VPP 322: Aquatic Animal Diseases, Health Care and Management (1+1)

LEUCOCYTES

• Leucocytes refer to non-pigmented, nucleated blood cells whose primary function is to combat infection and in some cases to phagocytose and digest debris. They differ from erythrocytes, in that they leave the vascular system to carry out their tasks by passing through the walls of small blood vessels. Phagocytosis is the intracellular uptake of solid particles by cells, either for nutritional purposes (e.g. food), or in the case of phagocytes such as macrophages and granulocytes as a defense mechanism (foreign bodies).
• Leucocytes or white blood cells can be categorized into four types namely
  • Granulocytes,
  • Lymphocytes
  • Monocytes
  • Thrombocytes
• These cells have their own specific features and it is therefore difficult to provide a detailed description of leucocyte features without referring to each cell type individually.
• In fish, the overall number of leucocytes varies in number; for example, the normal range of lymphocytes, the predominant type of leucocyte, in the salmonid, rainbow trout (Oncorhynchus mykiss), is between 7.8 and 20.9 x 10³ cells per mm³ .
• White blood cells account for a small proportion of all circulating blood cells, Eg. In seabass (Dicentrarchus labrax) it is 3.5% of blood cells.
• In higher vertebrates such as mammals, formation of white blood cells is restricted to bone marrow, the spleen and the lymph nodes. In fishes, organs such as the kidney, spleen and thymus take part in haematopoeisis or formation of all blood types. White blood cells account for a small proportion of all circulating blood cells
• Spleen is the organ comprising a mass of lymphoid tissue in the mesentery; unlike lymph nodes it is interposed in the blood circulation.
• Thymus is a paired lymphoid gland situated dorso-laterally in the gill chamber. The site of T-lymphocyte production, it is regulated by hormones produced by thymic epithelial cells. In fish (and in all vertebrates) the thymus gradually atrophies after the onset of sexual maturity but does not completely disappear