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Chapter 34 : The Zebrafish as a Model of Host-Pathogen Interactions
Category: Microbial Genetics and Molecular Biology; Bacterial Pathogenesis
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The zebrafish as a model for host-pathogen interactions has now matured to the point that we can reflect on what it truly has to offer, where it is helpful, and how it complements other models. The genetic advantages of zebrafish are often considered the strongest. Forward genetic screens are relatively easy, again because of the large number of progeny derived and the speed with which they can be assessed. Morpholino technology makes for a rapid and relatively inexpensive tool for the study of gene function for up to 7 to 10 days of life. Macrophages are the first phagocytes, and indeed the first immune cells, to appear in zebrafish development (although the precise timing of neutrophil development is not certain). Although the kidney is the site of hematopoiesis in the adult fish, the predecessors of the stem cells responsible for definitive hematopoiesis first appear in a region of the trunk called the aorta-gonad-mesonephros (AGM) at about 24 hpf, whence they move to another temporary hematopoietic site in the ventral tail called the caudal hematopoietic tissue. Some studies of embryonic macrophages have taken advantage of neutral red accumulation in macrophages, but this dye is toxic within hours of administration. Melanomacrophages are a subset of macrophages found in fish, amphibians, and reptiles. They are most commonly found as a part of melanomacrophage centers (MMCs) in the spleen, liver, and sometimes kidney, but are also seen singly. The chapter highlights the work done thus far with the various pathogen species.
General anatomy and location of myelopoiesis during progressive stages of development. (A–D) Line drawings based on sketches from Kimmel et al. (1995) . Path of hematopoietic cells as reported in Herbomel et al. (1999) and Murayama et al. (2006) . (A) Path of embryonic macrophages from lateral mesoderm to anterior yolk. (B) Before the onset of circulation, embryonic macrophages have spread over the yolk and begun to infiltrate the brain. (C) Definitive hematopoiesis begins in the aorta-gonad-mesonephros (AGM), but hematopoietic precursors soon migrate to the caudal hematopoietic tissue (CHT). (D) By 4 days postfertilization, hematopoiesis is taking place in the CHT, but hematopoietic cells are also transferring to the thymus and kidney. (E) Location of organs important to hematopoiesis and infection in the adult.
Examples of macrophages and neutrophils visible with DIC microscopy during embryonic and larval development. (A–C) Embryonic macrophages located near caudal vein (ventral is up), with muscle tissue nearby. (A) Early embryonic macrophage at yolk surface, ~30 hpf, just before the onset of circulation. (Scale bar, 20 μm; all panels, same scale.) (B) More mature embryonic macrophage in yolk circulation valley at ~48 hpf. (C) Two embryonic macrophages in yolk circulation valley, with many cellular processes and connected by a “tether.” (D–F) Neutrophils located just superficial to caudal hematopoietic tissue. Note the more slender proportions and plentiful cytoplasmic granules.
Larval zebrafish macrophages infected with Salmonella serovar Arizonae in vivo. (A) A macrophage in the yolk circulation valley contains three bacteria (white arrow) in a phagosome, ca. 1 h post-intravenous infection. (B) At ca. 18 hpi, a macrophage (outlined with white dots) near the CHT contains many bacteria, most within a large phagosome (white arrow). (C) At ca. 24 hpi, a macrophage crawling in a venule in the tail is overrun with intracellular bacteria. (All scale bars, 20 μm.)