The expression of nuclear and membrane estrogen receptors in the European eel throughout spermatogenesis

Stimulation of sexual maturation by weekly injections of human chorionic gonadotropin (hCG) induced a progressive decrease in E2 plasma levels, and different patterns of expression of ESRs and GPERs in the BPG axis. The expression of nuclear ESRs increased in some parts of the brain, suggesting a possible upregulation due to a local production of E2. In the testis, the highest expression levels of the nuclear ESRs were observed at the beginning of spermatogenesis, possibly mediating the role of E2 as spermatogonia renewal factor, followed by a sharply decrease in the expression of ESRs. Conversely, there was a marked increase observed in the expression of both membrane GPERs throughout spermatogenesis, suggesting they play a major role in the final stages of spermatogenesis.