Abstract in English:

Abstract 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.

Abstract in English:

Abstract Fertility is a key determinant of reproductive efficiency, economic performance, and sustainability in cattle production systems. Traditional fertility phenotypes that are still widely used today, such as conception rate, calving interval, non-return rate, and semen quality parameters, primarily describe reproductive outcomes rather than predict fertility. Consequently, important molecular and physiological differences between animals with contrasting fertility may remain undetected. Omics technologies provide new opportunities to identify reproductive phenotypes that more accurately reflect the biological mechanisms underlying fertility in cattle. Genomics enables the identification of single nucleotide polymorphisms, quantitative trait loci, and haplotypes associated with variation in fertility. Transcriptomics has revealed gene expression patterns related to spermatogenesis, sperm function, uterine receptivity, and embryo-maternal communication. Proteomics has identified protein profiles associated with oocyte developmental competence, fertilization capacity, and embryonic development, while metabolomics reflects the biochemical state most closely linked to the expressed phenotype, identifying biomarkers associated with oxidative balance and energy, lipid, and protein metabolism. This manuscript reviews selected studies that have applied omics technologies to identify measurable reproductive phenotypes that better capture the biological basis of fertility in cattle within the context of assisted reproductive technologies. While genomics remains the most widely applied omics approach due to the stability and accessibility of DNA, downstream omics may provide a more accurate representation of the dynamic biological processes defining fertility. Future research should focus on translating these findings into practical assays for rapid and accessible biomarker measurement, supporting informed decision-making at the farm level.

Abstract in English:

Abstract Embryo transfer technologies have become essential tools for accelerating genetic gain, disseminating superior genetics, and preserving valuable germplasm in livestock species. In small ruminants, embryo production has historically relied on multiple ovulation and embryo transfer (MOET), whereas in vitro embryo production (IVEP) has expanded substantially over recent decades. Despite this technological progress, the global embryo industry in sheep and goats remains dominated by MOET systems, in contrast to cattle, where IVEP has rapidly become the prevailing technology. This review critically examines the biological, technical, and logistical factors underlying the current balance between MOET and IVEP in small ruminants. We discuss the main biological determinants influencing embryo production efficiency, including oocyte developmental competence, species-specific reproductive traits, and the intrinsic sensitivity of small ruminant gametes and embryos to in vitro environments. A comparative analysis with bovine systems highlights that several operational advantages that have driven the expansion of IVEP in cattle, such as the possibility of repeated ovum pick-up without hormonal superstimulation, minimally invasive oocyte recovery, and efficient use of sex-sorted semen, are not readily transferable to sheep and goats. Conversely, recent advances in nonsurgical embryo recovery have reinforced the practical advantages of MOET in small ruminants by reducing procedural invasiveness and improving animal welfare. Emerging perspectives for IVEP are also discussed, including innovations in embryo culture systems and the integration of IVEP with genomic selection and sexed semen technologies. Overall, current evidence indicates that the predominance of MOET in small ruminants reflects not a technological delay but rather a complex interplay of biological robustness and operational simplicity. We believe that IVEP is more likely to expand as a complementary tool rather than to replace MOET as the dominant embryo production strategy in sheep and goats.

Abstract in English:

Abstract Seasonal heat stress (HS) is a pervasive environmental challenge with profound consequences for female reproductive physiology, affecting ovarian function, oocyte maturation, and early embryonic development. At the ovarian level, HS disrupts follicular growth, impairs steroidogenesis, and compromises granulosa cell function, thereby creating a suboptimal microenvironment that reduces oocyte competence. In oocytes, HS induces oxidative stress, mitochondrial dysfunction, endoplasmic reticulum (ER) stress, spindle abnormalities, chromosomal missegregation, and persistent epigenetic alterations. These disruptions extend into early embryonic development, where redox imbalance, apoptosis, ER stress, and altered lineage allocation reduce cleavage and blastocyst formation, compromise trophectoderm and inner cell mass integrity, and impair implantation potential. Maternal heat exposure further exacerbates embryonic vulnerability by altering the oviductal and uterine environment, reducing embryotrophic factors and antioxidant defenses, and ultimately influencing offspring phenotype and fertility, potentially across generations. Accordingly, this review aims to synthesize current knowledge on the physiological and molecular impacts of heat stress on ovarian function, oocyte maturation, and early embryonic development. To this end, we consider studies conducted under both in vivo and in vitro conditions, highlighting shared and distinct mechanisms of thermal stress at the organ and cellular levels to identify potential targets for intervention.