Light microscope appearance of adult zebrafish granulocytes.

(A) A heterophil (black arrow) and an eosinophil (arrowhead) in a single field of a May-Grünwald-Giemsa–stained zebrafish blood smear. (B) Detail of mature heterophils with double- and triple-lobed nuclei (May-Grünwald-Giemsa–stained blood smear). (C) Detail of mature eosinophils, illustrating characteristic pink cytoplasm and peripheral nucleus (May-Grünwald-Giemsa–stained blood smear). (D) Hematoxylin and eosin section of posterior kidney showing an interstitial niche of myelopoiesis lying between renal tubules (t) and blood vessels (v), including several heterophil granulocytes recognizable by their indented or segmented nuclei (arrow). (E-J) Myeloperoxidase histochemical staining of cytospin preparations of single-cell suspensions prepared from adult zebrafish kidneys. Giemsa-stained preparation (E) showing mature heterophil granulocytes serving as negative control for (F), which was stained histochemically for myeloperoxidase and counterstained with Giemsa (to display nuclear morphology), showing myeloperoxidase-positive cells with black cytoplasmic staining at various stages of heterophil development (eg, arrow). *Erythrocyte with weakly peroxidase-staining gray cytoplasm. Myeloperoxidase-positive heterophil granulocytes of various stages of development: promyelocyte (G), myelocyte (H), metamyelocyte (I), and segmented mature form (J). Myeloperoxidase positivity is strongest at the myelocyte and metamyelocyte stages of development. Scale bar = 10 μm in all panels except D, where it equals 24 μm.