The role of insulin 3, testosterone, Müllerian inhibiting substance and relaxin in rat gubernacular growth

Transabdominal testicular descent is influenced by various anatomical and hormonal factors and is mediated by gubernacular enlargement and regression of the cranial suspensory ligament, but its mechanism remains controversial. The aim of this study was to determine which hormones have a direct effect on the proliferation of cells in the day 17 fetal rat gubernaculum in vitro, using an organ culture system. The effects of synthetic rat insulin 3 (INSL3), inactive INSL3, dihydrotestosterone (DHT), DHT+INSL3, human Müllerian inhibiting substance (hMIS), hMIS+INSL3 and human gene 2 relaxin were tested, together with co-culture with fetal rat testis. Cell proliferation was assessed using a bromodeoxyuridine labelling index. The results showed that MIS and relaxin have a mild effect on gubernacular growth, whilst INSL3 and DHT have a more marked effect. The combination of MIS+INSL3 showed an effect close to that of co-culture with testis. However, the most pronounced effect was caused by DHT+INSL3. RT-PCR analysis indicated that the fetal rat gubernaculum strongly expresses putative INSL3 receptors, weakly expresses MIS type II receptors and does not express relaxin receptors. In conclusion, a number of different hormones directly influence growth of the gubernaculum in vitro, including the recently reported hormone INSL3. INSL3 shows a direct stimulatory effect on the swelling reaction, while DHT and MIS may have roles in augmenting this growth.