The development of the lymphoid organs of flounder, Paralichthys olivaceus, from hatching to 13 months

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Abstract

The growth of the lymphoid organs, such as head kidney, spleen and thymus were studied in flounder, Paralichthys olivaceus Temminck & Schlegel, from hatching to 13 months of age. Except for the thymus, all organs grew as the fish grew. By 2 months of age the lymphoid organs attained their maximum relative weight. The organ weight showed a closer correlation to body weight than they did to age. The total number of leucocytes in the lymphoid organs increased with age, but the number per milligram of lymphoid organ remained constant. A micro and ultrastructural study of the lymphoid organs showed that the full development of the lymphoid organs was not achieved until the juvenile stage. The spleen and head kidney had mixed populations of “red” and “white” cells. The head kidney was more lymphoid than the spleen. The thymus involuted quickly during the first 6 months. The blood components had no obvious relationship with age or season during the period studied.

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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    Citation Excerpt :

    In teleosts the spleen and head-kidney, which develop during the early stage post-hatching, and thymus, appearing at later stage of larval development, are the major lymphoid organs (Dijkstra, J. M et al., 2006; Liu, Y et al., 2004; Maddon, P. J et al., 1987; Suresh, N., and Ranganathan, L. S., 2008; Workenhe, S. T et al., 2010). Although thymus is one of the important lymphoid organs in fish, it gradually disappears until it can no longer be observed at around 7 months (Liu, Y et al., 2004). Hence, we compared the distribution of CD4-1-positive lymphocytes in gill, liver, spleen, head-kidney, trunk-kidney and intestine, but not thymus, and the tissue-specific expression of CD4-1-positive population was consistent with the observations seen in other teleost species (Bernard, D et al., 2006; Buonocore et al., 2008; Laing, Kerry J et al., 2006; Patel, S et al., 2009), mice and humans (Fournier-Betz, V et al., 2000).

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