Myelopoiesis in the zebrafish, Danio rerio

Genome-wide chemical mutagenesis screens in the zebrafish (Danio rerio) hav e led to the identification of novel genes affecting vertebrate erythropoie sis. In determining if this approach could also be used to clarify the mole cular genetics of myelopoiesis, it was found that the developmental hierarc hy of myeloid precursors in the zebrafish kidney is similar to that in huma n bone marrow. Zebrafish neutrophils resembled human neutrophils, possessin g segmented nuclei and myeloperoxidase-positive cytoplasmic granules. The z ebrafish homologue of the human myeloperoxidase (MPO) gene, which is specif ic to cells of the neutrophil lineage, was cloned and used to synthesize an tisense RNA probes for in situ hybridization analyses of zebrafish embryos. Granulocytic cells expressing zebrafish mpo were first evident at 18 hours after fertilization (hpf) in the posterior intermediate cell mass (ICM) an d on the anterior yolk sac by 20 hpf. By 24 hpf, mpo-expressing cells were observed along the ICM and within the developing vascular system. Thus, the mpo gene should provide a useful molecular probe for identifying zebrafish mutants with defects in granulopoiesis. The expression of zebrafish homolo gues was also examined in 2 other mammalian hematopoietic genes, Pu.1, whic h appears to initiate a commitment step in normal mammalian myeloid develop ment, and L-Plastin, a gene expressed by human monocytes and macrophages. T he results demonstrate a high level of conservation of the spatio-temporal expression patterns of these genes between zebrafish and mammals. The morph ologic and molecular genetic evidence presented here supports the zebrafish as an informative model system for the study of normal and aberrant human myelopoiesis.