Abstract
The testis develops in the abdominal cavity and descends into the scrotum. Although numerous theories have been proposed, the mechanism of descent and the reason for its inhibition remain unknown. Furthermore, none of the explanations account for the other occurrences related to the descent, such as failed obliteration of the processus vaginalis, or the reasons for the decrease in fertility and increase in the risk of malignancy associated with an undescended testis. The gubernaculum is a primitive mesenchymal tissue that was first described in 1786. However, the role of the gubernaculum in the descent process remains obscure. The testis descends through the processus vaginalis. Although the processus vaginalis (PV) is usually defined as a simple peritoneal protrusion, it actively develops into the gubernaculum. The gubernaculum gives rise to the smooth muscles that surround the processus vaginalis. The striated cremaster muscle (CM) is also derived from the gubernaculum. Because the testis descends through the processus vaginalis, the muscles develop to propel the testis. After propelling the testis, the smooth muscle (SM) undergoes programmed cell death. The initiation of programmed cell death through the intrinsic pathway requires activation of phospholipase C. A transient shift in the autonomic balance via a decrease in the sympathetic tonus and an increase in the parasympathetic tonus is essential for initiating this programmed cell death. Programmed cell death in the SM is the physiological pathway for the obliteration of the processus vaginalis. Differences in the timing, intensity, or duration of this physiological pathway result in pathological conditions. A shift before testicular descent diminishes the SM content that is required to propel the testis, and thus inhibits descent. The early shift persists throughout childhood and results in the decrease in fertility and increase in the risk of malignancy because of the differences in signal transduction. Despite a successful descent, persistence of the shift alters the contractility of the CM by increasing the cytosolic calcium levels. Contracted CMs retracts or even ascends the testis. Inadequate intensity or duration of the shift of autonomic tonus causes failure of the programmed cell death. Persistence of the SM hinders the obliteration of PV and gives rise to hydroceles or inguinal hernias depending on the amount of residual smooth muscles. Similar findings from different countries support these explanations. Thus, our proposed mechanism satisfactorily explains the process of descent while considering all the factors related to the process of testicular descent.