Mimicking the oviductal fluid viscosity improves the quality of in vitro–produced bovine embryos

Abstract The oviductal fluid (OF) provides essential nutritional, mechanical, and physical support during the first four days of embryonic development. Among its physical properties, viscosity has been largely overlooked in embryo in vitro culture (IVC) media due to limited available data. In this study, we measured the viscosity of bovine OF from ex vivo samples and mimicked it during in vitro embryo culture. OF viscosity remained consistent across estrous cycle stages and between oviductal antimeres, and was over three times higher than that of standard IVC media (3.4 ± 1 vs. 0.9 ± 0 mPa·s, P < 0.05). To replicate this property, in vitro bovine embryos were cultured in media supplemented with sodium alginate (0%, 0.25%, or 0.75% w/v), corresponding to low (control), physiological (3.3 ± 0.1 mPa·s), and supraphysiological (6.8 ± 0.02 mPa·s) viscosity levels. While cleavage and blastocyst rates were unaffected, embryos cultured at physiological viscosity (0.25%) showed improved quality indicators: increased total cell number, lower apoptosis, reduced reactive oxygen species, and decreased global DNA methylation compared to the high-viscosity (0.75%) group. Notably, embryos in the 0.25% group also showed lower apoptosis and inner cell mass methylation than controls. These findings suggest that standard IVC media fail to replicate key rheological features of the oviductal environment and that alginate is a safe, non-Newtonian viscosity modulator. Mimicking OF viscosity during IVC improves embryo quality and epigenetic outcomes and may represent a valuable refinement in assisted reproductive technologies.