The regulation of testicular descent and the effects of cryptorchidism.

The first half of this review examines the boundary between endocrinology and embryonic development, with the aim of highlighting the way hormones and signaling systems regulate the complex morphological changes to enable the intra-abdominal fetal testes to reach the scrotum. The genitoinguinal ligament, or gubernaculum, first enlarges to hold the testis near the groin, and then it develops limb-bud-like properties and migrates across the pubic region to reach the scrotum. Recent advances show key roles for insulin-like hormone 3 in the first step, with androgen and the genitofemoral nerve involved in the second step. The mammary line may also be involved in initiating the migration. The key events in early postnatal germ cell development are then reviewed because there is mounting evidence for this to be crucial in preventing infertility and malignancy later in life. We review the recent advances in what is known about the etiology of cryptorchidism and summarize the syndromes where a specific molecular cause has been found. Finally, we cover the recent literature on timing of surgery, the issues around acquired cryptorchidism, and the limited role of hormone therapy. We conclude with some observations about the differences between animal models and baby boys with cryptorchidism.