Journal of The School of Marine Science and Technology,Vol.3 No.1
The thymus development with growth of Japanese smelt, Hypomesus nipponensis
and the influence of water temperature on the gland volume
Daisuke HASEGAWA, Shin-ichi ONO and Hideo HARA
Abstract
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゜C)
and low water temperature(17.3゜C).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.