Structure and Function of the Male Canine Reproductive Tract

Below is information about the structure and function of the canine male reproductive tract. We will tell you about the general structure of the reproductive tract, how it work in dogs, common diseases that affect the reproductive tract and common diagnostic tests performed in male dogs to evaluate the reproductive tract.

What Is the Reproductive Tract?

The male canine reproductive tract consists of the male genital organs including the scrotum, the two testes (normally located within the scrotum), the epididymides, the deferent ducts (tube that leads from the epididymis to the urethra), the spermatic cords, the prostate (an accessory gland), the penis and the urethra (the common passageway of urine and semen).

Where Is the Canine Male Reproductive Tract Located?

The scrotum is located approximately two-thirds of the distance from the opening of the prepuce (the sheath of skin that covers the penis) to the anus. It lies on the back portion of the abdomen between the hind legs.

The testes, or testicles, are normally located within the scrotum. The left testicle usually lies slightly behind the right. Each testicle is oval in shape and thicker in the middle than from side to side.

The epididymis is an enlarged tube positioned along the edge of the testicle. Its beginning and end (head and tail) are located at the front and back of the testicle, respectively. There is one epididymis for each testicle.

The deferent duct or ductus deferens begins at the tail of the epididymis and runs along the border of the testicle, entering into the abdominal cavity through an area called the inguinal canal. It passes through the prostate and empties into the urethra.

The two spermatic cords are composed of the ductus deferens, and the vessels and nerves of the testicles. They are covered by a thin membrane. Each cord originates at the tail of the epididymis and extends back through the inguinal canal.

The prostate gland lies below the rectum and above the pelvic bone along the lower abdominal wall. It is normally located near the front of the rim of the pelvis at the back of the abdominal cavity. The prostate gland surrounds the neck of the bladder, the beginning portion of the urethra, and the termination of the ductus deferens.

The penis is located within the prepuce (a protective tubular sheath of skin). When the penis is not erect it is completely enclosed within the prepuce, which is attached to the lower outside abdominal wall.

The urethra is a hollow tube that extends from the urinary bladder to the very tip of the penis. It travels through the prostate, and carries urine from the bladder to the outside.

What Is the General Structure of the Canine Male Reproductive Tract?

The scrotum is a pouch divided by a thin wall into two cavities, each of which is occupied by a testicle, an epididymis, and the tail end of the spermatic cord. The skin of the scrotum is covered with fine hairs. The dartos of the scrotum is a layer of tissue that lies just under the skin and is made up of muscle and other tissue. Under the dartos is connective tissue and a shiny tissue that lines the scrotum.

Each testicle is oval in shape and thicker in the middle than from side to side. The testicles contain seminiferous tubules that manufacture sperm. Special cells near the seminiferous tubules, called Sertoli cells, support and supply nutrition to the sperm cells.

The epididymis is comparatively large in the dog and consists of an elongated structure composed of a long convoluted or twisted tube. It begins at the front end of the testicle and is positioned along the edge of the testicle.

The deferent ducts are thin muscular tubes that are made up of three layers of muscle.

The prostate gland surrounds the neck (thin part) of the bladder, as well as the end of the ductus deferens. A thin wall divides the gland into two equal-sized smooth, firm lobes. The prostate has multiple openings into the urethra.

The penis is a highly vascularized structure. It is composed of several parts, including the root, body and distal portion or glans penis. The root and body are made up of a spongy type tissue and a bone called the os penis. The glans penis is the soft portion of the penis. During copulation, the glans penis swells and causes the penis to become locked in the vagina of the female. The penis also surrounds the termination of the urethra and is important in directing the stream of urine to the outside of the body.

The prepuce is the tubular sheet of skin that covers the free part of the non-erect penis.

What Are the General Functions of the Male Reproductive Tract in Dogs?

• The scrotum, because of its thin, hairless skin, its lack of fat, and its ability to contract toward the body, functions as a temperature regulator for the testicle and epididymis. The temperature within the scrotum is generally several degrees lower than the abdomen, which is essential for the normal manufacturing and maintenance of sperm.
• The testicles perform two major functions that are largely complementary. They produce sperm and testosterone, a male hormone.
• The epididymides are the organs where sperm are stored before ejaculation. In addition, they slowly transport sperm to the ductus deferens. The length of the epididymis and the slow transport of sperm are important in allowing the sperm time to become mature.
• The ductus deferens transports sperm from the epididymis to the urethra by using strong contractions of the muscle wall.
• The function of the prostate is not entirely understood. It is believed to be essential in providing an optimum environment for sperm survival and motility. It may secrete substances that activate the sperm and make them motile. It may also secrete substances that protect sperm from the acidity in the urethra that comes from the passage of urine through the urethra.
• The penis is the male copulatory organ. It contains vessels and connective tissue specialized to produce an erection, which facilitates penetration of the penis into the vagina of the female.
• The prepuce acts as a moist protective covering for the non-erect penis, and is capable of secreting smegma, a thick, lubricating liquid.

What Are the Common Diseases of the Reproductive Tract in Male Dogs?

• In the puppy, the testicles are initially located within the abdominal cavity. Within several days of birth, they begin to move downward and outward through the inguinal canal in the groin. Within 6 to 8 weeks of age, they have moved into the scrotum. Failure of a testicle to move into the scrotum is called an undescended testicle, or cryptorchidism. The undescended testicle may lie in the groin just outside the abdominal wall or it may be retained within the abdomen. Because the temperature of an undescended testicle is higher than normal, it is infertile. Testicles retained in the abdomen also have a higher incidence of developing tumors later in life.
• Testicular tumors, torsions (twisting upon itself) and orchitis (inflammation of the testis) may also occur. Epididymitis (inflammation of the epididymis) is the most common disorder of the epididymis. Testicular tumors may not cause any clinical signs until they are large enough to be noticed. Torsion and inflammation of the testicle cause pain, irritation, and infertility. Inflammation of the testes may occur with bacterial infections, viral infections, trauma, and immune diseases.
• Benign prostatic hypertrophy (enlargement) is common in older unneutered male dogs. The prostate gland slowly enlarges as the dog ages due to the chronic influence of testosterone hormone. This enlargement does not occur in castrated dogs. Excessive enlargement of the prostate may lead to urinary and rectal blockage, resulting in clinical signs of straining to defecate, straining to urinate and painful urination.
• Other disorders of the prostate include the formation of abscesses following infection of the gland, the development of sterile cysts, and the growth of tumors. Most prostatic tumors in the dogs are malignant carcinomas.
• Several disorders of the prepuce are seen in the dog. Phimosis is the existence of an abnormally small opening in the prepuce, which prevents protrusion of the penis. Paraphimosis is the inability to withdraw the penis into the prepuce. Priapism is persistent erection of the penis that is not associated with sexual excitement. It usually develops following certain spinal cord injuries or urinary infections. Inflammation and infection of the prepuce may also develop in the dog. Tumors of the prepuce may also occur. A contagious tumor that is spread during breeding is the transmissible venereal tumor.
• Trauma may occur to the scrotum, prepuce and urethra. Trauma to the pelvis may affect the prostate gland. Such trauma may include blunt injuries from automobile accidents, bite wounds, and falling on to sharp objects.

What Types of Diagnostic Tests Are Used to Evaluate the Canine Male Reproductive Tract?

• A thorough physical examination and complete history are imperative for the successful management of any reproductive disorder. Diagnostic tests are tailored to the individual patient.
• A complete blood count, biochemical profile and urinalysis are important parts of any baseline workup. The urine is often cultured for bacteria. Infection, inflammation and other systemic disorders need to be considered and/or ruled out.
• Radiographs (x-rays) of the abdominal structures may be of benefit in many cases. An ultrasound may be performed on the prostate and testicles. This may require referral of your dog to a specialist in veterinary internal medicine or reproductive medicine.
• Aspirates for cytology (microscopic cellular evaluation) and/or biopsies (microscopic tissue evaluation) of any masses and tumors are important. Biopsies may also be performed on the prostate. Aspirates and biopsies are occasionally performed on the testicles to assess male dogs known to be infertile. If the testicle is surgically removed, then biopsy is always indicated.
• Semen analysis is sometimes performed to evaluate cases of infertility. Collection of a semen sample can be accomplished by allowing the male to mount a bitch (female dog) in heat, through the use of a teaser bitch, or by hand stimulation. Collected semen is examined microscopically and cultured for bacteria.
• Serologic testing for Brucellosis may be indicated. Brucellosis is caused by a bacterium that may infect the male reproductive tract.