate growth rates leading to oocyte maturation after long-term culture? DESIGN: Randomized-controlled trial of treatments in a bovine model. Secondary follicles (nâ¯=â¯213) isolated from bovine ovaries were cultured in a two-dimensional system (two-dimensional control) or three-dimensional levitation system with different concentrations (three-dimensional 50 µl/ml, 100 µl/ml and 200 µl/ml) of magnetic nanoparticles. Follicular growth (diameter, daily growth and growth patterns), morphology (normal, degenerated and extruded follicles), antrum formation, oocyte viability and chromatin configuration were assessed. RESULTS: Secondary follicles of three-dimensional 200-µl/ml treatment showed higher viability, antrum formation and lower degeneration rates than two-dimensional control. Also, follicles cultured in the three-dimensional 200-µl/ml treatment presented a most homogenous daily growth rate as shown by the lowest variance and standard deviation. Compared with the two-dimensional control, the proportion of non-growing and slow-growing follicles were 3.8-fold lower and 1.6-fold higher, respectively, in the three-dimensional 200-µl/ml treatment. After in-vitro maturation, the three-dimensional 200-µl/ml had a greater proportion of viable oocytes (1.7-fold) and meiotic resumption rates (2.4-fold) than the two-dimensional control treatment. CONCLUSION: The three-dimensional levitation culture system improves the viability of in-vitro development of bovine secondary follicles, antrum formation and lower extrusion and degeneration rates and adequate growth rate leading to relevant oocyte viability and meiotic resumption after in-vitro maturation. This approach does not require a specific medium, and has the potential as an alternative method to in-vitro follicle culture in several species, including humans.