In the fetus (dogs, cats, humans, etc), a structure called the gubernaculum connects the testicle (located next to the kidney during development) to the scrotum. If this structure fails to develop properly, the testicle will not end up in the scrotum, but will remain in the abdomen or the inguinal canal (which lies next to the prepuce in the dog). About 4% of mixed breed dogs and 9% of purebred dogs (particularly toy and miniature breeds) will be cryptorchid. Cats have a lower incidence–about 1% of male cats will be cryptorchid. It’s considered to be an X-linked, autosomal-recessive trait.
Cryptorchidism can be one-sided (unilateral) or bilateral. In unilateral cryptorchidism the right testicle is retained twice as often as the left. Bilaterally cryptorchid animals are usually sterile because the higher body temperature inside the abdomen is enough to prevent sperm production. (The animals will, however, still exhibit male behaviors.)
If an animal is cryptorchid, he should not be used for breeding. Dogs with cryptorchid testicles are prone to testicular torsion and testicular cancer, so these dogs should be neutered to prevent problems later. So when Pixel was 8 months old, had all his permanent teeth, and still only the left testicle in his scrotum, he came in to be neutered.
The descended testicle is generally removed first. An incision is made just above the scrotum and the testicle, spermatic cord, etc is exteriorized, tied off with suture material, and removed. Then the skin is closed.
The retained testicle was not found in the inguinal canal area, so an abdominal exploratory was done. This is similar to spaying a female dog–you need to find the retained testicle, which is often around the area of the uterus in a female. Once the testicle is found, it is tied off and removed like the other one.
It is a little hard to see on this picture, but the retained testicle is always smaller than the one that dropped into the scrotum.
Because I thought I could feel the retained testicle in the inguinal ring, I made an incision there. No luck, so then I had to make the incision into the abdomen to find it.
This is Pixel recovering after his surgery.