Hematopoietic and Vascular System Toxicity - Zebrafish: Methods for Assessing Drug Safety and Toxicity - page 73

(Langenau et al., 2004; Trede et al., 2004). T cells develop in the thymi throughout
adulthood, and mature T cells can also be found in the kidney.
7.2.6 Thrombocytes
The zebrafish equivalents to platelets are called thrombocytes, and unlike platelets
these cells maintain their nuclei (Fig. 7.1) (Jagadeeswaran et al., 1999). Thrombo-
cytes and their precursors, prothrombocytes and prethrombocytes, are found to
express high levels of CD41. CD41 expression studies have determined that throm-
bocytes do not develop in primitive or early definitive hematopoiesis, but begin to
appear at 2-3dpf (Lin et al., 2005).
7.2.7 Granulocytes
Zebrafish myeloid cell types differ slightly from mammalian myeloid cell types. In
zebrafish, three myeloid cell types are present: macrophages, neutrophils, and a third
cell type that has characteristics of both basophils and eosinophils (Bennett
et al., 2001) (Fig. 7.1). Macrophages are derived from monocytes, whereas the other
cell types are derived from a myeloblast precursor (Traver et al., 2003a). Neutrophils
have granules that are mpo positive, as well as segmented nuclei. The basophil/
eosinophil population can be characterized by a nonsegmented nucleus (Bennett
et al., 2001).
7.2.8 Erythrocytes
During zebrafish erythroid differentiation, a proerythroblast can be distinguished by
its large nucleus and is positive for gata1 and globin expression (Brownlie et al., 2003).
In contrast to mammalian red blood cells, when zebrafish erythroblasts differentiate,
the nucleus condenses and is retained (Fig. 7.1). These cells maintain expression of
gata1 and globins.
7.2.9 Vascular Development
The vascular system develops in parallel to hematopoiesis and HSC formation. Cells
from the blood islands form both HSCs and precursors termed angioblasts (Dooley
et al., 2005). Angioblasts differentiate into endothelial cells that line the blood vessels.
This differentiation is induced by the vascular endothelial growth factors (VEGFs)
(Lawson and Weinstein, 2002). VEGF can signal through multiple receptors,
including flk1, to induce differentiation and formation of the dorsal aorta (DA) and
posterior cardinal vein (PCV). After formation, these two vessels must be specified.
The DA and all artery precursors express ephrinB2, notch5, flk1, deltaC, and tbx20.
On the other hand, the PCVand all venous precursors express EphB4 receptor tyrosine
kinase and flt4. Notch5 and flt4 are both induced by VEGF signaling, though how