Morphologic and functional characterization of granulocytes and macrophages in embryonic and adult zebrafish

The zebrafish is a useful model organism for developmental and genetic stud ies. The morphology and function of zebrafish myeloid cells were characteri zed. Adult zebrafish contain 2 distinct granulocytes, a heterophil and a ra rer eosinophil, both of which circulate and are generated in the kidney, th e adult hematopoietic organ. Heterophils show strong histochemical myeloper oxidasic activity, although weaker peroxidase activity was observed under s ome conditions in eosinophils and erythrocytes. Embryonic zebrafish have ci rculating immature heterophils by 48 hours after fertilization (hpf). A zeb rafish myeloperoxidase homologue (myeloid-specific peroxidase; mpx) was iso lated. Phylogenetic analysis suggested it represented a gene ancestral to t he mammalian myeloperoxidase gene family. It was expressed in adult granulo cytes and in embryos from 18 hpf, first diffusely in the axial intermediate cell mass and then discretely in a dispersed cell population. Comparison o f hemoglobinized cell distribution, mpx gene expression, and myeloperoxidas e histochemistry in wild-type and mutant embryos confirmed that the latter reliably identified a population of myeloid cells. Studies in embryos after tail transection demonstrated that mpx- and peroxidase-expressing cells we re mobile and localized to a site of inflammation, indicating functional ca pability of these embryonic granulocytes. Embryonic macrophages removed car bon particles from the circulation by phagocytosis. Collectively, these obs ervations have demonstrated the early onset of zebrafish granulopoiesis, ha ve proved that granulocytes circulate by 48 hpf, and have demonstrated the functional activity of embryonic granulocytes and macrophages. These observ ations will facilitate the application of this genetically tractable organi sm to the study of myelopoiesis. (C) 2001 by The American Society of Hemato logy.