Fish White Blood Cells
White blood cells are less abundant (20,000 to 150,000
per mm3) than RBCs in fish blood and function in a variety of ways in ridding the body of foreign material (including invading pathogens),along with providing a mechanism for blood clotting. Measurements of change in total WBC number or in the percentages of the various types can often lead to a better understanding of the physiological or pathological state of the animal. Circulating WBC number can also vary through the year in some fish species. Bridges et al. (1976) describe pattern of total and differential (separate) WBC counts in winter flounder which generally shows an inverse relationship with fish condition or health. Sick individuals would presumably make more WBCs to synthesize antibodies, .phagocytes bacteria, etc. A thorough review of fish leukocytes has been prepared by Ellis (1977). Sample photo micro-graphs of winter flounder blood cells. The often several types of leukocytes found in fish blood, and roles have been attached to their presence. The principle ‘WBCs are lymphocytes, Thrombocytes.
Lymphocytes can vary in size (4.5 pm to 12 pm in diameter) among species. Their morphology, on the other hand, is more consistent. They are dominated by the nucleus, with only a narrow rim of basophilic cytoplasm in which there are a few mitochondria and ribosomes. The number of lymphocytes varies among species (and with counting technique!).Ellis (1977) found that 12 X 103 lymphocytes per mm3 was the representative value for plaice. However, the number of lymphocytes can vary with season, following general WBC seasonal trends. Teleostean lymphocytes appear to be produced by both thymus and kidney, although specific antigen responses may differ between cells produced by each organ (Ellis 1977).
The primary function of fish lymphocytes seems to be to act as the executive cell of specific immune mechanisms by means of anti- die production. Klotz (1972) reported a large increase in cells
small lymphocytes in the kidney of rainbow trout, which correlated with high antibody production, 2 to 3 days after antigen injection. Hardeman (1970) and Hogarth (1973) have described the
accumulation of cells resembling small lymphocytes at the graft points’ rejected tissue transplants in fish. There is also some evidence that fish lymphocytes may demonstrate phagocyte activity (engulfing reign cells) or give rise to cells (e.g., macrophages) which have this ability.
Thrombocytes of fishes may appear as spiked, spindle, oval and lone nucleus forms in stained blood smears, prepared on microscope-slides. The various Thrombocytes shapes may well be different forms of
e same cell, as changes in shape can be observed in live preparations. From his work on plaice, Ellis (1977) believes that thrombocytesoriginate in splendid tissue. Wardle (1971) has shown the function of1 Thrombocytes in place to be the clotting of circulating fluids. Clots are produced by spreading of the thrombolytic cytoplasm into long’ threads which cross-link the denuded nuclei, forming a fibrous network. Which traps circulating corpuscles?
Monocytes comprise a small proportion of the WBC population, unless foreign substances are present in the tissues or bloodstream. Thought4 o originate in the kidney, Monocytes (termed “macrophages” by some workers) concentrate at and phagocytize foreign particles. Therefore, the cell outline may be quite irregular from pseudo-do formation.
Granulocytes are formed in the kidney and the spleen to a lesser tent in teleports (Ellis 1977). Fanged (1968) determined the organ to be the site of granulocyte formation in elasmobranchs.ranulocytic function is still open to question. Neutrophilis apparently: grate to sites of bacterial infection, where they may be phagocyticinn. Europhilia (i.e., increased numbers osculating neutrophils) often corresponds to “stress” in rainbow trout.
Basophiles have been reported in goldfish, Australian lungfisheoceratodus .forester), carp (except an Israeli strain), and Pacific on, but absent in the blood of Anguilla eels, plaice, yellow perchlerca flavescens), brown and rainbow trout’s, and cyclostomes (1977). Ellis (1977) also reported much confusion and contradiction in the fish hematological literature as to the presence or absence ofeosinophils in fishes. Although still unclear, basophile and eosinophilfunctions seem related to antigen sensitivity, stress phenomena, andphagocytosis (eosinophils) (Ellis 1977).
Because of the presence of nuclei in both erythrocytes and leukocytes of fishes, electronic methods routinely used in clinical measurements of red and white cell number cannot he used for fish blood (Blackball 1972). The Neubauer-typehemocytometric method of blood dilution and counting in ruled areas in a microscope field is in common use. WBC abundance has been estimated by the height of the white cell layer (Buffy coat) which settles between the red cell pack and the plasma when capillary tubes (e.g., for hematocrit determinations) are centrifuged. This Buffy coat height or “leucocrit” has usefulness as a rough diagnostic tool in fish disease work. Microscopic examination is used to determine numbers of the various types of leukocytes.