Fertilizing capacity of the cryptorchid rat.

Rats were surgically made bilaterally cryptorchid and after 4-8 days the testes were returned to the scrotum. After 70 days fertility was tested by pairing with females. Fertility was restored in 5/6 rats with testes cryptorchid for 4 or 5 days, but only 2/9 were fertile when the duration of cryptorchidism was 6-8 days. The sterility was due to irreversible degeneration of the spermatogonial stem cells. The testes of infertile males were smaller and lighter than those of fertile males and the seminiferous tubule diameters were reduced.