The male reproductive system in a dog is made up of the scrotum, testicles, epididymides, deferent ducts, spermatic cords, prostate, penis and urethra. Understanding the structure and function of these organs can help you identify when any problems crop up which may need veterinary attention.
Elements of the reproductive tract
Hanging between the dog’s hind legs is a sac known as the scrotum which holds and protects the testicles. The scrotal pouch is divided into two separate areas by a thin wall running down the middle. Inside each area are one testicle, one epididymis, and the head of one spermatic cord. The scrotal sac, which does not contain any insulating fat, keeps the testicles several degrees cooler than the abdomen, which is important for sperm motility.
The testicles are oval shaped, with the left testicle usually hanging slightly behind the right. Inside the testicles are seminiferous tubules, responsible for the production of sperm. Sertoli cells fill the area around the tubules, supporting the tubules and providing nutrition to the sperm cells. The Leydig cells of the testicles are responsible for the production of the male hormone testosterone.
The epididymis (plural: epididymides) begins at the front of the testicle and runs along the edge to the back. It is responsible for storing and carrying sperm to the ductus deferens prior to ejaculation. The sperm stays in the epididymides for a relatively long period of time, allowing the cells to mature.
Spermatic cords are composed of vessels and nerves which nourish the testicles. The ductus deferens are also contained inside the spermatic cords. The ductus runs into the abdominal cavity through the inguinal canal, then leaves the abdomen, running through the prostate and ending at the urethra.
Although the exact function of the prostate gland is unknown, it is thought that it secretes substances necessary to protect and activate the sperm. This accessory sex gland contributes as much as 90% of the fluid that is released when the dog ejaculates.
Once they have matured in the epididymis, the sperm cells enter the ductus deferens, which has strong muscular walls. The muscular walls of the ductus contract to push the sperm into the urethra, which runs the length of the penis. The urethra then releases sperm into the female when the dog ejaculates.
The penis has three portions: the root, the body, and the glans. The root and body are made of spongy tissue surrounding a bone known as the os penis. The glans is soft but swells with blood during sexual stimulation. This swelling is important because it keeps the penis locked into the female dog’s vagina, allowing the dogs to “tie” to each other to promote successful passage of sperm from the male to the female.
Covering the penis is the prepuce, a tubular piece of skin which protects the penis. The prepuce secrets a lubricating liquid known as smegma which keeps the penis moist.
Hormonal control of the male canine reproductive system
The cells of the testicles are under the control of the hypothalamus and pituitary gland of the brain. The hypothalamus releases gonadal releasing factor, which stimulates the pituitary to secrete follicle stimulating hormone and luteinizing hormone. Both hormones travel to the testicles, where FSH stimulates the seminiferous tubules to produce sperm and LH stimulates the Leydig cells to produce testosterone.
Sperm, of course, is used to impregnate female dogs. Testosterone is responsible for the development of male sexual characteristics during puberty. This includes higher muscle mass, thicker bones, more red blood cells, and a higher metabolic rate.
Diseases of the male reproductive tract in dogs
Before a puppy is born, his testicles reside in his abdominal cavity. At birth, they begin to move slowly through the groin area and into the scrotum. The trip should be complete by 6 – 8 weeks of age. In some puppies, one or both of the testicles may fail to descend at all, or may get stuck part of the way through the trip. This condition, known as cryptorchidism or undescended testicle, keeps the testicle trapped in the relatively high temperature of the pelvic cavity, making the affected testicle incapable of producing sperm. Undescended testicles have a higher incidence of tumor development as the dog ages. This condition has a genetic component, so dogs with an undescended testicle should not be bred.
Other problems which may occur in the testicles are inflammation known as orchitis or torsion, which occurs when the testicle becomes twisted on itself. Both of these conditions cause your dog considerable pain as well as infertility. You may notice your dog walking strangely or wanting to lay on cold surfaces to relieve the pain.
The epididymis may become inflamed due to bacterial or viral infections, trauma, or diseases of the immune system.
The most common problem which occurs in the prostate is benign prostatic hypertrophy (BPH). In un-neutered dogs, the continual influence of testosterone causes the prostate to gradually enlarge. The enlargement may block both the urinary tract and the rectum causing the dog to strain when he relieves himself. Other problems of the prostate include infection, cysts, and tumors, which are usually malignant.
The prepuce can also be the site of several disorders. If the opening of the prepuce is abnormally small, the dog may not be able to protrude his penis through the opening, making sexual intercourse impossible. This condition is known as phimosis.
At the other end of the spectrum, if the dog cannot withdraw his penis into the prepuce, the condition is known as paraphimosis. This condition can be treated by applying ice to the dog’s penis to encourage it to become less erect, allowing the dog to retract it.
Priapism is a condition where the dog’s penis remains erect in the absence of sexual activity. This is typically the result of an infection or a spinal cord injury.
The prepuce may become inflamed due to infections, which may include sexually-transmitted diseases.
How will the vet check my dog for reproductive problems?
Your vet will first interview you to find out about the dog’s medical history so he can choose which tests will be most beneficial. It is important that you spend enough time observing your dog’s normal behavior in order to know when something has changed. You are your dog’s best advocate, so make sure you can give your vet an accurate description of any problems you are seeing.
Next, your vet may want to run an analysis of the dog’s blood and urine. This will help her rule out any systemic infections which may be causing the problems. Serologic testing of the blood can indicate the presence or absence of Brucellosis, a bacterial infection that is often the cause of reproductive tract disorders in male dogs.
X-rays of the abdomen can point out problems with internal structures, as can ultrasound inspection of the prostate and testicles.
If a tumor is suspected, your vet may take a biopsy of solid tissues or may aspirate the liquid from a fluid-filled tumor for microscopic analysis.
If you are concerned about your dog’s fertility, your vet may want to examine a sperm sample under the microscope to check for the quantity and quality / motility of the cells. Sperm may be collected by using what is known as a “teaser bitch”, which is just what it sounds like, or by the vet stimulating the dog by hand.
The importance of neutering
Neutering a dog involves the removal of both testicles, making the dog unable to produce sperm. Within a week or two after the surgery, your dog’s scrotal sacs will simply shrink and become unnoticeable. If you value the look of an intact dog, there are prosthetic testicles which can be inserted to maintain the scrotal sac size. The penis is not removed during neutering because it houses the urethra and has a function other than carrying sperm. It also carries urine from the bladder, directing it outside of the body.
Aside from reducing overpopulation problems, there are health reasons for neutering your dog as soon as possible after he joins your family. For example, a neutered dog cannot develop testicular cancer or benign prostatic hypertrophy. There is also some evidence that a neutered dog will be less aggressive than an intact dog.