Ultrastructure of haemopoietic organs of the Argentinean silverside (Odontesthes bonariensis)

VIGLIANO, F.A.; BERMÚDEZ, R.; NIETO, J.M.; QUIROGA, M.I.

Rosario, Argentina

Congreso; 10th Inter American Congress of Electron Microscopy and 1st Congress of the Argentine Society of Microscopy; 2009

Inter-American Committee of Societies for Electron Microscopy

In teleost fish, the main haemopoietic organs are the head kidney and the spleen. However, their ability to produce each blood cells lineage may vary between species. In addition, in the absence of lymph nodes and lymphatic vessels, the kidney and spleen are the principal sites for removal of particulate antigens from blood circulation in fish. Therefore, the aim of this study was to describe the ultrastructure of the head kidney and spleen of the Argentinean silverside (O. bonariensis) by means of an electron microscopy study in order to determine their vascular structure and the involvement of these organs in the haemopoiesis. Samples of head kidney and spleen were dissected out, cut in small pieces, and fixed in 2.5% glutaraldehyde in 0.1 M cacodylate buffer (pH 7.3), postfixed in 1% osmium tetroxide, and dehydrated in ethanol solutions. For scanning electron microscopy (SEM) study, samples were subjected to critical point drying with CO2, shadowed with gold, and observed with a JEOL JSM-6360LV. For transmission electron microscopy (TEM) study, samples were embedded in Epon 812. Thin sections were counterstained with uranyl acetate and lead citrate, and examined and photographed under a JEOL JEM-1011. The head kidney of O. bonariensis was composed of nephrons and intermingled haemopoietic cells at different maturation stages. The erythrocytic lineage consisted of erythroblast, proerythrocyte and erythrocyte. Erythroblasts were round and have a high nucleus/cytoplasm (N/C) ratio. Proerythrocytes were slightly larger than erythroblasts and some mitochondria, and free ribosomes were seen in their cytoplasm. Erythrocytes were oval with a heterochromatic nucleus and had the lowest N/C ratio of the series. In the lymphocytic lineage, lymphoblasts and lymphocytes were identified. The former were the smallest blast cell and had a lower N/C ratio than lymphocytes. The granulocytic series was composed of granuloblasts, progranulocytes and granulocytes type II. The main features of granuloblasts were a euchromatic and centrally located rounded nucleus, and a cytoplasm with free ribosomes and some cisternae of rER. Granulocytes were larger than the other cells of the lineage, with a euchromatic and pleomorfic nucleus, a large amount of round electron-dense granules within the cytoplasm and the lowest N/C ratio of the series. Haemopoietic stem cells and a meshwork of reticular cells were also seen. The most remarkable vascular structures of head kidney were the renal sinusoids which were often associated with different leukocytes. Although all the haemopoietic cells described above were also observed in the spleen, in this organ predominated the mature cells of each lineage associated with a mesh of reticular fibres. Plasmocytes and their precursor, proplasmocytes, were seen mainly distributed around blood vessels. According to their granule morphology, three types of granulocytes were distinguished in the spleen. Inactive macrophages were seen scattered through the parenchyma, while active macrophages with phagocytosed cells in their cytoplasm were observed associated with vascular structures, mainly ellipsoids and sinusoids. Thrombocytes were also observed. Among the circulatory structures of the spleen, the ellipsoids and sinusoids were the most outstanding. The former were composed by an outer layer of reticular cells and cuboid endothelial cells that delimit a narrow diameter. The splenic sinusoids were large vascular spaces limited by outer reticular cells. Within the sinusoids, inner reticular cells with long cytoplasmic projections interposed in flow circulation were seen. In this study, the presence of distinctive blast cells indicates that erythropoiesis, lymphopoiesis and granulopoiesis take place in both organs although mainly in head kidney of the Argentinean silverside. The vascular structure of these organs and their association with inmune-related cells suggest that they could play an important role in removing particulate antigens from blood.