Abstract:This work focused on the histological study of the ontogeny of lymphoid organs by paraffin tissue sections and HE staining techniques in Eleutheronema tetradactylum, from 1 to 60 dph (day post hatching). The results showed that in the conditions of salinity 9.0±0.5 and water temperature (28±2)℃, thymus primordium appears at 3 dph that composed of 4~6 layers of undifferentiated stem cells and lymphocyte-like cells; thymus differentiated rapidly and mainly filled with lymphocytes; at 25 dph, the cortex and medulla are clearly distinguished and thymus development was basically completed. Similarly, the head kidney primordium formed at 3 dph, it consists of pronephric tubules and a few of hematopoietic stem cells; at 5 dph, the head kidney began to lymphatize when the thymic lymphocytes migrate there, and hematopoietic stem cells differentiated into different types of cells as the fish grew; at 18 dph, pronephric tubules began to degenerate and disappeared completely until 53 dph, when the head kidney is mainly composed of lymphopoietic tissue supported by reticular endothelial system. At 7 dph, the spleen primordium appears and begins to lymphatize obviously until 16 dph. The endothelial system of spleen is more developed than that of head kidney, but its development speed is significantly slower than thymus and head kidney, and lymphocytes are less than in the thymus and head kidney. These observations suggested that the respective lymphoid organs primordium becomes the thymus, head kidney and spleen successively. The structure and function of the lymphatic organs in E. tetradactylum have not been fully developed and maybe one of the main causes of high mortality during the metamorphosis period. This study is of great significance for optimizing breeding conditions, improving juvenile cultivation and healthy aquaculture technology.
