Health Matters: Oh, No! Just One!
Debra Eldredge, DVM
Depending on your breed, that may be a familiar cry of woe. Cryptorchidism is one of those quiet defects lingering just under the surface of many breeds. Certainly there are other more devastating defects that interfere with a dog enjoying life even as a pet, such as hip dysplasia or epilepsy, but the lack of two descended testicles can destroy your hopes for a stunning male dog in the breed ring or for use at stud.
So how does this happen? It helps to understand the development of a normal male. The kidneys and the testicles develop very closely together in the canine embryo. In fact, an intermediate stage of kidney development, the mesonephron, regresses to become the testicles. Both the kidneys and the testicles are technically outside of the abdominal cavity, and are “retroperitoneal,” or behind the peritoneum, the membrane that lines the abdomen.That fact becomes important later during descent into the scrotum.
Since the testicles develop way up by the kidneys, that means they have a long way to travel to reach the scrotal sac. The right kidney is slightly more cranial, or towards the head, in location, which means the right testicle is also slightly more cranial. In fact it is felt that the right testicle is more often the one retained, or left inside the body, due to the longer journey it has to make to descend correctly. Once descended into the scrotum, the left testicle tends to be located slightly higher and behind the right one.
The testicles are pulled down into the scrotal sac by connective tissue-type ligaments called the gubernaculums. This cord regresses towards the scrotal sac, pulling the testicle along with it. Each testicle travels independently on its own side. Eventually the gubernaculums will exist only as a scar that fixes the testicle into its side of the scrotal sac. This action seems to be under the influence of testosterone, although simply giving testosterone injections will not help a wayward testicle.
The scrotal sac itself is continuous with the abdominal cavity, so when the testicles enter the scrotum through the inguinal canal (an opening in the muscle that allows the testicles to leave the main body cavity and enter the scrotum) they push the abdominal membranes with them. This can lead to inguinal hernias if the inguinal canals do not close by 6 months of age or if they are quite large to begin with. In these cases, intestines slip into the opening along with the testicles or in place of them.
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