Stallion Reproduction (AAEP Milne Lecture)

Dickson Varner, DVM, MS, Dipl. ACT (a specialist in reproduction), is a self-described "renegade" when it comes to presenting papers at veterinary gatherings. He is known for his irreverent humor, poetry, and clever turn of phrase. That being said, Varner is also a leading researcher in equine reproduction and has helped propel Texas A&M University into a leadership role in that field.

He was chosen to present the Milne State-of-the-Art Lecture at the American Association of Equine Practitioners (AAEP) convention held in December 2007. The lecture and award were named in honor of Frank Milne, the late and longtime editor of the annual AAEP Convention Proceedings. The lecture is designed to present in-depth information on a particular subject.

Varner lectured on the topic: "From a Sperm's Eye View--Revisiting our Perception of This Intriguing Cell."

Varner said the lecture was not a compilation of his work, but a presentation of the work from a great many scientists through the years. As if to prove that point, his paper published in the Proceedings contained more than 1,000 references.

Dickson's Ditty

It was truly an in-depth description of the sperm cell, but it also contained Varner's trademark brand of humor. He said that program chairman and 2008 AAEP President Eleanor Green, DVM, Dipl. ACVIM, ABVP, and 2007 AAEP President Doug Corey, DVM, had frequently expressed concern about what bent his humor might take and worried that he might get out of line. He jokingly offered to don a dog shock collar and said they could have the remote control button and activate it whenever he went over the edge in the humor department.

Among other things, Varner is a cowboy. He grew up working with his parents in a Wild West show, complete with rough stock and trick animal acts, and he went on from there to be a rodeo contestant before joining the academic path. Thus, it came as no real surprise when he announced that an animated cowboy would help him with his message during the Milne Lecture. He introduced "Dr. Dick," an out-of-work cowboy, who would ride a sperm through a series of slides depicting the cell's travels from the time it is formed until it ends up entering the female oocyte and establishing a pregnancy.

Everyone who knows Varner also knew he would do something unique when he arrived at the halfway point of his lecture and it was time for a break. He didn't let them down. He harked back to the 1998 Milne Lecture presented by O.J. Ginther, VMD, PhD, of the University of Wisconsin, and did him one better. During his break, Ginther, who was presenting a paper on equine pregnancy, picked up a guitar and sang a ditty titled "Mare is Four-Letter Word." It was a hit.

Varner brought to the stage two professional musicians from Texas, Aaron Watson and Dan McBride, as backup, and he sang a song titled "Stud is a Four-Letter Word." He also whipped out his trademark harmonica to play along, and the large crowd responded with a standing ovation.


Fertilizing an egg

Illustration of how a spermatozoon fertilizes an egg. a) The spermatozoon navigates through the egg�s protective barrier and b) adheres to the inner layer. c) The sperm cell�s protective covering begins to disintegrate. d) The sperm gains entry into the egg. e) The protective covering dissolves, allowing the sperm cell�s DNA to merge with that of the egg (f).

Origin of the Sperm

With the help of "Dr. Dick," whose spermatozoal ride got rather hectic and scary at times--especially during ejaculation--Varner described the origin of the sperm within the testis "the sperm factory."

The process is called spermatogenesis. "Spermatogenesis," he wrote in the paper he presented, "is an extremely complex process that involves germ proliferation, germ cell differentiation, and, paradoxically, programmed germ cell death (termed apoptosis). This lengthy process, 57 days in the stallion, is controlled by a vast array of cell-signaling messengers acting through endocrine, paracrine (when target cell is close to signal-releasing cell), juxtacrine (cells must be in direct contact), and autocrine (via secretion of a substance) pathways."

The seminiferous tubules within the testis are where the spermatozoa are manufactured, Varner said. If the tubules from two stallion testes were stretched out end to end, he said, they would be 50 football fields in length. As such, these tubules are capable of producing 60,000 to 70,000 spermatozoa per second. If one were to line up the number of sperm produced by a stallion in a normal lifetime, Varner said, they would stretch to the moon and back more than three times.


Dickson Varner

Dr. Dickson Varner

The main storage area for the sperm within the testis is the epididymis, which if stretched out would be about three-fourths of a football field in length. Each epididymis is capable of storing 40 to 45 billion sperm. During their time in the epididymis, the spermatozoa undergo a maturation process that is required for interaction with the mare's reproductive tract and establishment of fertilizing potential.

Prolonged storage of the sperm within the epididymis is not necessarily a good thing and problems can arise, Varner said, in the form of bent tails, detached heads, distal droplets (abnormalities on the sperm flagella, or tails), and decreased motility. Normally, spermatozoa enter the epididymis at a constant rate in a reproductively normal stallion, with about 5 billion arriving each day. The stay within the epididymis is from 8 to 11 days. After their stay in the epididymis, sperm are either ejaculated or spontaneously ejected into the urethra and expelled. This latter mechanism ensures that viable sperm are available for fertilization at the time of mating or artificial insemination.

A Sperm's Tale

Varner described an individual spermatozoa as being shaped like a tadpole, with a head and a flagellum. Stallion spermatozoa are about 70 to 80 microns in length. They are only about half as long as mouse spermatozoa.

Here is how Varner described the makeup of the spermatozoa: "The head contains the nucleus, overlying acrosome (a membrane-bound compartment at the tip of the sperm head that contains enzymes to digest the outer surface of the egg), and a reduced complement of cytosolic (fluid portion of cytoplasm) elements. The head can be subdivided into an acrosomal region, equatorial segment, post-acrosomal region, and posterior ring, which demarcates the junctions between the head and flagellum. The posterior ring is the site of plasma membrane anchoring to the nuclear envelope and is thought to produce a tight seal that separates cytosolic components of the head and flagellum. The flagellum can be subdivided into a connecting piece, middle piece, principal piece, and end piece. These various parts of the spermatozoon are surrounded by a common plasma membrane."

The spermatozoa are virtually immobile in the epididymis, Varner told the group, but develop motility when ejaculated. After ejaculation, the sperm make their way from the uterus into the oviduct on a journey that allows them to meet up with the descending egg. Many of the sperm, Varner said, do not complete the journey and are lost along the way, with only about 0.0006% of spermatozoa gaining access to the oviducts following insemination. The remainder are lost through the cervix. Approximately four hours are required for sufficient sperm to ascend from the uterus into the oviduct to establish pregnancy at a normal rate.

The ejaculated spermatozoa cannot fertilize an egg right away. Varner described it this way: "In mammals, freshly ejaculated spermatozoa are not immediately capable of fertilizing an oocyte. Early studies showed that spermatozoa require residence time in the female reproductive tract to gain this capability, later termed capacitation."

The principal location where capacitation occurs, Varner said, is the caudal (toward the rear) segment of the oviduct. "Interactions with an ovulated oocyte, however, require spermatozoal migration to the ampullar region of the oviduct, and only a small percentage of spermatozoa that gain access into the oviduct will eventually arrive at this fertilization site. The precise mechanisms by which spermatozoa migrate to the ampullar region of the oviduct remain speculative, but contractile movements of the oviduct and hyperactivated spermatozoal motility are thought to play key roles in this migratory phase."

After capacitation, the spermatozoa must undergo an "acrosomal reaction" before they can pentrate the vestments of the egg required for fertilization. This is a rather complicated reaction that occurs upon spermatozoal contact with the zona pellucida, a thick, transparent outer envelope that encases an egg. In this reaction, the outer membrane of the acrosome fuses at multiple points with the overlying plasma membrane of the sperm head. The process enables the release of enzymes from the acrosome that are necessary to enable the sperm to enter the egg.

Once that entry has been made, the sperm's journey has come to an end. And so it was for "Dr. Dick," who was able to head off into the sunset, looking for new adventures.

Stallion Fertility

Varner directed the final two hours to a discussion of stallion fertility and told the group, for example, that morphologically abnormal sperm often do not have a negative impact on normal sperm. "Therefore," he said, "the total number of morphologically normal sperm in ejaculates may provide more information regarding the fertility of a stallion than the percentage or absolute number of morphologically abnormal sperm."

He also discussed ways to refine a stallion breeding soundness examination, and he described newly developed laboratory assays that might improve the predictive value of such an examination.

Varner concluded his talk by telling the group that, while scientists have achieved much concerning research on sperm, more study is needed.

"Goals might include devising methods for long-term cooled semen preservation, improving the preservation of cryopreserved (frozen) semen, and incorporation of in vitro fertilization (both conventional in vitro fertilization--using multiple sperm--and intracytoplasmic sperm injection--where only one sperm is injected in the egg) in commercial programs," he said. "Although these might seem to be lofty goals, a more absolute understanding of the spermatozoal structure and function would certainly take a lot of the 'guesswork' out of current approaches to analysis and manipulation of equine spermatozoa."

About the Author

Les Sellnow

Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at or by calling 800/582-5604.

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