Expression and localization of aquaporin 1b during oocyte development in the Japanese eel (Anguilla japonica)

To elucidate the molecular mechanisms underling hydration during oocyte maturation, we characterized the structure of Japanese eel (Anguilla japonica) novel-water selective aquaporin 1 (AQP1b) that thought to be involved in oocyte hydration. The aqp1b cDNA encodes a 263 amino acid protein that includes the six potential transmembrane domains and two Asn-Pro-Ala motifs. Reverse transcription-polymerase chain reaction showed transcription of Japanese eel aqp1b in ovary and testis but not in the other tissues. In situ hybridization studies with the eel aqp1b cRNA probe revealed intense eel aqp1b signal in the oocytes at the perinucleolus stage and the signals became faint during the process of oocyte development. Light microscopic immunocytochemical analysis of ovary revealed that the Japanese eel AQP1b was expressed in the cytoplasm around the yolk globules which were located in the peripheral region of oocytes during the primary yolk globule stage; thereafter, the immunoreactivity was observed throughout the cytoplasm of oocyte as vitellogenesis progressed. The immunoreactivity became localized around the large membrane-limited yolk masses which were formed by the fusion of yolk globules during the oocyte maturation phase. These results together indicate that AQP1b, which is synthesized in the oocyte during the process of oocyte growth, is essential for mediating water uptake into eel oocytes

ニホンウナギの人為催熟における卵質の季節変化

Feminized 2-year-old eels were reared in freshwater at 26℃ until October, then the water temperature was gradually decreased to 16℃ to December, and gradually increased to 26℃ from February to April. They received weekly injections of salmon pituitary extract (body weight, 15 or 30 mg/kg) to induce sexual maturation in September, December, or April. The quality of the eggs obtained in each season was evaluated by the resulting fertilization, hatching, and 8-day survival rates. High quality eggs were obtained via high-dose injections in most experimental groups. In the high-dose injection groups, eels were induced to the final maturation phase in all seasons; however, egg quality obtained in April was extremely low in contrast to that obtained in September and December. Histological observation of oocytes just before the process of artificial maturation was commenced indicated that the developmental stages of oocytes varied by season: oil droplet in September, early vitellogenic in December, and regressive from vitellogenesis in April. The extremely low egg quality in April is probably due to the fact that the oocytes had already entered the regressive stage. These results indicated that better quality of eel eggs can be obtained by starting the process of artificial maturation between September and December.雌化ウナギ2 歳魚を9 月，12月または4 月にサケ脳下垂体注射によって人為催熟し，得られる卵質の差を受精率，孵化率および8 日後生存率で評価した。9 月，12月は比較的良質な卵が得られる傾向が認められたが，4 月では卵は得られるものの，その卵質は極めて悪かった。催熟前の卵母細胞を組織学的に観察すると，12月に卵黄形成開始が観察されが，4 月には退行している様子が観察された。4 月に催熟を開始しても良質な卵が得られない原因として，催熟前に既に卵母細胞が退行しつつあることが考えられた。以上の結果から，雌化ウナギ2 歳魚の人為催熟は9 月から12月の間にかけて開始することが良質な卵を得るために適当であると結論された