Wikipedia has a great article on Cryptorchidism. It's a clinical read, and gives the basics.
Cryptorchidism literally means "hidden testicle". It exists when one or both testicles are absent from the scrotum.
In most cases of cryptorchidism, the descent of the testicle into the scrotum has been arrested and can usually be found somewhere along the inguinal canal, a passage that exists between the abdominal cavity and the scrotum. The canal usually closes after the testicle descends into the scrotum in fetal life. When that process fails and the canal remains open, the condition is called an inguinal hernia.
The old adage about a boy's "balls dropping" at puberty is without basis in reality. Almost all boys have descended testicles at birth.*
The very few that don't are usually unilaterally cryptorchid, or one testicle is not visible. A much smaller group is bilaterally cryptorchid, where both testicles are hidden. The more premature a birth is, the more likely a baby will be born with bilateral cryptorchidism, since testicular descent occurs late in pregnancy.
So what is the upshot of this?
It is important that both testicles be visible and palpable (able to be felt) in the scrotum, first because undescended testicles are rarely viable if undescended past roughly two years of age. Secondly, an undescended testicle has an elevated risk of becoming cancerous, and testicles in the scrotum can be monitored for changes during testicular self examination.
The surgical procedure to relocate an undescended testicle to the scrotum is Orchidopexy. In cases where one or both testicles is absent, the condition is called Anorchia. Anorchia can be unilateral or bilateral. A person with one testicle is said to be Monorchid.
In most cases of Anorchia, the single testicle will be larger than a typical testicle, due to compensatory hypertrophy. The sole testicle enlarges to perform the functions that normally two testicles do. This physiologic response is an accurate diagnostic sign of unilateral anorchia in up to 95% of patients who are imaged or explored surgically to confirm an absent testicle.
Cancer concerns and risks.
A patient who has undergone Orchidopexy has an elevated risk of testicular cancer. A patient who has had an undescended unviable testicle removed, (Orchiectomy) remains at elevated risk of cancer in the normally located testicle. In other words, for any person with a history of cryptorchidism, testicular self examination is of increased importance than for men in the general population
"I had an undescended testicle removed and I'm very self conscious about it. What can I do?"
A prosthetic testicle can be implanted to restore the appearance of your genitals. Implants on the market now closely mimic the feel of a natural testicle, and are not hard like a bowling ball any more. You may wish to delay this procedure if you are an adolescent, until your natural testicle reaches full size. The only way to adjust your prosthetic testicle is to exchange it for a larger one, as your testicle grows.
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*This may have come about because a boy's scrotum seems to go from a tight ball of 'stuff' to a slacker dangly thing containing whatever testicles he owns at the onset of puberty. The scrotum, thus the balls, seem to descend. This has nothing whatsoever to do with cryptorchidism, though ~ Webmaster.