Testicular Descent

A discussion of cryptorchidism in young stallions is fraught with controversy. Everyone agrees on what constitutes cryptorchidism, but that sometimes is as far as agreement goes. We know that the condition involves the retention of one or both testicles in a stallion, either in the abdominal cavity or the inguinal canal. Just exactly why the descent of the testis or testes into the scrotum fails to occur is not definitely known, athough there are several theories.

The real controversy begins when the subject of treatment is brought into the conversation.

There are individuals, for instance, who feel a discussion on treatment protocols is unwarranted because the condition is hereditary. The only recourse, they would maintain, is castration.

Others will argue that it has not been proven scientifically that cryptorchidism is inherited. Unless the testis is retained within the abdominal cavity, they would likely advocate the use of hormonal therapy to stimulate descent of the testis.

The two hormones that have been used with some success are gonadotropin releasing hormone (GnRH) and human chorionic gonadotropin (HCG). A discussion on how they work and what the success rates have been would be inappropriate without first understanding what occurs when testes descend or fail to descend.

Testicular Descent

The testes are the male gonads that produce both spermatozoa and the predominant male sex hormone, testosterone. In addition, they produce a hormone that is more often considered relevant to the female reproductive system—estrogen. The testes are walnut-shaped structures. The testes of mature stallions range greatly in size and weight. On the average, however, an adult testis might measure 80 to 140 millimeters in length by 50 to 80 millimeters in width and weigh about 225 grams.

During the first 18 to 24 months after birth, the testes are idle as far as sperm production is concerned. Starting at approximately 18 months of age, the testes grow and develop rapidly, and several months later, they gradually begin to produce sperm.

The timetable for sexual development for individual stallions varies, as many horse owners have learned when allowing a yearling stallion to run with open mares.

Before this development can occur normally, the testes must descend from the abdomen. Nature has declared that both testes wind up in the scrotum, an outpouching of the skin that is composed of two scrotal sacs, one for each of the testes. The scrotum is comprised of four layers. The outermost layer, of course, is the skin. Beneath it is a layer of smooth muscle. The third layer is loose connective tissue that allows the testis great mobility within the scrotal sac. The innermost layer is the parietal vaginal tunic, a membranous sac that extends from the abdominal cavity through the inguinal canal.

The inguinal canal is the passageway between the abdomen and the vaginal cavity that surrounds the spermatic cord in the adult male. At the upper and lower ends of the inguinal canal are the inguinal rings, which form the upper and lower openings of the inguinal canal.

In the normal colt, both testes should descend through the inguinal canal and into the scrotum between 30 days before birth and 10 days after birth. When this doesn’t occur on that timetable, the horse owner is faced with the fact that he or she owns a cryptorchid.

A succinct description of what happens when the testes descend into the scrotum is provided by Bill Pickett, PhD, Colorado State University, who was the lead author of the book Management of the Stallion for Maximum Reproductive Efficiency.

Pickett gives this description:

"By Day 40 of gestation, the testis is suspended from the abdominal wall and the mesonephric duct, which later gives rise to the epididymis and deferent duct, (which) leads into the pelvic area. A narrow evagination, termed the vaginal process, starts to form about Day 43 of gestation and progressively develops to form the inguinal canal.

"Around Day 150, the developing tail of the epididymis is drawn to or just within the inguinal ring, but the testis is very large and cannot enter the inguinal canal.

"Entrance of the testis into the inguinal canal typically begins between 270 and 300 days of gestation. This occurs only after the vaginal process and internal inguinal ring have been stretched sufficiently by the enlarging tail of the epididymis to allow entrance of the testis that has diminished in size. Pressure from the fluid in the abdominal cavity, and possibly from the intestines, forces the testis down through the inguinal canal."

For some reason, the right testicle advances before the left.

Pickett reported that in 32 fetuses between nine months of gestation and birth, descent of the right testis was further advanced than the left in 78% of the fetuses. The left testis was more advanced in only 3% of the fetuses.

Pickett also reported that of 12 fetuses collected at term, 42% had completely descended testes; 25% had both testes within the inguinal canals and equally distant between internal and external inguinal rings; 17% had one testis in the scrotum and one in the inguinal canal, and 17% had both testes within the abdominal cavity.

In another bit of research involving nine colts less than one week old, it was found that five of the nine had both testes completely descended.

Why do the testes not descend in some cases?

Pickett tells us that researchers have come up with at least four basic theories:

1. Insufficient abdominal pressure to expand the vaginal process properly.

2. Stretching of the gubernacular cord. (The gubernacular cord is the fetal ligament attaching the epididymis to the scrotum.)

3. Insufficient growth of the gubernaculum and tail of the epididymis so that they are unable to expand the inguinal ring sufficiently to allow entrance of the testis.

4. Displacement of the testis to a position where the pressure of the intestines prevents gubernacular tension from pulling the testis into the vaginal process.

The four above possible causes generally are attributed to one or more of the testes that are suspended somewhere along the inguinal canal. It is with cryptorchids of this type that hormonal therapy sometimes can be successful. Hormonal therapy would be useless where one or both of the testes are retained within the abdominal cavity because the inguinal ring would have closed.

Abdominal retention apparently occurs, Pickett reports, when the testis fails to enter the inguinal canal before closure of the internal (upper) inguinal ring during the first two weeks after birth.

Is He A Gelding, Or…?

When a colt is born on the farm and is monitored from birth, there is little doubt in the diagnosis of cryptorchidism. Either both testicles have descended into the scrotum or they have not.

It can be more difficult when a colt is purchased and has supposedly been castrated. If the "gelding" exhibits stallion-like behavior, the suspicion will arise that he is a cryptorchid. In cases where the testis is suspended in the inguinal canal, palpation of the scrotal area might be all that is needed to provide a definitive diagnosis. If the testis has been retained near the inner inguinal ring in the abdomen, rectal palpation and ultrasound sometimes can provide a diagnosis, although they by no means provide a surefire approach.

However, if the testis has been retained elsewhere within the abdominal cavity, none of the above techniques will work. In those cases, administration of the hormones used for therapy is used for diagnosis.

The two blood tests that have become traditional in determining cryptorchidism have been based on the age of the horse. In horses three years of age and older, a single plasma sample for estrone sulfate can be evaluated. Horses lacking testicular tissue normally will have estrone sulfate concentrations that are less than 100 picograms per milliliter. Cryptorchids, on the other hand, would have concentrations of estrone sulfate in excess of 400 picograms per milliliter.

The common hormonal assessment for horses younger than three years of age involves administration of human chorionic gonadotropin (HCG) and measurement of the resutling rise in testosterone. The procedure involves first obtaining a base reading prior to administration of HCG. Then, 6,000 international units of HCG are administered. Between 30 and 120 minutes after the injection, blood samples are drawn and analyzed. Horses that have been gelded basically do not respond to HCG, while cryptorchids will show testosterone concentrations at about 100 picograms per milligram.

(HCG is a hormone that has come into strong favor in helping to stimulate ovulation in mares. The hormone can be used to reduce the length of estrus in cycling mares. It also provides a means of synchronizing ovulation with a stallion’s breeding schedule. Ovulation occurs 24 to 48 hours after the administration of HCG, provided, of course, that the follicle is at the correct stage of maturity. Generally speaking, a follicle of 35 millimeters in diameter that is a developing or dominant follicle will respond.)

At the 1998 AAEP gathering in Baltimore, G. F. Carneiro, DVM, University of California, Davis, reported on a study that adds another dimension to diagnosis. Carneiro presented a report outlining success for diagnosing cryptorchidism in stallions of all ages by using an enzyme immunoassay for the measurement of estrone sulfate in stallions. Used in the research was the enzyme-linked immunosorbent assay (ELISA) method.

Involved in the research were 48 known cryptorchid horses, 49 geldings, and six stallions ranging in age from one to 10 years.

The estrone sulfate levels were found to be significantly different in cryptorchids and stallions when compared to those for geldings.

"The evidence presented here in our experiment," Carneiro reported, "suggests that a single-sample plasma level of estrone sulfate, as performed by an ELISA, was reliable, less time consuming, and equally accurate for the detection of the presence of functional testicular tissue in cryptorchids as the paired-sample HCG stimulation test in horses of all ages. Since the stallion testis produces high amounts of estrogen, it seems reasonable to diagnose the presence of testicular tissue by estrone sulfate measurement alone. In this study, the ELISA was able to detect estrone sulfate in donkeys and horses of all ages. To our knowledge, this is the first report comparing estrone sulfate levels in geldings, stallions, and cryptorchids by an enzyme-linked immunosorbent assay."

Thus, we learn, the diagnostic approach has become even more sophisticated.

Then What?

Now comes the tough question. What do we do when we have determined that the animal is a cryptorchid?

The answer is relatively simple if the testis has been retained in the abdominal cavity. The only way to remove it in that case is through surgery.

However, if the testis is located somewhere along the inguinal canal, other options are available. As mentioned earlier, one school of thought takes the position that the condition is genetic and that it is questionable logic to use a cryptorchid stallion for siring foals because the condition will be passed on to another generation of males.

Before going further on that point, it should be noted that stallions with one descended testis can be fertile. However, if both testes are retained in the abdominal cavity, the stallion cannot be fertile because the higher temperature within the abdomen will prevent spermatogenesis—the development of sperm. The testis or testes retained in the abdominal cavity still can produce testosterone, which, in turn, can result in stallion-like behavior.

That being said, we still are faced with the question of whether to treat the cryptorchid in an effort to get a testis retained in the inguinal canal to descend, or simply to perform surgery that will remove both testes.

Juan Samper, DVM, MSc, PhD, Diplomate ACT, of Abbotsford, British Columbia, is a specialist in the field of reproduction who is in private practice and who was one of the presenters at the Stallion Reproduction Symposium staged by the Society for Theriogenology, held in conjunction with the AAEP meeting in Baltimore. He reports that, while the consensus among many in the equine community is that cryptorchidism is hereditary, it has not been definitely proven to be so.

In one of his reports, Samper had this to say:

"Although it has been speculated that cryptorchidism is an hereditary condition, none of 56 colts sired by a cryptorchid Quarter Horse had abnormalities in their testicular descent. If the condition is heritable, perhaps it is controlled by a gene with very low penetrance or, as has been shown in men, associated with several autosomal genes."

Is the condition prevalent? The answer is somewhat ambiguous because the condition seems to vary breed by breed. It is reported to be most prevalent in Percherons and least prevalent in Thoroughbreds, Samper says.

With Standardbreds, it can run as high as 10% of all males.

Does the very fact that it shows up rather consistently in some breeds and rarely in others lend weight to the argument that heredity is involved?

In some cases, when valuable horses are involved, the owner might not opt for castration and seek to solve the problem with hormonal therapy.

Steve Conboy, DVM, of Lexington, Ky., a private practitioner who deals with many Standardbreds, has devised a protocol for treatment that has been successful.

When young stallions reach about 12 months of age and a testis has not descended, Conboy will administer 2,500 international units of HCG twice a week for four to six weeks.

He reports that over an 11-year period on a Standardbred farm where he handles the veterinary duties, 876 colts were born. Of that number, 73, or 8%, were cryptorchids. Nineteen of the 73 colts were treated with HCG and 10 of the 19 developed palpable testicles for a 58% success ratio. The incidence of success relates to cryptorchids of all types, he says, as no effort was made among the group treated to determine whether the problem involved abdominal retention or whether the testis was suspended in the inguinal canal.

Of the 54 colts not treated with HCG, only three, or 6%, developed palpable testicles.

Conboy continues to use HCG on cryptorchids with inguinal suspension at the above rate of administration and reports that the success ratio runs as high as 60-70%.

How does it work? Conboy believes that administration of HCG might have a beneficial effect on the cremaster muscle of the spermatic cord and the vaginal tunic, a covering of the testis, which can suspend the retained testicle within the inguinal canal, but neither he nor anyone else seems to know for sure exactly what occurs.

Conboy will be the first to say that reports of success in the wake of HCG injections are primarily anecdotal as there have been no studies to date to determine the effect of HCG administration in a group of cryptorchids as compared to a control group.

There definitely are more questions than answers concerning hormonal therapy for cryptorchids. It is known that injections of HCG and GnRH appear to stimulate the production of testosterone, but that is about all anyone will say for certain. What exactly does this testosterone accomplish? Does it cause the retained testis to increase in size and weight, thus providing additional pressure for descent? Does it cause the cremaster muscle to stretch and relax so that the testis can descend? Does it cause the inguinal canal itself to relax and enlarge, facilitating the descent? The questions go on, but definitive answers are few.

Dickson Varner, DVM, MS, Diplomate ACT, of Texas A&M University, says that administration of hormonal therapy does sometimes have the effect of increasing the size of the retained testis, but that it generally will never, after descending as the result of therapy, develop to the size of the testis that descended normally.

We now turn to GnRH therapy for cryptorchids. Again, there are anecdotal reports of success, but the protocol has never been submitted to a controlled study.

Advocating the GnRH form of therapy is BET Reproduction Laboratories in Lexington, Ky.

A word about GnRH. It is a hormone that is secreted by the hypothalamus gland. It travels from the hypothalamus to the anterior pituitary gland. There it stimulates the secretion of gonadotropins—follicle stimulating hormone (FSH) and luteinizing hormone (LH). These gonadotropins travel to the testes, where FSH exerts its effects on the Sertoli’s cells and LH on the Leydig’s cells.

The Leydig’s cells produce both testosterone and estrogen. The prime job of the Sertoli’s cells is to support spermatozoa development. Sertoli’s cells also secrete a number of proteins that function in the regulation of FSH release from the anterior pituitary gland and proteins that bind testosterone.

With that explanation in mind, it becomes obvious that administering GnRH would cause an increase in the horse’s testosterone level.

The recommended approach by BET is to administer 500 micrograms of GnRH twice daily. If there is no testicular change after 31/2 weeks, it is unlikely that the treatment will be successful. If, however, there is testicular change, the treatment is continued until the testis descends into the scrotum. Once there, treatment often is continued for a time to stimulate the testis to increase further in size.

The laboratory reports that if the testis is palpable before treatment, the success ratio has been running between 60-70%.

So, we come back to square one. Should the horse owner make use of hormonal therapy when faced with a cryptorchid? Only the owner can make that decision. Conboy is hopeful that ongoing research at the University of Kentucky aimed at developing a gene map for the horse will be able to help answer the genetic questions. Until there is a definitive answer, it remains a personal decision.

The good news for those who decide to treat the condition is that hormonal therapy can be helpful in many instances.

About the Author

Les Sellnow

Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.

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