Adaptation of fish lymphomyeloid organs to polar water

Lymphomyeloid organs of three common Antarctic fish species, Trematomus bernacchii, Trematomus nicolai and Chionodraco hamatus, were analysed. Contrary to species living in temperate sea water, the thymus of polar fishes were flattened, incompletely lobated and scarcely distinguishable by normal histology into cortical and medullary regions. Functional regionalisation, however, was suggested by differences in the sizes of thymocytes from the outer to the inner thymus zone. Another particularity was observed in the thymus of Trematomus species: next to lymphocytes, numerous erythroid cells circulated and differentiated in the parenchyma. Only two main types of epithelial cells could be found by cytological analysis: (i) limiting cells that surround the haematopoietic tissue and (ii) reticular cells that constitute the frame where the lymphoid and erythroid cells can proliferate and differentiate. The reticular cells could not be distinguished in cortical and medullary subtypes as observed in temperate-water fish. Numerous Hassall’s corpuscles, probably with a scavenging role, were also observed in the thymus. The head kidney housed haematopoietic tissue, lacked any excretory tubules, and had a huge blood supply, characteristic of polar fish species. It appeared mainly lymphopoietic in C. hamatus but contemporary erythropoietic and lymphopoietic in Trematomus species. The ultrastructural analysis revealed the presence of both reticular and limiting epithelial cells. Reticular epithelial cells (REC) characteristically showed numerous vesicles with a granular content and cell debris. Numerous lymphoblasts, lymphocytes and plasma cells were observed among the REC. Erythropoiesis occurred in all polar species analysed, but in C. hamatus the erythroblasts did not differentiate because they had a fast senescence. The spleen appeared mainly erythropoietic, with scarcely developed areas of white pulp, in Trematomus species; the erythropoiesis was scarcely evident in C. hamatus. Small vascular ellipsoids showed numerous melano-macrophages in Trematomus, while large haematopoietic areas were organised around the capillaries in C. hamatus. Utrastructural analysis revealed, in all species examined, two main types of epithelial cells: reticular, close to the ellipsoids, and limiting-subcapsular, which surround the organ. A large blood supply and extended capillary frame were also observed in polar species. The possible adaptation of lymphoid organs to the low temperatures of polar water is discussed.