Let the female tract choose: finding the sperm to enhance in vitro embryo production

In vitro embryo production (IVEP) has become an essential tool for livestock breeding, genetic dissemination, and research. Despite its widespread use, overall efficiency remains limited, as only approximately 25% of fertilized oocytes develop into transferable embryos, and pregnancy outcomes are consistently lower than with in vivo-derived embryos. While oocyte quality is a major determinant, fertilization success and early embryonic development are also strongly influenced by sperm functional competence, which is only partially captured by conventional selection methods based primarily on motility and morphology. In vivo, the female reproductive tract imposes stringent physical and molecular barriers, with fewer than 0.01% of inseminated sperm reaching the fertilization site. This selection involves key processes, including i) passage through the utero-tubal junction, ii) formation and regulation of the oviductal sperm reservoir, and iii) guided sperm navigation toward the oocyte. These mechanisms ensure functional selection of sperm and synchronized meeting with oocytes, features that are largely absent or poorly replicated in conventional in vitro fertilization (IVF) systems. Current sperm selection methods include swim-up and density gradient centrifugation. Emerging biomimetic approaches aim to better replicate physiological conditions, notably through microfluidic platforms and oviduct cell-based selection systems. These approaches have shown promising improvements in IVEP, although their routine application remains constrained by limited throughput, standardization, and scalability. This review synthesizes current knowledge on both the in vivo mechanisms of sperm selection and emerging biomimetic techniques, particularly in cattle, where improving sperm selection remains a key challenge for IVEP efficiency.