Male Reproductive Tract - Horse Anatomy

Testes and Epididymis

Scrotum

Outer-most layer is the scrotal skin, which contains many sweat glands, thermosensitive receptors and sympathetic nerves. The tunica dartos is a smooth muscle layer found below the skin of the scrotum. In horses, this muscle forms a septum which divides the scrotal sac into right and left compartments. Smooth muscle tone is controlled by androgens released in response to temperature changes, detected by sensory nerves within the scrotal skin. The tunica dartos is responsible for holding the testes close to the body during cold temperatures, as it can maintain sustained contraction, and letting the testes lower in hot weather, increasing the surface area for heat loss from the skin. The next layer is the scrotal fascia.

The testes are contained within the tunica albiguinea, which is a fibrous capsule composed of collagen. It also contains the testicularis arteries and veins, which are visible on the surface of the testicle. This capsule is covered by the tunica vaginalis, which is an outpocketing of the abdominal peritoneum. The tunica vaginalis can be further divided into the visceral and parietal tunics. The visceral tunic is adherent to the testicle, ducts and vessels. The parietal tunic is continuous with the parietal peritoneum. The parietal tunic adheres to the tunica dartos muscle, scrotal fascia and scrotal skin. The tunica albuginea sends projections from the capsules into the testicular parenchyma that join up with the mediastinum of the testis forming lobules. Contraction and relaxation of the smooth muscle fibres within the tunica albuginea in response to acetylcholine and norepinephrine results in a pumping action facilitating the flow of spermatozoa through the tubules of the testes.

The cremaster muscle is the caudolateral extension of the internal abdominal oblique muscle. It is continous with the parietal layer of the tunica vaginalis and inserts at the caudal pole of the testes.

The scrotum has important functions:

• Temperature sensor
• Protection
• Cooling of the testes - evaporative heat loss due to lots of sweat produced by glands in the skin.

Vaginal Process

The vaginal process is formed by the evagination of peritoneum from the abdomen through the inguinal canal. It encloses the vaginal cavity, which communicates with the abdominal cavity through the vaginal opening at the internal opening of the inguinal canal. Inguinal Hernia is where a loop of intestine or omentum herniates into the vaginal process, this is more common in horses than in other species because they have a wide vaginal ring.

Testes

In the horse, the left testicle is usually larger and more caudal than the right testicle. It is also usually suspended more ventrally.

The parenchyma of the testes is composed of the semniferous tubules, rete testis and efferent ducts. The semniferous tubules are the site of spermatogenesis. The walls of the semniferous tubules contain spermatogenic cells and their sustentacular cells, which are known as sertoli cells. The sertoli cells give support and also produce hormones. They regulate spermatogenesis and the release of spermatozoa into the lumen of the semniferous tubules. Each semniferous tubule opens into a network of tubules called the rete testis. Each rete testis is drained by multiple efferent ducts which perforate the capsule of the testes to enter the head of the epididymis. The interstitial tissue between semniferous tubules contains Leydig cells. These cells produce androgens, such as testosterone.

The Blood-Testes Barrier is formed by tight junctions between neighbouring Sertoli cells. This barrier prevents leukocytes from entering the adluminal compartment of the seminipherous tubules and autoreacting to the haploid spermatids within it.

Epididymis

The Epididymis provides the environment for final maturation of spermatozoa. It is here that they aquire motility and fertility. The Epididymal duct is very long, between 72-81 meters in the horse. It is divided functionally into 3 regions; the Caput (head), Corpus (body) and Cauda (tail). The caput of the epididymis is connected to the efferent ducts of the testes. The Cauda connects to the Ductus Deferens of the spermatic cord.

Caput (Head)

Attached to the testicular capsule, receives the efferent ducts. After efferent ducts enter the head of the epididymis, they join to become the duct of the epididymis. Fluid secreted in the rete tubules of the testes is reabsorbed here and epididymal fluid is secreted. This fluid secretion is important in order to facilitate movement of the sperm through the epididymis.

Corpus (Body)

Formed by the convuluted ducts. Nuclear chromatin condenses via the addition of di-sulphide bridges. This condensation stabilises the nuclear material and can be observed histologically as darker staining material. Stabilising surface proteins are added to the sperm membranes. Sperm aquire fertility by the addition of membrane receptors to female tract ligands. This enables them to bind to the zona pellucida and oocyte membranes. Sperm cells are dehydrated and sperm acquire forward motility.

Cauda (tail)

Continuation of the duct of the epididymis, which continues as the ductus deferens. It is attached to the caudal pole of the testis by the proper ligament of the testis and to the vaginal process by the ligament of the tail of the epidymis. Sperm metabolic activity is depressed in order to prolong life as they are stored here until ejaculation.

Histology

The Epididymal duct is surrounded by smooth muscle fibers. These fibers contract rhythmically facilitating sperm movement through the duct.

Caput

• Columna epithelium with basal nuclei.
• Small steriocillia present, these have an absorbtive function

Corpus

• Low columna epithelium with very prominant stereocillia.
• Small groups of sperm can be observed within the lumen of the duct.

Cauda

• Cuboidal/ Low columna epithelium. However, unlike the caput and corpus regions of the epididymis the cauda epithelium has no stereocillia.
• Large numbers of sperm observable here as this is the site of sperm storage within the testes.

Ductus Deferens

The ductus deferens is a direct continuation of the duct of the epididymis from the cauda (tail) of the epididymis. It runs cranially within the spermatic cord to enter the abdominal cavity via the inguinal canal. It loops within a fold of peritoneum and passes under the ureter to reach the dorsal surface of the bladder. It enters the prostate gland and opens at the seminal hillock in the proximal urethra. The ampulla is a thickening that arises at the termination of the ductus deferens adjacent to the ampullary gland. In the horse, the excretory duct of the vesicular gland joins the ductus deferens close to its termination. The shared duct is known as the ejaculatory duct.

Spermatic Cord

The spermatic cord comprises the ductus deferens, testicular vessels and nerves, lymphatics and their serous coverings. It runs from the abdomen caudally, through the inguinal canal. The inguinal canal is formed by the deep and superficial inguinal rings.

• Deep inguinal ring: Opening between the internal abdominal oblique, rectus abdominis, prepubic tendon and inguinal ligament.
• Superficial inguinal ring: Opening in the external abdominal oblique.

Testicular Vasculature

The testicular artery is a direct branch of the abdominal aorta and runs together with the testicular vein. This has important implications when performing castration procedures as the pressure within the artery will be very high and so it is imperitive to tie the artery off properly. Within the spermatic cord, the testicular artery becomes highly convoluted. Branches of the testicular artery supply the epididymis and ductus deferens. The testicular veins form a network called the pampiniform plexus around the convoluted arteries. Within the spermatic cord, there are arteriovenous anastamoses between the testicular arteries and surrounding veins. The pampiniform plexus becomes a single vein, the testicularis vein,which drains into the caudal vena cava. The network of testicular veins that surround the testicular artery as the pampiniform plexus, provide the testes with a countercurrent heat exchanger. The testes require a temperature of 33°C in order for maintaining spermatogenesis, 3-6°C lower than core body temperature. This is achieved by the countercurrent exchange system between the pampiniform plexus and the testicular artery. Heat is transfered from the warmer blood in the testicular artery to the cooler blood returning from the testes in the testicular veins. The testicular veins run close to the surface of the scrotum, so the blood within them is cooled by heat radiation from the skin of the scrotum.

Lymphatic Drainage

The testicular lymphatics drain to the lumbar aortic lymph nodes and the medial iliac lymph nodes. Lymphatics from the scrotum drain to the scrotal or superficial inguinal lymph nodes.

Innervation

• Parasympathetic: Vagus nerve and pelvic plexus
• Sympathetic: Caudal mesenteric plexus and pelvic plexus

Nerve supply to scrotum and surrounding structures is provided by the ventral branches of the lumbar nerves, iliohypogastric nerve, ilioinguinal nerve and genitofemoral nerve.

Penis and Prepuce

The penis is the male copulatory organ. It is formed from three parts; two Corpora cavernosa, comprising of cavernous tissue and a connective tissue sheath the tunica albuginea, and the single Corpus Spongiosum which contains the urethra encased in a vascular tissue sleeve. The penis can also be divided into the root, body and glans. The horse has a musculovascular penis. In the musculovascular penis, there is only a small amount of connective tissue, allowing them to be flaccid when not erect. Erection is accomplished by engorgement of the tissue with blood and a relatively larger ammount of blood is required to achieve erection than for a fibroelastic penis (ruminants, boar). Engorgement with blood increases the length and diameter of the penis.

Root

The root of the penis is the most proximal part, attached to the ischial arch. It consists of the crura and bulb. The crura consist of a cavernous tissue core surrounded by tunica albiguinea. The two crura converge to form the body of the penis. The bulb of the penis is an expansion of the corpus spongiosum around the pelvic urethra. The bulb lies between the two crura, close to the ischial arch.

Body

This is the main part of the penis, between the root and the glans.

Corpus Cavernosum

The corpus cavernosum is made up from the paired columns of erectile tissue (cavernous bodies) with enlarged venous spaces. Proximally, the right and left cavernous bodies separate, forming the two crura of the penis at its root.

Corpus Spongiosum

The corpus spongiosum is a vascular tissue sleeve surrounding the urethra. It commences at the bulb of the penis as an enlargement of the spongy tissue of the pelvic urethra. It is another column of erectile tissue, more delicate than the cavernous bodies. It contains larger blood spaces, divided by thinner septae. At the end of the penis the corpus spongiosum expands over the distal end of the corpus cavernosum to form the glans penis, bringing the urethra to the extremity of the penis.

Glans Penis

The glans is the distal, free part of the penis. In the stallion, the glans penis is shaped like a mushroom, with the widest part being the corona. It then continues towards the body of the penis as the neck of the glans. The free end of the corona has a central fossa, the fossa glandis, which receives the terminal urethra. The glans penis ends with the urethral process, which is a protrusion of the terminal urethra.

Prepuce

The prepuce is the skin sheath that conceals the penis when it is flaccid and is formed by an invagination of the abdominal skin. The prepuce is hairless and contains many smegma secreting glands important for lubrication between the shaft of the penis and the prepuce during copulation. Within the prepuce are varying amounts of striated muscle fibres; the cranial prepucial muscles responsible for retracting the prepuce and the caudal prepucial muscles responsible for protracting the prepuce. Unlike other species, the prepuce of the stallion contains a second prepucial fold in order to accommodate the length of the penis in this species. The external fold of the prepuce corresponds with the prepuce of other species. The prepucial fold is a second fold inside the prepucial sheath, which ends at the prepucial ring cranially.

Muscles of the Penis

Bulbospongiosus

A single muscle that covers the root and ventral surface of the penis as well as the bulbourethral glands, extending to the apex of the penis. It is a continuation of the urethralis muscle. The function of this muscle is to empty the extrapelvic urethra of sperm in a similar way to the urethralis muscle emptying the pelvic urethra.

Ischiocavernosus Muscles

These are paired muscles located at the root of the penis which arise from the ischial arch and enclose the crura. They connect the penis to the ischial arch of the pelvis.

Retractor Penis Muscles

These paired muscles originate on the caudal vertebrae and insert on the ventrolateral surfaces of the penis. They follow the bulbospongiosus muscle to the apex of the penis.

Vasculature

The artery of the penis is a direct branch off the internal pudendal artery. It splits into three branches:

• Artery of the bulb – supplies the corpus spongiosus
• Deep artery of the penis – supplies the corpus cavernosum
• Dorsal artery of the penis – supplies the glans penis

The prepuce covering the flaccid penis is supplied by anastamosis between the external pudendal artery and the artery of the penis. In the stallion, there are additional anastamoses between the dorsal artery of the penis and the obturator artery.

Innervation

• Mostly parasympathetic from the paired pudendal nerves
• The glans penis and internal lamina of the prepuce are heavily infiltrated by sensory nerve endings, responsible for stimulating ejaculation

Lymphatics

Lymph from the penis and prepuce drains into the superficial inguinal lymph nodes.

Accessory Sex Glands

Accessory sex glands are all found along the length of the pelvic urethra. They produce secretions containing lots of different components many of which can be found also the blood. One of the most important substances is fructose which provides an energy source for sperm. Along with providing nutrition for spermatozoa sex gland secretions facilitate movement of the sperm as well as providing a physiological buffer against the acidic environment of the female tract. Development and function of the accessory sex glands is androgen dependant.

Bulbourethral Glands

The paired bulbourethral glands are found near the ischial arch either side of, and dorsal to, the urethra. They are approximately the size of a cherry. These are very dense glands containing a lot of fibrous connective tissue.

Prostate Gland

The prostate is a singular gland, found at the junction between the bladder and the urethra. The stallion has only the compact part of the prostate (corpus prostate), which is a compact body of tissue found external to the urethra.

Ampullary Glands

These paired glands are found lying on the dorsal neck of the bladder. They develop due to a glandular enlargement within the wall of the ductus deferens as it crosses the dorsal surface of the bladder. The glands are connected to the urethra via the ejaculatory duct.

Vesicular Glands

The paired vesicular glands are found dorsocranial to the pelvic urethra. In the horse, the excretory duct from the vesicular glands join the ductus deferens to form the ejaculatory duct. They are also relatively large in the horse and are hollow with a thick, muscular wall and smooth surface.