Developmental changes in pituitary-thyroid axis, and formation of gonads in leptocephali and glass eels of Anguilla spp.

Pituitary, thyroid gland and gonads in leptocephali of Japanese eel Anguill a japonica (19.8-32.6 mm in total length), A. obscura (45.0 mm), and A. bic olor pacifica (49.5 mm) and those in glass eels of the Japanese eel were hi stologically and immunohistochemically examined in order to observe the dev elopmental changes of these endocrine organs in the Anguillidae. The pituit ary, consisting of adenohypophysis and neurohypophysis in Japanese eel lept ocephali over 22.5 mm, did not contain thyroid stimulating hormone (TSH) im munoreactive cells. Such cells were, however, detectable in the more develo ped pituitaries of leptocephali of A. obscura and A. bicolor pacifica and i n those of glass eels. Conversely, thyroxine (T-4)-immunoreactive thyroid f ollicles could be detected in all specimens, both leptocephalic and glass e el. Only in glass eels, gonads were found in the body cavity, and these gon ads harbored one or two primordial germ cells (PGC) per cross-section. Our results indicate that thyroid hormones (TH) production started prior to TSH production, and that TSH and TH are both secreted during the metamorphosis from leptocephalus to glass eel. Therefore, it is plausible that the TSH-T H axis is involved in the metamorphosis from leptocephalus to glass eel, bu t not in the early growth from preleptocephalus to leptocephalus.