Effect of 17 alpha-methyltestosterone on the skin and gonads of freshwater male silver eels.

Male silver eels kept in fresh water (FW) were injected with 17 alpha-methyltestosterone (MT). They received 5, 10, 15, or 20 injections (8 micrograms/g every other day) or 6 or 9 injections (4 or 2.5 micrograms/g once a week) and were killed 24 hr after the last injection. Compared to solvent-injected controls, the treated eels appeared darker, the eye diameter was enlarged, the head and the pectoral and caudal fins were black and the melanophore index was maximal. The thickness of the skin (epidermis plus dermis) was significantly increased. Macroscopically, the organs of Syrski (juvenile testes) were slightly larger and less translucent, but the maximal gonosomatic index reached only 1-1.2 in two fishes. Stimulation started in the anterior part of the gonad. Microscopically, all stages of spermatogenesis and spermiogenesis were observed. After 20 injections (high dose, 40 days) or 6 (one case only) and 9 injections (low dose, 57 days), spermatids and spermatozoa were present. These positive results are compared to the negative data previously reported in other countries. The mode of action of the androgen is discussed. It is suggested that it acts for the most part directly on the gonad, but a participation of the pituitary cannot be ruled out, although gonadotrophs appeared only slightly stimulated.