What Boys Are Made Of
- Jun 1, 2003
An appreciation of normal reproductive anatomy is essential to the competent physical examination of the stallion's reproductive tract. If you are involved in breeding, then you should be familiar with what is considered normal so you know when something is wrong. This information will also help mare owners make informed decisions about a particular stallion which might have physical problems outside the norm.
The genital tract of the stallion consists of two testes and attached epididymides contained within a scrotum; paired deferent ducts (or ductus deferens), which traverse the spermatic cord to enter the abdomen through the inguinal rings, then penetrate the genital fold to terminate in the pelvic urethra; four distinct accessory genital glands; a musculocavernous penis; and its protective prepuce. In this article, we will review each anatomical feature and describe its function. The language at times might seem technical, but if you are going to deal with breeding stock, you might as well familiarize yourself with proper anatomical names so you can understand the discussions around you.
Scrotum--The scrotum of the stallion, situated high between the hind limbs, is slightly pendulous (hanging downward) and has a globular shape. It contains two distinct pouches, each investing and protecting a testis, epididymis, spermatic cord, and cremaster muscle. The scrotum plays a vital role in testicular thermoregulation (more on this in a moment).
The wall of the scrotum is composed of four layers. From the outside inward, these layers are the skin, a muscle layer (called the tunica dartos), a connective tissue layer (called scrotal fascia), and an interior lining (called the parietal layer of the vaginal tunic). The scrotal skin is normally thin, pliable, and relatively hairless, and it contains numerous sweat and sebaceous glands (that produce oil--hence its smooth, oily texture). Directly underlying and adherent to the skin is the tunica dartos muscle. The smooth muscle and fibroelastic tissue of this layer bestow contractile properties to the scrotum, thereby permitting changes in its shape, size, and position.
Testes--The paired testes are situated in the scrotum, with their long axes directed horizontally. Testes have two functions: Production of sperm (which is called spermatogenesis), and production of hormones and other factors important in spermatogenesis, sexual differentiation, development of secondary sexual characteristics, and libido. On average, mature, reproductively normal stallions produce five to six billion sperm per day. This is approximately 60 to 70 thousand sperm per second!
Epididymides--Each epididymis is an unbranched tortuous (winding) duct that spans 230-262 feet (70-80 m) in the stallion. The epididymis is divided anatomically into a caput or head, corpus or body, and cauda or tail. The head is closely applied to the front pole of the testis, the body courses over the top surface of the testis, and the bulbous tail is attached loosely to the back pole of the testis.
The epididymis not only serves as a conduit for spermatozoa, but also absorbs fluids within the epididymal lumen (cavity) to concentrate the spermatozoa, and it provides a special place that enables spermatozoa to mature and achieve fertilizing capacity. Spermatozoa entering the epididymis are incapable of unassisted fertilization; they acquire this capacity only after transit through the epididymis (at least through the caput and corpus segments of the epididymis).
The cauda epididymis serves as a storage depot for mature spermatozoa. In sexually rested adult stallions (five to 16 years old), the combined caudae epididymides typically contain more than 50 billion spermatozoa. Spermatozoa that aren't removed from the caudae epididymides by ejaculation are usually emitted intermittently into the urethra and voided during urination.
Spermatozoal transit time through the caput and corpus segments of the epididymis is constant at approximately four days. Peristaltic epididymal contractions (moving in waves along the epididymis) and speed of spermatozoal transit within the caput and corpus are unaffected by extrinsic factors, such as frequency of ejaculation, age, or season.
The rate of spermatozoal passage through the cauda epididymis is variable and is a function of ejaculation frequency. Spermatozoal transit time through the cauda epididymis is constant at about 10 days in sexually rested stallions, but is reduced by two to three days in stallions which are sexually active.
Deferent Ducts (Ductus Deferens)--The two deferent ducts serve as a passageway for mature spermatozoa by connecting the tail of the epididymis with the pelvic portion of the urethra. These long, highly muscular ducts ascend through their respective spermatic cords to emerge via the vaginal rings into the abdominal cavity. They then pass caudally (rearward) through the genital fold (peritoneum) to enter the pelvic cavity and terminate as slit-like openings near a small protuberance, the seminal colliculus, situated on the top surface of the pelvic urethra.
Spermatic Cords--The two spermatic cords extend from the vaginal rings to the testes. Each spermatic cord encloses a testicular artery, testicular veins (pampiniform plexus), lymphatic vessels, nerves, small bundles of smooth muscle, and a deferent duct with its associated nutrient vessels. The cremaster muscle, a striated muscle that originates from the abdominal wall, attaches to each spermatic cord and permits voluntary upward movement of the testes within the scrotum.
Accessory Genital Glands--Ejaculated semen is comprised primarily of secretions from the accessory glands. Aside from providing a vehicle for transport of spermatozoa from the stallion to the mare's reproductive tract during mating, the exact functions of the secretary products from the different accessory glands remain obscure. Accessory glands in stallions include the paired ampullae and vesicular glands (or seminal vesicles), a bilobed prostate gland, paired bulbourethral glands, and disseminate urethral glands. The ampullae and lobes of the prostate gland can be identified consistently by rectal palpation. The vesicular glands evade identification because of their thin walls, but usually can be detected by rectal palpation when they are distended through sexual stimulation.
The bulbourethral glands usually are not palpable per rectum because they are covered by thick musculature. All accessory genital glands (except the disseminate urethral glands) can be identified by transrectal ultrasonography.
Urethra--The urethra is a membranous canal extending from the bladder to the end of the penis, and it is a passageway for both urine and semen. The pelvic portion of the urethra is surrounded by a layer of erectile tissue (the stratum cavernosum), which itself is enveloped by the thick striated urethralis muscle. The extrapelvic or penile portion of the urethra begins at the penile attachment to the pelvis, then courses along the penis in the ventral groove of the corpus cavernosum penis to terminate as the urethral process at the tip of the penis.
Penis--This erectile copulatory organ is well developed in stallions. It consists of a root (radix penis), body (corpus penis), and glans penis. The root of the penis is securely attached to the ischial arch of the pelvis by two strong ligamentous crura containing erectile or vascular spaces. These crura fuse in the root of the penis and extend distally to occupy the body of the penis as the expansive corpus cavernosum penis. The sizable glans penis is located distal to the body, capping the free end of the penis. The cranial (forward) surface of the glans has a deep depression, the fossa glandis, from which the urethral process protrudes. The fossa glandis contains two smaller recesses on its bottom side and a large dorsal diverticulum (urethral sinus).
The penis of the male horse is of the musculocavernous type, because the erectile tissue is made up predominantly of cavernous spaces rather than trabeculae of connective tissue. This is in contrast to the fibroelastic penis of male ruminants (i.e., bulls).
The penis is composed of three cavernous bodies--the corpus cavernosum penis, corpus spongiosum penis, and corpus spongiosum glandis. The corpus cavernosum penis spans the entire length of the penile body and is covered by a thick capsule (tunica albuginea). Precopulatory erection in the stallion is achieved through engorgement of the corpus cavernosum penis. A longitudinal groove on the ventral (lower) surface of the corpus cavernosum penis shields the penile urethra. The corpus spongiosum penis surrounds the penile urethra along its entire length. The corpus spongiosum penis is continuous with the cavernous spaces of the corpus spongiosum glandis, located in the glans penis. It is capable of great expansion and is responsible for the marked "belling" or "flowering" of the glans penis during coitus.
Prepuce--The prepuce shields the non-erect penis. Unlike that of other domestic species, the prepuce of the horse telescopes around the non-erect penis to form two distinct folds, creating both an external and internal prepuce.
Selected Physiologic Events
Testicular Descent--The testes develop embryologically near the kidneys, then traverse the abdominal cavity with their attached epididymides and descend through the inguinal canal to reach a final position within the scrotum by late fetal or early postnatal life (between 300 days of gestation and 10 days after birth). The mechanism controlling testicular descent is not well understood, but gonadotropin-induced androgen production by the developing testes plays a major role in the process.
The fetal gonads (either testes or ovaries) become unusually large during gestation. They grow rapidly between Days 100 and 200 to become larger than the gonads of the dam, then begin to decrease in size after Day 250, such that their weight at birth is only about 10% of their maximal weight during gestation. Inguinal passage of the testes does not begin until about 270-300 days of gestation, after the testes begin to diminish in size. The vaginal ring constricts soon after birth. After this occurs, it is very unlikely that an abdominal testis would subsequently descend into a scrotal position. Although considered abnormal, descent of inguinal testes into the scrotum has been observed at up to two to three years of age in stallions.
Puberty--Puberty in the male is the age when he can first impregnate a female. Sexual desire, ability to copulate, and presence of viable spermatozoa in the ejaculate are required to accomplish this feat. Age at puberty in stallions ranges from 12 to 24 months; breed, nutritional plane, and season of birth might influence this process. Rapid testicular growth and development of secondary sex characteristics are the most easily recognizable traits of the prepubertal period (before puberty). Puberty in stallions should not be confused with sexual maturity, as spermatogenic potential and size of the testes continue to increase after puberty is attained.
Efficiency of spermatogenesis (determined by daily spermatozoal production per gram of testicular parenchyma) reaches adult levels by 2 1/2 to three years of age, whereas daily spermatozoal production per testis and testicular weight continue to increase until four to five years of age.
Thermoregulation of the Testes--
The testes and epididymides of domestic mammals require a temperature below that of normal core body temperature to function properly. Elevated temperature in this portion of the reproductive tract impairs spermatogenesis and sperm-fertilizing capability. The combined actions of the scrotum, pampiniform plexus, and cremaster muscle regulate gonadal temperature to provide a suitable environment for spermatozoal production.
The scrotum allows the testes and epididymides to assume a position outside the body proper. The scrotal wall is thin, with very little fat, and it contains numerous sweat glands that help lower the temperature of its contents through evaporation. Temperature receptors are also present in the scrotal skin and influence general body thermoregulatory mechanisms.
In addition, the tunica dartos muscle in the wall of the scrotum relaxes in warm weather to produce a more pendulous scrotum and thereby provides more surface area for evaporative cooling.
The testicular vein leaves each testis and divides to form many intercommunicating vessels that, in turn, intertwine with the highly coiled testicular artery en route to the testes. The numerous smaller vessels branching from this vein form the pampiniform plexus. The temperature of blood in the superficial branches of the testicular vein is lower than body temperature because of the loss of heat by evaporation. The cooled venous blood enters the pampiniform plexus, which then serves as a countercurrent heat-exchange area. As the pampiniform plexus and testicular artery intertwine, heat is transferred from the warm testicular artery to the cooler pampiniform plexus, reducing the temperature of the arterial blood by several degrees as it leaves the area. More heat might be lost from the testicular artery as it passes over the testicular surface.
The cremaster muscles can contract to raise the testes up nearer to the body wall in cold weather, but only for short periods of time. Complete relaxation of these muscles permits the testes to assume a more distal scrotal position, which facilitates cooling.
With this text and accompanying illustrations, you should be better prepared to deal with breeding your stallion or mare. When discussions with your veterinarian involve normal (or abnormal) stallion anatomy, feel free to take this article and let your veterinarian point out to you on the illustrations what he is discussing. With the proper terminology and preparation, you should be a more knowledgeable breeder, and therefore do a better job.
About the Author
Dickson Varner, DVM, PhD, Dipl. ACT, is based at Texas A&M University. He is a member of the AAEP Editorial Advisory Board of The Horse.
POLL: University Equine Hospitals