Since the thymus in fishes is the organ in which hematopoietic stem cells differentiate into lymphocytes, it is believed that thymus is an important gland in biophylaxis.In the present study, we investigated the existing region, the shape, and histological structure of the thymus and the change of thymus volume in Japanese smelt, Hypomesus nipponensis, in order to clarify the correlation between the gland development and growth. In addition, the influence of water temperature on the thymus of Japanese smelt was examined. The thymus of Japanese smelt is a paired organ, rod-formed or pyriform, located on the rear of the dorsolateral region in the gill chamber. The histological structure of the thymus, without the inner and outer zones, consists of the thymus parenchyma surrounded by an epithelial cell layer and is filled with lymphocytes. The thymus of Japanese smelt was observed 15 days after the fish were hatched and the thymus volume increased with growth until September when the maximum volume was reached. The thymus volume differed considerably when Japanese smelt was reared at high water temperature (20℃) and low water temperature (17.3℃). In the case of fish sampled in September, these reared at low water temperature showed 2.3times increase in the organ volume as compared to those reared at high water temperature. Histological changes were observed in fishes in which thymus volume decreased in October. The changes were thymus involution as hyperplasia of epithelial-like cells and formation of cystic space in the parenchyma, and they were prominent when the fishes were reared in the high water temperature. These results suggested that the thymus volume and the tissue structure were influenced by water temperature.

魚類の胸腺は造血幹細胞がリンパ球へ分化する場であり, 生体防御において重要な器官であると考えられている. 本研究は, ワカサギの胸腺の存在位置, 形状および組織学的構造を明らかにすると共にワカサギの成長に伴う胸腺体積の推移を調べた. また, ワカサギの胸腺に対して飼育水温が及ぼす影響について検討した. ワカサギの胸腺は左右一対で鰓腔の背側後部に位置し, その形状は棒状または洋梨形を呈していた. 胸腺の組織学的構造は, 上皮細胞に包まれた胸腺実質はリンパ球様細胞で満たされ, 内域と外域の境界は不明瞭であった. ワカサギの胸腺は孵化後15日目頃に確認され, 成長と共にその体積は増加し9月で最大となった. それ以後胸腺は退縮し始め, 産卵期の3月の個体ではほとんど消失していた. この胸腺の退縮には性成熟の関与が示唆された. ワカサギを高水温 (20℃), 通常の飼育水温 (低水温 : 平均水温17.3℃)で飼育したとき, 9月に採取した低水温飼育の個体では胸腺体積は高水温飼育の個体の2.3倍となり大きな差が認められた. 胸腺体積が減少した10月の個体では, 胸腺実質内に上皮性細胞の増生や囊胞の形成などの胸腺退縮を示す組織変化が確認された, これらの組織学的変化は高水温飼育の個体で顕著であった.