Spermatozoa are highly specialized male cells that are characterized by a unique ability to move, which is a critical factor for successful fertilization. The relative simplicity of motility assessment, especially in livestock, has made it a widely used parameter for evaluating ejaculate quality or cryopreserved semen in the clinical field, and an advanced tool in reproductive physiology and toxicology research. Technological advances in image analysis and computational methods have substantially increased its accuracy through the use of computer-assisted sperm analysis (CASA) to minimize subjective bias in motility assessments. Nevertheless, this more objective method still presents some significant challenges, including variability in the sample preparation, imaging conditions, and analytical parameters. These issues contribute to inconsistency and impair the reproducibility and comparability of data between laboratories. The implementation of standardized protocols, combined with comprehensive training and rigorous evaluation, can serve to mitigate some of the emerging inconsistencies. In addition, the in vitro conditions under which CASA analyses are performed often differ significantly from the natural environment of the female reproductive tract in vivo. This review discusses the methodologies, critical issues, and limitations of sperm motility analyses using CASA, with a particular focus on the boar as an important agricultural and biomedical model species in which this system is widely used.

Sperm capacitation is essential for fertilization and involves coordinated changes in membrane organization, ion fluxes, and intracellular signaling. However, commonly used detection methods may reflect different biological events, which can be strongly influenced by experimental methodology. This study critically evaluated fluorescence-based approaches for assessing capacitation in boar spermatozoa, focusing on their specificity, interpretative limits, and methodological sensitivity. Ejaculated boar spermatozoa were incubated under in vitro capacitating conditions in TALP medium. Selected samples were subsequently treated with calcium ionophore to induce the acrosome reaction (AR). Phosphotyrosine (PTyr) immunofluorescence was assessed using five fixation and labeling protocols, acrosin redistribution was evaluated with the ACR.2 antibody, calcium ion redistribution was assessed using chlortetracycline (CTC) fluorescence, and acrosomal responsiveness was monitored by peanut agglutinin (PNA) lectin labeling. PTyr immunofluorescence was highly dependent on fixation protocol, indicating marked methodological sensitivity. Acrosin immunodetection revealed a clear capacitation-associated redistribution from weak or diffuse staining to a well-defined acrosomal pattern, whereas ionophore treatment caused a pronounced signal loss consistent with acrosomal exocytosis. PNA labeling confirmed that capacitation alone did not increase spontaneous acrosome loss, whereas ionophore treatment induced a robust AR. CTC staining showed a significant shift from whole-head pattern to acrosome in TALP-treated spermatozoa, indicating capacitation-associated Ca²⁺ redistribution. Together with CTC and Western blot data, these findings show that sperm capacitation status should be evaluated using multiple complementary markers rather than a single gold-standard assay.

Capacitation is a key maturation process that enables spermatozoa to acquire fertilizing ability and can be induced in vitro using capacitation media. Because capacitation protocols differ markedly among laboratories, we compared three compositionally distinct Hepes-, Tris-, and TALP-based media. This study was performed in boar spermatozoa using 3–6 biological replicates of pooled ejaculates depending on the assay, with 46 ejaculate samples from 12 boars in total. The aim was to determine whether such non-standardized conditions differentially affect signaling pathways leading to capacitation and thereby influence the detection of commonly used capacitation markers. We found clear differences among the tested media. All three induced capacitation-associated events, but their functional and molecular effects were not equivalent. The Hepes-based medium supported sperm motility most effectively, increasing total and progressive motility to 60.0% and 48.7%, respectively, after 1 h of incubation and maintaining the highest motility throughout the incubation period. In contrast, the Tris-based medium maintained lower but relatively stable motility, whereas the TALP-based medium showed a rapid decline in total motility from 53.1% to 15.2% during the first hour. The TALP-based medium induced the highest and most sustained protein kinase A (PKA) activity, reaching 0.047 U/mL at 0 h and 0.040 U/mL after 3 h, whereas the Hepes- and Tris-based media showed lower and less sustained activity ranging from 0.003 to 0.030 U/mL during incubation. In addition, distinct patterns of protein tyrosine phosphorylation were observed depending on the medium used. In particular, the TALP-based medium containing bicarbonate and bovine serum albumin (BSA) and the Hepes-based medium with the highest BSA concentration were associated with the highest levels of total protein tyrosine phosphorylation. Phosphoproteomic analysis further revealed condition-specific phosphorylation events, indicating that sperm maturation is dynamically regulated by the surrounding molecular environment. In contrast, no significant differences were detected in oxidative phosphorylation or in electron transport system complexes among the tested media. These findings show that differences in capacitation media composition, particularly in bicarbonate and BSA content, can markedly alter signaling outcomes and the interpretation of capacitation markers, with important implications for reproductive technologies and experimental standardization.

The initial molecular events mediating mammalian sperm binding to the zona pellucida (ZP) of the oocyte are highly complex and still not fully elucidated. Recent advances have identified multiple candidate sperm surface proteins, often functioning as part of high-molecular-weight complexes that mediate this critical fertilization event in a species-specific and coordinated manner. To address a significant gap in the literature, we provide an in-depth overview of the functional assays employed to investigate sperm–ZP interactions, emphasizing their underlying principles, potential applications, and key methodological strengths and limitations. The techniques discussed range from classical in vitro sperm–oocyte and hemizona binding assays, including antibody-blocking and competitive strategies, to cutting-edge in vivo genetic models, each contributing unique insights into the physiological relevance of the proposed ZP receptors. Robust experimental design, including the use of appropriate controls and validation strategies, is essential for accurately interpreting the role of candidate sperm receptors. This review provides a structured overview of current methodologies to support researchers in critically evaluating and applying functional assays in future studies.