Anatomy and Normal Reproductive Physiology

By | March 9, 2015

1) When should I expect my male dog to be fertile?

That depends on his breed. Animals of most species go through puberty and become sexually mature when they reach about 80% of their adult body weight. For a Maltese, that may occur when he is 4 months old. For a Leonberger, that may not occur until he is almost 2 years old. The only ways to know for sure that a male is fertile are either to breed him or to collect semen for evaluation.

2) My dog has poor libido. Does he have a testosterone deficiency?

No. I have never documented lack of testosterone as a cause of poor libido in dogs nor has anyone else, to my knowledge.

Embryology and anatomy of the male reproductive tract

Determination of gender is dependent on the type of sex chromosomes present in the embryo. All dogs have 78 chromosomes, two of which are sex chromosomes; in females both are X chromosomes, and in males there is one X and one Y chromosome. Two tubular tracts are present in the developing embryo: the mullerian (paramesonephric) duct system, which goes on to form the female reproductive tract, and the wolffian (mesonephric) duct system, which goes on to form the tubular portions of the male reproductive tract.

In the presence of a Y chromosome, the indifferent gonad is stimulated to form a testis. A gene on the Y chromosome, called the SrY gene, encodes for production of a protein, sometimes called testis-determining factor, which stimulates formation as a testis. The testis secretes testosterone and mullerian-inhibiting factor. Testosterone is metabolized to a similar compound, dihydrotestosterone. Both these compounds must be present for normal development of the male reproductive tract to occur. Mullerian-inhibiting factor prevents development of the female ductal system. The mesonephric ducts form the epididymis and vas deferens. The genital tubercle forms the penis. The genital swellings close to form the scrotum.

The reproductive anatomy of the male dog consists of the paired testes-epididymes within the scrotum, the vasa deferentia, the prostate, the penis, and the prepuce (). The testes consist of multiple seminiferous tubules in which spermatogenesis takes place. The seminiferous tubules empty into a fibrous center space, the rete testis. The rete testis connects to the head of the epididymis. The testicular tissue is surrounded by a tight capsule, such that the testes have the consistency of a peeled hard-boiled egg when palpated. An epididymis is tightly adhered to each testis. Fluid and spermatozoa from the testis enter the head of the epididymis; move through the body of the epididymis, which lies laterally on each testis; and are stored in the tail of the epididymis.

The tail of the epididymis becomes the vas deferens, the tubule running from the epididymis to the urethra in the spermatic cord. Other tissues within the spermatic cord include the testicular arteries and veins. The vas deferens on each side empties into the urethra at the level of the prostate through multiple small openings collectively called the seminis colliculus. The prostate encircles the urethra at the neck of the urinary bladder.

The urethra runs through the center of the penis. It is surrounded throughout its length by cavernous tissue. Cavernous tissue is an empty honeycomb space within the penis that fills with blood during erection. The urethra also is surrounded by part of its length within the penis by a bone, the os penis (). The os penis allows the male to introduce the penis before erection is complete; this allows the extreme increase in size of the penis to occur within the vagina, permitting formation of the copulatory lock, or tie.

The proximal portion of the penis, which always is larger than the rest of the shaft of the penis, is the bulbus glandis. This tissue will treble in size when the penis is erect and is the portion of the penis that is caught within the vulvar lips during the tie that usually occurs during breeding (). The tip of the penis contains the external urethral orifice, visible as a small opening or slit. When the penis is erect, the tip takes on the appearance of a thick leaf; it is thought that this shape helps promote ejaculation of spermatozoa toward the cervix of the female during natural service. The prepuce is a piece of haired skin that encloses the flaccid penis at rest.


Male dogs are considered to have entered puberty when they have continuing spermatogenesis evidenced by normal semen quality and when they can exhibit normal breeding behavior. Small-breed dogs enter puberty earlier than do giant breed dogs. The average age at onset of puberty is about 10 months, but some small-breed dogs are capable of reproduction at 4 to 5 months of age, and some giant-breed dogs are not fertile until almost 2 years of age.

Spermatogenesis review

The seminiferous tubules within the testes are the site of formation of spermatozoa, or spermatogenesis, in males. The seminiferous tubule contains immature cells, the spermatogonia, which lie against the basement membrane, or lowest level, of the tubule. There are two populations of spermatogonia; the reserve population, which is resistant to damage to the testes from radiation, toxins or other insults, and the proliferating population. Spermatogonia from the proliferating population divide several times such that each spermatogonium forms multiple spermatozoa. The developing spermatozoa are embedded in Sertoli cells within the tubule. Within a given seminiferous tubule, cells at various stages of development will be present at all times in a normal male such that new spermatozoa always are being produced. Spermatogenesis continues whether or not the dog is ejaculating regularly.

Once the spermatozoa are completely developed, they are released into the center of the seminiferous tubule and through the center of the testis into the epididymis. The spermatozoa are not motile as they leave the testis but gain motility and fertilizing ability as they move through the head and body of the epididymis. Spermatozoa are stored in the tail of the epididymis. It takes 62 days from division of a spermatogonium to ejaculation of a motile spermatozoon. Therefore, any insult to the testis will not be reflected by a change in semen quality for at least 62 days.

Erection and ejaculation

Erection and ejaculation are not hormonally mediated processes. Both are under control of the autonomic nervous system, the portion of the central nervous system that controls subconscious processes such as breathing and beating of the heart. The autonomic nervous system is made up of the parasympathetic and sympathetic systems. The latter is associated with release of adrenaline (epinephrine) from the adrenal gland during the “fight or flight” response.

Erection of the penis is due to parasympathetic nervous stimulation. Blood flow into the penis is stimulated, and blood flow from the penis is inhibited such that the cavernous spaces fill with blood and the pressure within the penis increases, making the penis rigid. The erect penis is, however, still flexible. During natural service, at the formation of the tie, the male steps over the hindquarters of the female with his penis still entrapped within the vagina, twisting the penis by 180 degrees.

Ejaculation is due to sympathetic nervous stimulation. Fluid from the epididymis and prostate fills the urethra at the level of the prostate, forming an area of high pressure. The neck of the bladder is closed off, preventing movement of fluid into the urinary bladder, and synchronous contractions of the urethra and surrounding musculature are stimulated, causing ejaculation. Semen is ejaculated in three fractions. The first, or presperm fraction, is clear fluid that arises from the prostate and clears the urethra. The second, or sperm-rich fraction, arises from the epididymis and contains spermatozoa. The third, or prostatic fraction, is a large volume of clear fluid that helps promote movement of spermatozoa cranially in the vagina of the bitch during natural service (22-4).

Because erection and ejaculation are not hormonally mediated, they can occur occasionally in castrated male dogs. The fluid portion of semen arises from the prostate, which atrophies in castrated males. Intact males that do not exhibit erection and ejaculation, and are therefore presumed to have poor libido, may suffer from pain when attempting to mount or breed bitches or may have a psychological block to mating. Hormone insufficiency as a cause of poor libido never has been documented in the dog. Animals with poor libido are better evaluated for abnormalities of the hindlimbs, spine, or prostate. Semen collection or breeding with nondominant bitches or in different surroundings may be attempted. Too frequent ejaculation may be a cause of poor libido (Table Causes of Poor Libido in Dogs).

Table Causes of Poor Libido in Dogs

Cause of Apparent Poor Libido Diagnostics Test Required/ Management
Pain in rear limbs or spine Radiographs of rear limbs and spine, neurologic examination
Painful prostate Palpation of prostate, culture and cytology of prostatic fluid, ultrasound of prostate
Psychological block to breeding Attempt collection/breeding with different female, change location of collection/ breeding attempt, change humans present during breeding
Too frequent semen collection/breeding Dogs undergoing semen collection or natural breeding twice to three times daily exhibit decreased libido within days. Most dogs tolerate daily semen collection or breeding, but semen quality declines with daily ejaculation

Dogs with poor libido for which no apparent cause can be identified may be induced to ejaculate by administration of testosterone or of a hormone that causes testosterone release, such as gonadotropin-releasing hormone (GnRH) or human chorionic gonadotropin (hCG). Testosterone can be given immediately before an attempted collection or breeding. GnRH or hCG should be given one hour prior to attempted collection or breeding. This technique should not be used regularly. Artificially induced increases in serum testosterone concentration exert negative feedback on the pituitary and testes, decreasing the regular cyclic release of testosterone necessary for continuing spermatogenesis.

Test your understanding

1) A friend calls you with the following information and asks your advice: His valuable 9-year-old weimeraner has not settled any bitches he has bred in the last 2 years. He went to a vet, who told him the dog’s testes were small and soft and recommended testicular biopsy. He reads you the biopsy report: “Numerous seminiferous tubules are present on the section. No maturing spermatozoa and no spermatogonia are present.” What is the prognosis for this animal’s future fertility?

The prognosis is grave. Spermatogonia must be present for spermatozoa to be formed. New spermatogonia do not appear in testes after embryonic development. Some episode caused destruction of the proliferating and reserve populations of spermatogonia in this dog, and he will never be fertile again.

2) Your neighbor claims that your castrated male dog tried to breed his valuable show bitch. Is that possible?

Yes. Erection is not a hormonally mediated process. Dogs that have been castrated can become erect and may even show mounting and thrusting behavior. They cannot, however, get bitches pregnant unless they were very, very recently castrated.


Selections from the book: “The Dog Breeder’s Guide to Successful Breeding and Health Management”, 2006.