The zebrafish (Danio rerio) has emerged as a valuable model organism that is amenable for large-scale chemical and genetic screens. The ability of zebrafish to produce large quantities of synchronized, externally fertilized, transparent embryos makes them ideal for screens, which often are not possible in mammalian models. Signaling pathways important for hematopoiesis are well conserved between zebrafish and mammals, making many targets identified in zebrafish screens applicable to mammals. Hematopoiesis in zebrafish occurs in two waves: the primitive or embryonic wave and the definitive or adult wave. Definitive hematopoietic stem cells arise in the aorta-gonad-mesonephros region (AGM) and express conserved markers such asrunx1andc-mybthat allow for the detection of stem cells by whole-mount in situ hybridization (WISH). In this protocol, we will discuss a chemical screen in zebrafish embryos to detect compounds that expand or deplete hematopoietic stem cells (HSCs) in vivo. This type of screen represents a powerful tool to study HSCs in zebrafish.
