Developmental biology of zebrafish myeloid cells.

The zebrafish (Danio rerio) has emerged as an informative vertebrate model for developmental studies, particularly those employing genetic approaches such as mutagenesis and screening. Zebrafish myelopoiesis has recently been characterized, paving the way for the experimental strengths of this model organism to contribute to an improved understanding of the genetic regulation of myeloid development. Zebrafish have a multi-lineage myeloid compartment with two types of granulocyte (heterophil/neutrophil and eosinophil granulocytes), and monocyte/macrophages, each with characteristic morphological features and histochemical staining properties. Molecular markers have been characterised for various myeloid cell types and their precursor cells, for example: stem cells (scl, hhex, lmo2), myeloid lineage precursors (spi1/pu.1, c/ebp1), heterophil granulocytes (mpx/mpo), macrophages (L-plastin, fms). In zebrafish, the sites of early myeloid and erythroid commitment are anatomically separated, being located in the rostral and caudal lateral plate mesoderm respectively. Functional macrophages appear before cells displaying granulocytic markers. By the second day of life, cells expressing granulocyte- and macrophage-specific genes are scattered throughout the embryo, but tend to aggregate in the ventral venous plexus, which may be a site of their production or a preferred site for their residence. Even in early embryos, macrophages are phagocytically active, and granulocytes participate in acute inflammation. Equipped with an understanding of the developmental biology of these various myeloid cells and a set of tools for their identification and functional study, we will now be able to exploit the experimental strengths of this model organism to better understand the genetic regulation of myelopoiesis.