The thymus development with growth of Japanese smelt, Hypomesus nipponensis and the influence of water temperature on the gland volume
2005
Hasegawa, D.(Tokai Univ., Shizuoka (Japan). School of Marine Science and Technology) | Ono, S. | Hara, H.
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 deg C) and low water temperature (17.3 deg C). In the case of fish sampled in September, these reared at low water temperature showed 2.3 times 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.
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