The Sertoli cells themselves mediate parts of spermatogenesis through hormone production. They are capable of producing the hormones estradiol and inhibin. The Leydig cells are also capable of producing estradiol in addition to their main product testosterone. Estrogen has been found to be essential for spermatogenesis in animals.   However, a man with estrogen insensitivity syndrome (a defective ERα ) was found produce sperm with a normal sperm count , albeit abnormally low sperm viability ; whether he was sterile or not is unclear.  Levels of estrogen that are too high can be detrimental to spermatogenesis due to suppression of gonadotropin secretion and by extension intratesticular testosterone production.  Prolactin also appears to be important for spermatogenesis. 
One final (and interesting) cause of azoospermia is anabolic steroid abuse. Some men with azoospermia may have used testosterone or other steroids as part of their strength and conditioning training. High doses of these steroids can suppress sperm production. Sperm production can be reinitiated in such patients by stopping the steroids and starting gonadotropin therapy (analogous to ovulation induction therapy in women). Although clomiphene citrate has been used to improve sperm quality in men, most studies reveal it to have little to no benefit.
The Mesterolone hormone is not estrogenic. It does not aromatize and it carries no progestin nature. As a result, the side effects of Proviron will not include any related effects such as gynecomastia or excess water retention. Such adverse effects are impossible with this steroid. This will also greatly reduce the risk of high blood pressure as high blood pressure associated with anabolic steroid use is often due to extreme water retention. In fact, Proviron should provide an anti-estrogenic effect by preventing testosterone to estrogen conversion or at least tremendously slow it down.