Improved sperm selection a chance to boost assisted reproduction in horses – review

Opportunities for improved fertilization, embryo development, and pregnancy rates lie ahead over the next decade, according to the authors.
Opportunities for improved fertilization, embryo development, and pregnancy rates lie ahead over the next decade, researchers say.

The rapid expansion of novel semen selection techniques represents an opportunity to significantly improve assisted reproduction in horses over the next decade, according to scientists.

It will open the door to improved fertilization, embryo development, and pregnancy rates, the authors of a just-published review wrote in the journal Animals.

Morgan Orsolini, Stuart Meyers and Pouya Dini said producers may choose to select the best sperm within stallion ejaculate in order to improve fertilization and pregnancy rates when employing artificial insemination or in vitro fertilization techniques.

In their paper, the researchers, all with the University of California, Davis, set out to review conventional and novel methods of sperm selection.

The trio noted that the use of assisted reproductive techniques is expanding in the horse industry, providing valuable opportunities to produce foals from animals who are unable to breed, conceive, or carry a pregnancy to term, because of either sub-fertility or logistical management issues.

Techniques such as artificial insemination and embryo transfer are practiced with consistent success, and have already become an indispensable part of equine reproduction.

Newer techniques, such as transvaginal oocyte aspiration and in vitro embryo production, are also being implemented worldwide, allowing veterinarians and farm managers to maximize the reproductive performance of horses.

Conventional and novel methods of sperm selection used to select for superior quality sperm. Image: Orsolini et al.
Conventional and novel methods of sperm selection used to select for superior quality sperm. Image: Orsolini et al.

The authors noted that most of the efforts for optimizing in vitro embryo production outcomes are focused on egg maturation protocols. However, sperm quality should not be overlooked in terms of its fertilization and development potential, they said, as there is varying sperm quality within ejaculate that may influence ideal embryo production.

In animals, it is believed that sperm are naturally “selected” as they navigate through the female reproductive tract, resulting in only the most competent spermatozoa reaching the oviduct and ultimately fertilizing an ovum.

However, these natural sorting procedures are bypassed during laboratory-based embryo production which could contribute to the suboptimal outcome of these techniques.

The review team noted that laboratory-based embryo production is only about one-fifth as efficient as natural reproduction due to damage from cryopreservation and overall worsened sex-cell competence.

Inefficiencies have been documented in many species, they noted.

Although it is possible to produce healthy embryos and offspring from low quality sperm samples, it is preferable to process and select superior sperm to maximize the chances of successful fertilization and embryo development.

The authors traversed the range of sperm selection methods developed to maximize the chances of selecting highly viable sperm. These have been employed with variable success. Some conventional options, such as the Swim Up method, are simple and cost effective, while new techniques have also been developed, such as Magnetic Activated Cell Sorting and Zeta Potential Selection.

They also traversed the science around separating sperm based on whether they will produce male or female offspring. Currently, flow cytometry is the only vetted method for separation of X and Y chromosome-bearing spermatozoa, they noted.

They said that while various technical advancements have been made with flow sorting, the method is inefficient in producing doses adequate for artificial insemination, and can cause damage to the sperm, which in turn can reduce their longevity.

“Although a variety of techniques exist to separate and select for spermatozoa based on quality and viability, as well as sex chromosome, not all methods have yet been translated to the equine industry,” the review team noted.

As the implementation of laboratory-based embryo production in horses expands, so does the need for highly affordable and efficient semen selection techniques.

The inconsistent efficacy of traditional sperm selection methods, such as Swim Up and Density Gradient Centrifugation, presents a barrier to the optimization of lab-based techniques.

The introduction of techniques, such as Microfluidics, that are used clinically in other species may consequently improve embryo production in the horse.

There is also a need to assess the effectiveness of newly developed sperm selection methods to determine their potential for use in clinical settings.

“Ultimately, the rapid expansion of novel semen selection techniques provides many opportunities for improved fertilization, embryo development, and pregnancy rates within the equine breeding industry over the following decade.”

The review forms part of a special issue of Animals, “Assisted Reproductive Techniques in Equids.”

Orsolini, M.F.; Meyers, S.A.; Dini, P. An Update on Semen Physiology, Technologies, and Selection Techniques for the Advancement of In Vitro Equine Embryo Production: Section II. Animals 2021, 11, 3319.

The review, published under a Creative Commons License, can be read here

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