Electron microscope appearance of adult zebrafish granulocytes and macrophages.

(A) Electron micrograph overview of hematopoietic area of adult posterior zebrafish kidney, illustrating heterophil granulocytes (eg, black arrow) as the most prevalent cell, a rarer eosinophil granulocyte (white arrow), and a macrophage (white arrowhead) containing numerous cytoplasmic phagosomes including electron-dense material of similar appearance to erythrocyte cytoplasm. (B-D) Electron microscope appearance of zebrafish heterophil granulocytes. (B) Immature heterophil promyelocyte with large nucleus and few electron-dense, cigar-shaped cytoplasmic granules. (C) Heterophil metamyelocyte with cytoplasm densely packed with electron-dense granules and peripheral nonsegmented nucleus. (D) Higher-power view of the cigar-shaped, electron-dense heterophil granulocyte cytoplasmic granules, showing their axial electron-denser lamellations. (E-G) Electron microscope appearance of zebrafish eosinophil granulocytes. (E) Immature eosinophil promyelocyte with large nucleus and few round cytoplasmic granules of variable electron density. (F) Eosinophil metamyelocyte with cytoplasm densely packed with round and oval-shaped granules and peripheral nonsegmented nucleus. (G) Higher-power view of the characteristic granules of eosinophils, larger than heterophil granules (D) and with marbled variable electron density. (H, I) Electron micrograph incubated for peroxidase (I) and negative control (H) showing the peroxidase reactivity of eosinophil granules (white arrow), evidenced by their darker color in panel I than in panel H, and erythrocyte cytoplasm (black triangle) under conditions of this stain. The already electron-dense heterophil granules (black arrow) are not discernibly darker under the peroxidase reaction conditions. (J, K) Macrophage in kidney (J) and spleen (K) of adult zebrafish, with phagosomes suggestive of erythrophagocytosis. Scale bar = 5 μm in all panels except D and G, where it equals 0.5 μm.