The Male Dog’s Reproductive System

Anatomy and physiology of the male dog’s reproductive system.

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by Randy Kidd, DVM, PhD

The reproductive system has what seems to be a relatively simple task: to provide the mechanisms necessary to maintain survival of the species. However, the mechanisms involved are complex, and maintaining a functional balance of the hormonal, mechanical, and physiological requirements for reproductive success can be difficult at best. What’s more, while dogs (and for that matter, all animals) are hard-wired to maintain reproductive viability at all costs, very small alterations in the balance of any one of the functions involved with reproduction can produce profound results throughout the body.

This month, we’ll examine the male reproductive anatomy and physiology, and deal with the female next month.

Anatomy

The scrotum is the pouch of lightly haired skin that contains the paired testes and their attached epididymides. The testes are the functional units that generate the production of sperm; they also contain cells that produce a variety of hormones.

The reproductive “plumbing” begins with the seminiferous tubules, which coil throughout the interstitial tissues of the testes. Each seminiferous tubule empties into an epididymis, which, in turn, empties into a vas deferens, which in turn opens into the urethra. The primary function of the testes is to produce the male gametes or sperma-tozoa; this process is termed spermatogenesis and takes from 55 to 70 days in the dog.

The exact timing of testicular descent into the scrotum has not been established, likely due to the small size of neonates, the softness of immature testes, and the tendency of the cremaster muscle to hold immature gonads in the inguinal region.

In one study of Beagles and mixed-breed dogs, the testicles were in their final scrotal position at seven weeks of age. In another study (of German Short-Haired Pointers) there was a correlation between the time of passage of testes through the inguinal canal and completion of deciduous dentition – they both occurred at about 30 to 35 days of age. Unilateral or bilateral cryptorchidism results if one or both testes remain within the abdomen after closure of the inguinal canal.

The penis develops from the same embryologic tissue as does the female’s clitoris, and the testes are derived from the embryologic tissue that produces the female ovaries. Development into male or female depends on the presence of the Y chromosome, which fosters the production of testosterone, thus promoting the development of the male sexual characteristics. In the male canine embryo, the mesenchymal tissue inside the glans penis ossifies to form a bony os penis.

The penis of the adult dog is composed of the proximal body and the distal glans penis, which includes the bulbus glandis and the pars longa glandis. The bulbus glandis is a cavernous expansion of the corpus cavernosum urethrae. It surrounds the proximal portion of the os penis, and when it fills with blood during erection, it expands into a bulb-like structure that is held within the female’s vagina during copulation.

Disorders of the male canine reproductive system
There are several disorders of the canine male reproductive tract that deserve mention. They can be categorized into congenital defects, acquired disorders, neoplasias, and functional disorders.

Congenital defects of the penis are relatively rare, but include the following:

• Penile hypoplasia (incomplete or underdevelopment), usually due to aberrations of the XY chromosomes.

• Hypospadia, a defect that results in an abnormal location of the urethral orifice.

• Persistent penile frenulum. Normal separation of the glans penis from preputial epithelial cells occurs before birth. If it doesn’t, a tag of skin that restricts movement may cause the penis to deviate.

• Congenital preputial stenosis (a narrowing of a duct or canal) often occurs with concurrent phimosis (entrapment of the penis within the prepuce).

Cryptorchidism is a congenital condition in which the testes fail to descend into the scrotum. The condition may be bilateral or unilateral (75 percent of the cases are unilateral). It is considered the most common reproductive disorder in dogs, affecting between 1 to 15 percent of dogs, and it is inherited as a sex-limited autosomal recessive trait. Since cryptorchid testes have a much greater risk (6- to 13-fold) for developing Sertoli cell tumors compared to normal testes, and since the trait is inherited, bilateral castration is recommended, even in cases of unilateral cryptorchidism.

Acquired disorders include:

• Injury/contusion to either the penis or testicles, perhaps as a result of fighting or failure to jump cleanly over a fence.

• Fracture of the os penis, again from trauma or as a result of a breeding accident.

• Balanoposthitis, inflammation of the glans penis and prepuce, generally from a bacterial infection.

• Paraphimosis, a failure of the glans penis to retract normally into the prepuce.

• Inflammation or infection of the testes is termed orchitis; epididymitis or inflammation of the epididymis may also occur. Either of these can be caused by any number of bacteria.

Of particular interest is brucellosis, an infection that causes resorption of the fetuses early in gestation or sudden abortion during the last trimester of pregnancy. Caused by the Brucella canis organism (or occasionally by B. abortus, B. suis, or B. melitensis), it is a highly contagious disease that can spread rapidly through a kennel by contact with infected fetuses, vaginal discharge, or occasionally by venereal means. Infected male dogs may develop generalized swellings of the lymph nodes, and they frequently show signs of a painful orchitis, epididymitis, or prostatitis.

Neoplasias of the penis run the gamut of common types of tumors seen at other body sites, and, except for transmissible venereal tumors (TVTs), they usually occur in older dogs, beginning at around 10 years of age or older. TVTs generally occur in younger dogs since they are transmitted at coitus. TVTs grow, invade, and metastasize most widely in an immune-compromised individual, so immune-supportive therapy is indicated as a part of any holistic protocol for treating them.

Tumors of the canine testes are the second most common neoplasm affecting the male dog, representing about 5 to 15 percent of diagnosed tumors. Tumors may develop in any of the tissue types that occur in the testes, but the most common tumors involve the germ cells (seminomas) and tumors of the Leydig and Sertoli cells. Dogs with undescended testicles (cryptorchid) are much more likely to develop testicular tumors, specifically Sertoli cell tumors and seminomas, than are normal dogs.

Testicular tumors may be asymptomatic; many occur as a difficult-to-palpate, discrete mass within the testis. Most do not grow rapidly nor metastasize to other body sites. Some (slightly more than half of all) Sertoli cell tumors result in a feminization syndrome as a result of the production of estrogenic steroids by the tumor tissue. The feminization syndrome may cause significant dermatologic changes such as hair loss without itching and hyperpigmentation; behavior changes may also be noted. Seminomas may be associated with other clinical problems such as prostate disease, alopecia, perineal hernia, and other tumors (especially perianal gland adenomas).

Prostatic tumors include adenomas, which are generally benign, and adenocarcinomas, which can be quite difficult to treat since they tend to metastasize to other tissues rapidly and because complete surgical excision is rarely possible.

Tumors are treated via Western medicine by the usual means: surgical excision, and/or some form of chemo- or radiation-therapy. Alternative therapies for tumors of any type include homeopathy or acupuncture; nutritional supplements and herbal remedies may be included to support the primary therapy of choice.

Functional disorders
Failure to achieve an erection is a fairly common functional disorder of the male dog. Reasons for this condition include:

• The female is not at the receptive stage of heat (this is the most common cause).

• Pain (from prostatitis, for example, or arthritis or other conditions of the hips, legs, or lower back), which prevents mounting of the female.

• Fear/anxiety about the female or the breeding environment.

• Drugs that have anti-androgen activity – for example, ketaconazole, a commonly used drug for treating fungal infections.

• Priapism, which is a persistent erection in the absence of sexual stimulus, is occasionally seen in dogs with spinal lesions. Rarely, it occurs due to thrombo-embolism of penile vasculature. This condition differs from the frequent erections seen in young excitable small breed dogs, which respond to behavioral modification, castration, and/or progestin therapy.

Prostate problems
The prostate, an accessory sex gland in the male dog, is located just caudal to the bladder in the area of the bladder neck and proximal urethra. It encircles the urethra and has several ducts that enter the urethra throughout its circumference. It produces prostatic fluid, which acts as a transport and support medium for sperm during ejaculation.

The prostate increases in weight with age until it reaches its normal size; it then stabilizes through adulthood until it begins to grow again in aged animals. However, if the dog is castrated before sexual maturity, normal growth of the prostate is completely inhibited. If the dog is castrated as an adult, the gland will decrease in size to about 20 percent of its normal adult size.

Some practitioners refer to age-related prostatic weight increase as benign prostatic hyperplasia (BPH), a condition commonly seen in aged humans. In contrast to human males, however, the uncastrated dog’s prostatic enlargement usually does not cause dysuria (difficulty in urination) due to urethral obstruction; the dog’s prostrate tends to enlarge outward, away from the urethra. Treatment may be required only if the enlarged prostate causes abnormal signs such as dysuria or straining when defecating.

Other diseases of the prostate are fairly common, especially in the older dog, and may lead to fertility problems. In the aging dog, fluid cysts may develop in association with age-related hyperplasia.

Urinary incontinence
True incontinence (involuntary urine leakage) must be differentiated from behavioral urinary issues such as lack of housetraining, submissive urination, territorial marking, or senile loss of housetraining due to canine cognitive dysfunction.

There are several causes of incontinence, including excessive water consumption; infection of the urinary tract or bladder stones (either of which can cause irritation and stimulate the “need to pee” feeling); spinal cord irritation; weak bladder sphincter; and several diseases, including diabetes mellitus and diabetes insipidus, Cushings syndrome, and kidney failure.

Western medicine treatments have included hormone therapy, alpha-adrenergic agonists, anticholinergics, and surgery. All of these except the anticholinergics work by attempting to restore neuromuscular control and tone to the bladder sphincter. The anticholinergics work by relaxing the bladder’s muscle fibers, thus facilitating urine storage.

All of these treatments (with the excep-tion of surgery, which has not been very successful without additional medications) have proven to be fairly effective. The problem is, all the medications listed above may cause severe side effects, and some have been removed from the market. Both phenylpropanolamine, the popular alpha-adrenergic agent, and diethylstilbestrol (an estrogen), are now available from special pharmacy compounding outlets.

My choice for treating urinary incontinence is to first of all perform a chiropractic evaluation and adjustment if indicated. Many patients respond to this initial treatment, and periodic adjustments seem to keep their spinal cord fit enough to allow for urine retention. If indicated, I also add acupuncture treatments for their ability to help enhance the balance of hormones throughout the body, as well as for their ability to treat spinal or other inflammatory conditions that might exist.

There are also several herbal remedies that have been especially developed to help with treatment of this condition, and in my mind the idea of treating all castrated individuals with herbs that contain steroidal precursors warrants further study.

In any event, only after I’ve tried all the alternatives am I tempted to resort to Western medicine drugs.

Fertility evaluation
There are several ways to evaluate the functional capacity or breeding soundness of the male dog, including observation of sexual behavior (libido), general health, soundness of rear quarters, spermatogenic capacity, and functional evaluations of other, associated organ systems.

Daily spermatozoa production is highly correlated with testicular weight and this latter highly correlated with scrotal width. Scrotal widths are measured with calipers.

The ejaculate should be evaluated for sperm count, motility, and morphology. Frequency of ejaculate does not affect daily sperm production, but it depletes the sperm reserves contained in the epididymis. Thus, when high sperm concentrations are desired (for example, when sperm are being collected for preservation), the number of sperm is maximized by having the dog ejaculate only at four- to five-day intervals.

Healthy animals have a high percentage of sperm that are actively and rapidly swimming in a freshly collected sample. Sperm morphology is evaluated by staining the cells and observing them under a microscope. Stained slides may also reveal the abnormal presence of high numbers of bacteria, white blood cells, or red blood cells. At least 75 percent of the sperm cells should be morphologically normal.

Sometimes, fertility evaluations include an examination of the bacteria that resides in the prepuce and distal penile urethra of the male. However, even perfectly healthy and fertile dogs have a normal population of bacteria in those locations. There are often at least three or four different species of bacteria found.

On the other hand, some studies indicate that a high bacterial count, especially if there are high numbers of gram negative bacteria along with large numbers of white blood cells, indicates infection. If total numbers of bacteria are high, rule out infection as a potential cause of infertility.

Outside factors
Poor thyroid function is known to adversely affect libido and breeding soundness in animals, and other organ systems will likely be shown in the future to have intimate connections to the reproductive system. A complete breeding soundness exam will certainly include an evaluation of thyroid function, and an evaluation of other organ systems may also be indicated.

Recently, there has been much to-do about estrogenic factors in the environment – airborne “sexual toxins” such as pesticides, herbicides, and manufacturing by-products of plastic production. This news is worth following.

Some final breeding hints:

• Be certain the dog has reached puberty.

• The dog’s libido and efficiency will increase with age and experience – at least, until the onset of old age problems.

• Be certain that the female is truly in standing heat. To be sure, use a combination of hormonal, cytological, endoscopic, and behavioral evaluations for the female.

• Wherever possible, stay out of the way and let nature take its course. The second most common cause of breeding failure is well-meaning folks who feel the need to interfere and thus disrupt the “ambiance” necessary for good reproductive contact.

• Realize that sometimes the mating was simply not meant to be. Sometimes, for whatever the reason, the couple may not be attracted to each other, and they may never be able to “hit it off.”

Alternative treatments
I think acupuncture and chiropractic are essential when treating at least two conditions of the reproductive system: incontinence and breeding soundness. I’ve had very good results in some (but certainly not all) cases of urinary incontinence using chiropractic adjustments alone or in combination with acupuncture. The typical animal who will benefit from chiropractic adjustments will have a palpable subluxation somewhere between T-12 and L-3.

Acupuncture and chiropractic have also cured many an animal from “infertility” that was caused by pain in their hindquarters. Acupuncture has been shown to enhance sperm production, libido, and overall vitality.

Herbal remedies offer a good alternative way to treat bacterial infections. Antibiotic herbs such as goldenseal (Hydrastis canadensis) and Oregon grape root (Berberis aquifolium), and immune-stimulating herbs such as echinacea (Echinacea spp.) apply here.

In addition, I think it is healthy to support male hormone production after an animal is castrated, even though I realize this is controversial. The major controversy seems to be that the phyto-hormones are not actually hormones, but rather, precursors to hormones. Those stuck on a biochemical paradigm say that only preordained amounts of the biochemical will be effective.

I happen to like the herbal remedies because they are precursors of the steroidal hormones; I like the idea that the animal’s body can select the precursor it needs along with the amount it needs. To my way of thinking, this way of supplying a hormone has the best chance for being applied in a healthy manner by the dog’s own systems, and it has the least chance to interfere with the intricate feedback methods already in place in the body.

Some plants that provide anabolic steroidal precursors include wild yam (Dioscorea villosa), sarsaparilla (Smilax officinalis), licorice root (Glycyrrhiza glabra), damiana (Turnera aphrodisiaca), and saw palmetto (Serenoa serrulata). Check with an herbalist experienced with using herbs for treating animals for proper dosages and delivery methods.

Also With This Article
Click here to view “To Neuter or Not to Neuter Your Dog, That Is the Question”
Click here to view “New Reasons to Spay, Neuter”

-Dr. Randy Kidd earned his DVM degree from Ohio State University and his PhD in Pathology/Clinical Pathology from Kansas State University. A past president of the American Holistic Veterinary Medical Association, he’s author of “Dr. Kidd’s Guide to Herbal Dog Care” and “Dr. Kidd’s Guide to Herbal Cat Care”.

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