Spermatozoal acrosome dysfunction and its role in stallion subfertility.
Authors: Hernández-Avilés Camilo, Ramírez-Agámez Luisa, Varner Dickson D, Raudsepp Terje, Love Charles C
Journal: Journal of equine veterinary science
Summary
Impaired Acrosomal Exocytosis (IAE) represents a distinct form of stallion subfertility that typically manifests in animals with otherwise normal or excellent sperm motility and morphology, making it easily overlooked without specific functional testing. Originally characterised in the 1990s, this condition is particularly prevalent in Thoroughbreds carrying the homozygous A/A-A/A susceptibility genotype in FKBP6 exon 5, though recent clinical observations by Hernández-Avilés and colleagues have identified acrosomal dysfunction in subfertile stallions from non-Thoroughbred registries lacking this specific genetic marker. The authors' review synthesises current diagnostic approaches for assessing acrosome function and presents clinical phenotypes of IAE across breed populations, establishing that acrosomal dysfunction exists on a spectrum beyond the well-documented Thoroughbred presentation. For equine practitioners managing subfertile stallions with seemingly normal semen parameters, this work highlights the critical importance of incorporating acrosome-specific functional assays rather than relying solely on conventional semen analysis, particularly when fertility remains inexplicably poor despite excellent morphology and motility metrics. Understanding the genetic and functional bases of IAE expands breeding soundness evaluation protocols and may identify previously undiagnosed subfertility cases across diverse breed populations.
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Practical Takeaways
- •Breeders and veterinarians should consider acrosome function testing for subfertile stallions, particularly Thoroughbreds with normal semen parameters, as genetic screening for the FKBP6 A/A-A/A genotype can identify high-risk individuals before breeding losses occur.
- •Acrosome dysfunction may not always be accompanied by obvious semen quality defects, so subfertility without corresponding morphology or motility issues warrants investigation of acrosomal exocytosis capability.
- •Management strategies should account for the possibility of acrosome dysfunction in non-Thoroughbred stallions even without the identified Thoroughbred-specific genotype, as alternative causative mechanisms may be at play.
Key Findings
- •Acrosome dysfunction-related subfertility in stallions has been recognized since the 1990s, with cases occurring in both stallions with poor sperm quality and those with normal-to-excellent parameters.
- •Thoroughbred stallions with the homozygous susceptibility genotype (A/A-A/A) in FKBP6 exon 5 are uniquely predisposed to impaired acrosomal exocytosis (IAE).
- •Non-Thoroughbred stallions displaying reduced acrosomal exocytosis ability have been identified clinically without carrying the A/A-A/A FKBP6 genotype, suggesting alternative genetic or non-genetic mechanisms.
- •Clinical assessment methods and diagnostic tools for estimating acrosome function in subfertile stallions are available and can differentiate IAE cases.