Embryology and physiology of testicular development and descent.

Sexual differentiation starts with the development of bipotential gonads that further differentiate into testes or ovaries. The fetal testis secretes hormones that guide the differentiation of internal and external sex organs, whereas the fetal ovary remains rather inactive hormonally. Defects in gonadal differentiation or hormone secretion and action result in disorders of sex development (DSD). Testicular descent is a continuum that has often been described to occur in two main phases: the transabdominal phase and the inguinoscrotal phase. The first phase is according to animal studies dependent on Leydig cell-derived insulin-like peptide 3 (INSL3) that induces male-like development of the gubernaculum. This phase is rarely disrupted in man. The inguinoscrotal phase is dependent on androgens, also secreted by Leydig cells.