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The descent of the testicles consists of the opening of a connection from the testis to its final location at the anterior abdominal wall, followed by the development of the gubernaculum, which subsequently pulls and translocates the testicle down into the developing scrotum. Ultimately, the passageway closes behind the testicle.
Cryptorchidism, or "undescended testicles", is when the testicle does not descend into the scrotum of an infant boy. [30] Testicular enlargement is an unspecific sign of various testicular diseases, and can be defined as a testicular size of more than 5 cm × 3 cm (short axis). [42]
It is the most common birth defect of the male genital tract. [1] About 3% of full-term and 30% of premature infant boys are born with at least one undescended testis. [2] However, about 80% of cryptorchid testes descend by the first year of life (the majority within three months), making the true incidence of cryptorchidism around 1% overall.
The main male sex organs are the penis and the scrotum, which contains the testicles that produce semen and sperm, which, as part of sexual intercourse, fertilize an ovum in the female's body; the fertilized ovum develops into a fetus, which is later born as an infant. The corresponding system in females is the female reproductive system.
The upper part of the gubernaculum degenerates. The lower part persists as the gubernaculum testis ("scrotal ligament"). This ligament secures the testis to the most inferior portion of the scrotum, tethering it in place and limiting the degree to which the testis can move within the scrotum.
In the male, the paramesonephric ducts atrophy (but traces of their anterior ends are represented by the appendix of testis of the male), while their terminal fused portions form the prostatic utricle in the floor of the prostatic urethra. This is due to the production of anti-Müllerian hormone by the Sertoli cells of the testes.
Mammals are the only animals in which the testes descend from their point of origin into a scrotum.Concurrently, mammals are the only class of vertebrates to evolve a prostate gland starting with prostate evolution in monotreme mammals.
In a ridgling, either the testicles fail to descend, or they are trapped behind the external inguinal ring and cannot emerge. Usually both testicles in a horse have descended by the age of 16 months, [5] but as many as 15% of all two- and three-year-old male horses may have a retained testicle. [6]