Bovine reproductive fluid viscosity: measurement and biomimicry with alginate

Dear editor, Assisted reproductive technologies (ARTs) such as in vitro maturation (IVM), fertilization (IVF), and embryo production (IVP), are widely used in human and animal reproduction but often fail to mimic the mechanical characteristics of the natural reproductive environment. Mechanical forces, including fluid viscosity, play a critical role in oocyte development by influencing folliculogenesis and maturation through mechanosensitive receptors and intracellular signaling [1]. Standard in vitro systems typically lack the necessary rheological complexity, behaving as low-viscosity, Newtonian fluids, which contrasts with the viscoelastic, non-Newtonian, shear-thinning behavior observed in vivo [2, 3]. Although viscosity of reproductive fluids has been studied in relation to sperm motility [4, 5], we are still lacking information on its effect on gamete and embryo development. In a study by Fontes et al., culturing bovine embryos in media that matched oviductal fluid viscosity improved blastocyst quality by increasing the total cell number, reducing apoptosis, and decreasing global DNA methylation compared to standard media, highlighting the effects of viscosity on developing embryos [3]. This provided a strong rationale for investigating this overlooked physical parameter. Therefore, this study sought to quantify this key missing physical feature, viscosity, in bovine reproductive fluids, mirroring the natural conditions experienced by gametes and embryos during early stages of development. For this, we measured the viscosity of bovine dominant follicular fluid (domFF) and oviductal (OF) and uterine (UF) fluids at defined stages of the estrous cycle and identified alginate concentrations required to replicate these viscosities in standard IVM, IVF, and IVC media.