The Goitrogen 6-n-Propyl-2-Thiouracil (PTU) Given during Testis Development Increases Sertoli and Germ Cell Numbers per Cyst in Fish: The Tilapia (Oreochromis niloticus) Model

The main objectives of the present study were to investigate the effects of 6-n-propyl-2-thiouracil (PTU) on Sertoli cell proliferation, germ cell number, and testis size in Nile tilapias (Oreochromis niloticus). In this regard, young fish (approximately 1 g BW and approximately 3.5 cm total in length) were treated for a period of 40 d with different concentrations (100 and 150 ppm) of PTU. The animals were killed and analyzed on d 1, 30, 40, 98, and 208 after the beginning of the treatment. On d 30 and 40 the spermatogenic process was delayed in fish treated with PTU compared with the control group. Also at these periods, treated tilapia had decreased (P0.05) body weight and total length. On d 98 body weight and total length had recovered in PTU-treated fish and were similar (P0.05) to those of the controls. However, testis weight and gonadosomatic index (testis mass/body weight) were approximately 100% higher (P0.05) in treated tilapia. Similarly, the area occupied by seminiferous tubules, the number of Sertoli cells and germ cells per cyst, and the number of Leydig cells per testis were significantly (P0.05) greater in treated fish. Nevertheless, nuclear volume and individual Leydig cell volume were significantly lower (P0.05) in tilapia receiving PTU treatment. Compared with controls, at 208 d all parameters analyzed presented the same trend as that observed at 98 d. In general, at 98 d the different PTU concentrations used during the treatment period induced similar effects. However, at 208 d the mean values observed for several parameters were significantly higher (P0.05) in fish exposed to 150 ppm. Probably due to the higher density of Sertoli cells per cyst in treated tilapia, these cells presented a smaller (P0.05) nucleolus and a trend to decrease its support capacity (efficiency). However, the meiotic index (germ cell loss during the two meiotic divisions) was similar (P0.05) in the three groups of fish investigated. Remarkably, the results found in tilapia were similar to those found for rats treated with PTU. This suggests strongly that the mechanisms of control of Sertoli cell and Leydig cell proliferation seem to be preserved during vertebrate evolution.