Development and differentiation of gonadotropin hormone-releasing hormone neuronal systems and testes in the Japanese eel (Anguilla japonica)

In this study we investigated the relationship between the development of the olfactory, preoptic, and midbrain gonadotropin-releasing hormone (GnRH) neuronal systems and testicular differentiation in eels (Anguilla japonica) from embryonic stages through adulthood (5.4-50 cm body length). GnRH-synthesizing neuronal populations were first observed in the youngest fish (~5.0 cm) at the rostrobasal and caudalmost olfactory bulbs immunoreactive to a 'promiscuous' (nonspecific) GnRH antiserum (635.5), and in the preoptic area and midbrain tegmentum immunoreactive to chicken GnRH II antiserum. The eel brains lacked salmon and seabream GnRH immunoreactivity. The evidence from our study suggests that the olfactory, preoptic, and midbrain GnRH populations have origins independent from those of proliferative periventricular zones within the brain. However, the olfactory GnRH neurons could have migrated out of the olfactory placodes during ages earlier than those observed in this study. Although all three GnRH neuronal populations contribute to pituitary innervation to some degree, the preoptic GnRH innervation was pronounced in the pituitary when primordial germ cells (animals ~5.0 cm) differentiated into male germ cells (animals 14-16 cm) and, therefore, an association can be assumed between preoptic GnRH expression and testicular differentiation in the Japanese eel.