Efferent duct differentiation during female-to-male sex change in honeycomb grouper Epinephelus merra

Efferent duct (ED) differentiation was examined histologically during female-to-male sex change in the honeycomb grouper Epinephelus merra. During natural sex change, ED differentiation began with the appearance of slit-like structures between the stromal tissue and the tunica ovary and small oval-shaped spaces within the wall of the ovarian cavity, accompanied by oocyte degeneration and the initiation of spermatogenesis, i.e. the early transitional phase. In the late transitional phase, ED structure formation expanded and further ED differentiation occurred, including the rapid multiplication of spermatogonial germ cells. In sex-changed males, the slit-like structures increased in size, fused with each other and finally formed a well-developed ED. The oval-shaped spaces also increased in size and fused to form an ED. In contrast, during artificial sex change, induced by aromatase inhibitor (AI, 1 mg kg−1), ED differentiation in E. merra was first observed as the appearance of slit-like structures and small oval-shaped spaces in the restructuring gonads in the third week after AI treatment. These were accompanied by oocyte degeneration and the proliferation of gonial germ cells into spermatogonia. In the fifth week, the rapid multiplication of spermatogonial germ cells, increases in 11-ketotestosterone (11-KT), and further differentiation of EDs were observed. Sex-changed males had testes containing sperm in the completely differentiated EDs; the significantly highest levels of 11-KT were observed in the sixth week. Simultaneous increases in 11-KT and initiation of ED differentiation were observed, suggesting a role of 11-KT in ED differentiation during sex change. There were no basic differences in the mechanisms of natural and artificially induced ED differentiation. Two types of structure led to the formation of EDs in two different areas of the newly formed testis during sex change.