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A recent study of extracellular vesicles in commercial boar insemination doses asks what sperm concentration misses.
A commercial semen dose looks like a finished unit. It has a defined volume, a standardized sperm concentration, an extender, and a storage condition. Those numbers make the dose practical to manufacture and compare. They can also create the impression that standardizing the sperm has standardized the biology.
A recent paper in *Scientific Reports* examines what remains outside that count: extracellular vesicles in the seminal plasma of diluted boar artificial-insemination doses. These membrane-bound particles carry proteins and small RNAs, and they remain in contact with sperm during storage. The researchers asked whether vesicles survive commercial dilution, whether their cargo differs between boars with high and reduced field fertility, and whether adding them back to sperm changes measurable physiology. The study is peer reviewed and currently published as an Article in Press, with a permanent DOI and an open full-text paper (Martín-San Juan et al., 2026).
The question matters because dilution does two things at once. It adjusts the number of sperm in a dose, and it reduces everything in seminal plasma that is not used as the standardizing variable. A process designed around concentration may therefore be consistent in one dimension while remaining biologically different in another.
The authors began with commercial doses from Landrace boars whose fertility had been estimated from more than 250 inseminations per male. Their classification model adjusted farrowing rate for sow, farm, season, semen age, and other production factors before assigning six boars to a high-fertility group and six to a reduced-fertility group. That field record gives the comparison more weight than a laboratory label alone, although it does not turn every molecular difference into a cause of fertility.
The vesicles were isolated from diluted seminal plasma by ultrafiltration and size-exclusion chromatography. Transmission electron microscopy, nanoparticle tracking, protein markers, and checks for common contaminants were used to establish that the preparations contained intact extracellular vesicles. The researchers then combined proteomics with small-RNA sequencing across three compartments: vesicles, free transcripts in seminal plasma, and spermatozoa. A molecule present in the dose is not necessarily packaged, protected, or delivered in the same way across those compartments.
The functional experiment added another layer. In five independent replicates, sperm pooled from four reduced-fertility males was washed free of its original seminal plasma and incubated at 38 °C with low or high concentrations of vesicles from either fertility group. A control received extender and filtered buffer without vesicles. Over 24 hours, the team measured motility, acrosome integrity, membrane fluidity, mitochondrial activity, and oxidation using computer-assisted sperm analysis and flow cytometry. Fluorescent labeling was also used to examine where vesicles associated with sperm cells (full methods and results).
This design does more than compare two omics profiles. It asks whether material recovered from an ordinary commercial dose is still structurally present and biologically active after dilution.
Vesicle concentration and mean size did not differ significantly between the fertility groups. The separation appeared later, in what the vesicles carried. Across twelve samples, the proteomic analysis identified 1,943 proteins. Of 108 proteins that differed between groups, 97 were more abundant in vesicles from high-fertility boars and 11 in those from reduced-fertility boars. Some of the enriched proteins were associated with sperm-egg recognition, zona pellucida binding, vesicle transport, cytoskeletal regulation, and metabolic functions. Those pathway assignments are useful leads, not proof that each protein produced the field-fertility difference.
The RNA result sharpened the importance of compartment. The researchers found 80 differentially expressed microRNAs in vesicles, 52 among free seminal-plasma transcripts, and only three in spermatozoa. Sixty-nine of the vesicle-associated microRNAs were unique to that compartment. Four known microRNAs shared by vesicles and seminal plasma even changed in opposite directions between fertility groups. Measuring a transcript in whole seminal plasma could therefore hide whether it is free or enclosed within a vesicle, even though enclosure may affect stability and access to a recipient cell.
The coincubation results were not a simple contest in which every high-fertility vesicle improved every sperm measure. Vesicles from reduced-fertility boars produced the clearest early increases in total and progressive motility. Vesicles from high-fertility boars were more consistent in improving early viability, preserving intact acrosomes, supporting mitochondrial activity, and reducing sperm or mitochondrial oxidation. Many effects were strongest during the first hours and weakened later. The pattern suggests that motility, oxidative state, membrane behavior, and fertility classification cannot be collapsed into one quality score.
A dose can meet a familiar concentration standard while retaining another layer of variation that is molecular, compartment-specific, and functional. The study makes that layer measurable.
The title calls the cargo "fertility-predictive," but the experiments identify candidates rather than a finished prediction test. The omics comparison used six males per group. The functional work pooled sperm and vesicles from several boars, which reduces the influence of one unusual animal but also hides individual pairings between cargo, sperm response, and fertility. The authors explicitly note this limitation, along with variation introduced by the original dilution rate and the need to validate candidate proteins and microRNAs independently.
The functional assay also remained in vitro. It showed that vesicle supplementation changed sperm measurements during incubation; it did not test whether supplementing a commercial dose increased farrowing rate or litter size. Fluorescence localized labeled vesicles to the acrosome, midpiece, head, and tail, demonstrating association with sperm, but not resolving the exact route of cargo transfer. Predicted microRNA targets and enriched pathways still require direct mechanistic tests.
These boundaries change the next experiment. A convincing biomarker study would need a larger, independent cohort in which candidates are measured before fertility outcomes are known. A functional study would keep individual males separate, test defined vesicle doses across extenders and storage intervals, and connect early sperm responses to fertilization and field outcomes. The distinction between vesicle-bound and free cargo should remain intact rather than being averaged back into whole seminal plasma.
For research concerned with boar semen preservation, antioxidant protection, and fertility endpoints, the paper also raises a practical question: how much of an apparent extender or supplementation effect depends on the seminal material that remained in the dose before the experiment began? Washing, dilution, storage, and freezing do not merely change sperm exposure to a treatment. They also change exposure to the sperm's original molecular surroundings.
The answer is not to add extracellular vesicles to commercial doses immediately. The study is too small and too early for that. Its value is more disciplined. It shows that a semen dose contains biologically active information that a sperm count does not capture, and it supplies a route for testing whether that information can improve prediction or intervention.
Standardization is strongest when it defines what has been made equal and keeps measuring what has not. In this case, the overlooked part of the dose did not disappear during dilution. It remained present, variable, and capable of changing how sperm behaved.
- Martín-San Juan A, Cerrato Martín-Hinojal C, Martinez-Alborcia MJ, Nieto-Cristóbal H, de Mercado E, Álvarez-Rodríguez M. "Seminal extracellular vesicles from boar AI doses contain fertility-predictive protein and miRNA cargo and improve sperm physiology." *Scientific Reports*. Published online September 26, 2026. DOI: 10.1038/s41598-026-71880-w. Open full-text PDF.