Immunocytochemical studies of the ontogeny of peripheral blood leucocyte subpopulations in Japanese flounder (Paralichthys olivaceus)

Ontogenetic studies of peripheral blood leucocyte (PBL) subpopulations were conducted in Japanese flounder (Paralichthys olivaceus), from the post-flexion larval stage to the juvenile stage, using three monoclonal antibodies (MAbs) that were specific for membrane components of myeloid cells (JFW1), thrombocytes (JFW10) and immunoglobulin (JFW20). Total leucocyte concentrations decreased between the post-flexion stage (19.1%) and when body length was 29.0 mm (1.4%), remaining unchanged thereafter (1.1–2.8%). The percentage of JFW1+ cells relative to the other leucocytes was high in the early life stages (6.4–36.6% in larva, 1.1–9.5% in juvenile). No differences in the phagocytic activity of JFW1+ leucocytes from larval and juvenile flounder were observed, and the ultrastructure of JFW1+ cells was equivalent to that of granulocytes in adult fish. Conversely, JFW20+ cells were detected in fish with body lengths of 29.0 mm, and the percentage of the JFW20+ cells increased with age. No developmental changes were observed in the proportion of the JFW10+ leucocytes, which had ultrastructural characteristics that differed from the thrombocytes of adult fish. It therefore appears that instead of immature lymphocyte-mediated specific immunity, the non-specific defense mechanism involving myeloid cells plays a major role in host defenses in the early life stages of Japanese flounder.