The development of the lymphoid organs of flounder, Paralichthys olivaceus, from hatching to 13 months
Introduction
The lymphoid organs in fish include thymus, spleen and head kidney. There are several early reports on the growth of the thymus in fish [1], [2] and others have reported on the growth of the spleen and head kidney [3], [4], [5]. There is a detailed report on the growth of the lymphoid organs of the rainbow trout, over a period from 1 to 15 months of age [6]. Histological changes in the lymphoid system of fish with respect to age, seasonal and endocrine changes have been reported [7] as well as seasonal changes in the lymphoid tissues and humoral immune response in Sebasticus marmoratus [8]. Studies have also been done on the larval stages of sea bass [9].
In recent years, with the development of new culture methods, many fish have been produced in China. Flounder, Paralichthys olivaceus Temminck & Schlegel, is a main cultured marine species in north China. This paper presents basic data on the growth of the lymphoid organs of the flounder over a period from hatching to 13 months of age.
Section snippets
Materials and methods
Fish were obtained at monthly intervals from the Huaxin Fish Farm, Weihai City, Shandong Province, for the 13-month period from March 2001 to June 2002. The fish were from the same batch from the hatchery, and the fish selected were always the biggest and healthiest fish in order to minimize any variation.
From hatching until 21 days of age, 20 fish were sampled every 2 days. Thereafter until 2 months of age sampling was conducted at 5-day intervals. The smaller samples were fixed whole in
Kidney
The kidney was already present when hatching. It is composed of straight tubules running along the notochord. As the fish aged, a glomus appeared in the anterior part of the kidney and the tubule in the posterior part folded. The mass of haemopoietic tissue increased dramatically as the fish aged, especially in the anterior part of the kidney. The kidney was a symmetrical organ by 30 days post-hatching, though the fish had undergone metamorphosis. By 2 months, the pronephric kidney had many big
Discussion
The kidney is a paired excretory organ, but the foremost part of the kidney in teleosts always lacks excretory tissue and is called head kidney or pronephric kidney, which always serves as a secondary lymphoid organ [10]. In this study the excretory cells first appeared at the foremost part of the kidney and with development, many haemopoietic cells concentrated in the cranial regions and formed the head kidney.
It has been questioned if lymphoid stem cells become seeded in the thymus. Because
Acknowledgements
The authors want to thank Jiang M., Bai S.L., Zhang F.L., Qu H.B., Wang L.J., Qu H.X. and Sun Q.J. for technical assistance and Huaxin Haida Marine Biology Company of Weihai for providing the experimental fish. This work was funded by the National Natural Science of China (Grant No. 30000129) and Ministry of Science and Technology of China (G1999012005).
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