Morphological and cytochemical characterization of the peripheral blood cells in Paralichthys olivaceus

The flounder (Paralichthys olivaceus) is a highly nutritious, cultured bony fish with a high economic value. The health of the fish is closely related to its blood cells, which are critical for oxygen transport, natural defense, and immunity. The microstructures, classification, counting and size of peripheral blood cells in P. olivaceus were observed and measured by Wright-Giemsa staining, and the cytochemical characteristics of peripheral blood cells were investigated by different cytochemical staining methods including periodic acid-Schiff (PAS), Sudan black B (SBB), acid phosphatase (ACP), alkaline phosphatase (ALP), peroxidase (POX), and α-naphthol acetate esterase (NAE). Besides, the ultrastructures of different cells were detected by the transmission electron microscope. The results showed that erythrocytes, thrombocytes, lymphocytes, monocytes, and neutrophils constituted the peripheral blood cells in P. olivaceus. More heterochromatins were found in erythrocytes, thrombocytes and neutrophils; however, more organelles with fewer heterochromatins were found in monocytes. Endoplasmic reticulums and phagocytic vesicles were abundant in neutrophils. Lymphocytes were the most abundant in leukocytes, followed by monocytes and neutrophils. The cytochemical staining results showed that all leukocytes were positive for SBB. Most of the lymphocytes were positive for PAS, and monocytes were positive for PAS, ACP, and POX. As for neutrophils, ACP and POX were positive. Both monocytes and neutrophils showed positive for SBB, ACP and POX, indicating that the two kinds of cells play a vital role in phagocytosis and bactericidal action. Only lymphocytes were positive for ALP, indicating that they were important in inflammation and immune response. Some characteristics similarities in peripheral blood cells were showed in P. olivaceus just as the other fishes.