Oncogene (2004) 23, 7233–7246 & 2004 Nature Publishing Group All rights reserved 0950-9232/04 $30.00 www.nature.com/onc The ‘definitive’ (and ‘primitive’) guide to zebrafish hematopoiesis Alan J Davidson1 and Leonard I Zon*,1 1Division of Hematology/Oncology, Department of Medicine, Children’s Hospital and Dana Farber Cancer Institute, Howard Hughes Medical Institute, Boston, MA 02115, USA Progressive advances using zebrafish as a model organism to bilateral stripes of cells in the posterior meso- have provided hematologists with an additional genetic derm. These cells migrate to the trunk midline system to study blood cell formation and hematological and differentiate into ‘primitive’ proerythroblasts malignancies. Despite extensive evolutionary divergence and endothelial cells of the trunk vasculature. In between bony fish (teleosts) and mammals, the molecular cross-section, the endothelium of the axial vein pathways governing hematopoiesis have been highly encapsulates the converged mass of erythroid cells, conserved. As a result, most (if not all) of the critical thereby resembling the cellular architecture of the hematopoietic transcription factor genes identified in mammalian yolk sac blood island (Al-Adhami and mammals have orthologues in zebrafish. As in other Kunz, 1977; Willett et al., 1999). vertebrates, all of the teleost blood lineages are believed to originate from a pool of pluripotent, self-renewing hematopoietic stem cells. Here, we provide a detailed Ventral mesoderm patterning review of the timing, anatomical location, and transcrip- With the advent of modern molecular and genetic tional regulation of zebrafish ‘primitive’ and ‘definitive’ techniques, it has been possible to develop a more hematopoiesis as well as discuss a model of T-cell comprehensive understanding of the origin and forma- leukemia and recent advances in blood cell transplanta- tion of the ICM. Fate mapping analyses have demon- tion. Given that many of the regulatory genes that control strated that the first circulating erythroid cells descend embryonic hematopoiesis have been implicated in onco- from mesoderm on the ventral side of the early gastrula genic pathways in adults, an understanding of blood cell stage embryo (Kimmel, 1990; Warga and Nusslein- ontogeny is likely to provide insights into the patho- Volhard, 1999). Following the extensive morphological physiology of human leukemias. movements of gastrulation, ventral mesoderm even- Oncogene (2004) 23, 7233–7246. doi:10.1038/sj.onc.1207943 tually comes to occupy the posterior and lateral regions of the embryo (Figure 2). In addition to blood and Keywords: zebrafish; blood; hematopoietic stem cell; vascular precursors, the posterior mesoderm also gives embryo; leukemia; hematopoiesis rise to the embryonic kidney (pronephros) that forms as bilateral stripes of cells lateral to the ICM precursors (Figure 3e). How the ventral mesoderm is patterned into different cell fates is still largely unclear. However, secreted ‘Primitive’ hematopoiesis signaling molecules belonging to the bone morphoge- netic protein (BMP), fibroblast growth factor (FGF), Successive waves of ‘primitive’ and ‘definitive’ hemato- and Wnt families have been implicated as early inducers poiesis, occurring in anatomically distinct sites, are and patterning factors of ventral mesoderm (reviewed characteristics of vertebrate embryos (Figure 1 and by Mun˜ oz-Sanjua´ n and Hemmati-Brivanlou, 2001). At reviewed by Galloway and Zon, 2003). In mammals and present, it is widely believed that a morphogenic birds, ‘primitive’ hematopoiesis occurs in the extraem- gradient of BMP signaling across the dorsal–ventral bryonic yolk sac blood islands and predominantly axis during gastrulation is primarily responsible for generates erythroid cells. The equivalent site in zebrafish patterning mesoderm into discrete cell fates. A recent is located intraembryonically and is known as the re-evaluation of the Xenopus fate map has shown that intermediate cell mass (ICM). Historically, the forma- the dorsal–ventral axis of the early gastrula embryo tion of the ICM has been studied for over 130 years more accurately reflects the arrangement of tissues in a number of diverse fish species (Oellacher, found along the anterior–posterior axis (Lane and 1872; Stockard, 1915; Al-Adhami and Kunz, 1976; Sheets, 2000; Lane and Sheets, 2002). In general, Al-Adhami and Kunz, 1977). These early light mesoderm on the dorsal side of the early gastrula gives microscopy studies traced the origin of the ICM rise to anterior structures such as the head, while ventral mesoderm generates more posterior tissues. These *Correspondence: L Zon, Children’s Hospital, KARP7, 1 Blackfan observations are also consistent with the zebrafish fate Circle, Boston, MA 02115, USA; E-mail: [email protected] map (Kimmel, 1990; Warga and Nusslein-Volhard, Zebrafish hematopoiesis AJ Davidson and LI Zon 7234 Transcriptional regulation of ICM-derived hematopoietic stem cells (HSCs) Many of the transcription factor genes that play a critical role during mammalian hematopoiesis have been identified in zebrafish (Table 1) and have spatiotemporal expression patterns comparable to their mammalian counterparts. The stem cell leukemia (scl) gene, encoding a basic helix–loop–helix transcription factor, has been extensively characterized in both mammals and zebra- fish, largely due to its early and essential role in HSC formation (Robb et al., 1995; Shivdasani et al., 1995) and its involvement in acute lymphoblastic leukemia Figure 1 Approximate time of appearance and duration of hematopoietic activity in different tissues during zebrafish embry- (Begley et al., 1989; Brown et al., 1990). It is important ogenesis (adapted from Galloway and Zon, 2003). to note, however, that scl is also an early molecular ICM ¼ intermediate cell mass; AGM ¼ aorta-gonad-mesonephros marker of vascular precursors (angioblasts). In the region equivalent (corresponding to the ventral wall of the dorsal posterior mesoderm, expression of scl is first evident aorta); hpf ¼ hours postfertilization; dpf ¼ days postfertilization around the 2-somite stage in bilateral stripes of cells flanking the paraxial mesoderm at the future level of somite six (Davidson et al., 2003 and Figure 3f). This expression pattern of scl overlaps with other transcrip- tion factor genes, including cdx4, fli1a (previously known as fli1), gata2, hhex, lmo2, spadetail/tbx16, and tif1g (Table 1 and Figure 3b, c). The coexpression of scl, lmo2,andgata2 is consistent with biochemical studies showing that these transcrip- tion factors can interact to form a multimeric complex (Valge-Archer et al., 1994; Wadman et al., 1997) and overexpression experiments have suggested crossregula- tory interactions between scl, hhex,andfli1a (Gering et al., 1998; Liao et al., 2000b; Gering et al., 2003). Whether the scl þ ICM precursors at this stage represent hemangioblasts (a common progenitor to HSCs and angioblasts) or independently derived populations of HSCs and angioblasts that simultaneously develop adjacent to each other is currently unresolved. Aside from dorsal–ventral patterning mutations, which glob- Figure 2 Schematic representation of zebrafish embryos during gastrulation and somitogenesis showing the rostral and caudal sites ally affect the patterning of ventral mesoderm, only of ‘primitive’ hematopoiesis (blue stripes). ICM precursors are the mutants cloche, spadetail/tbx16, and kugelig/cdx4 derived from a subset of ventral mesoderm (red) during gastrula- display defects at this early stage of HSC formation tion, which forms the posterior lateral mesoderm. RBI precursors (see below). are derived from more anterior lateral mesoderm. Both blood cell The expression pattern of scl in ICM precursors at the populations migrate during subsequent stages of development (indicated by white arrows). At the 20-somite stage (bottom right 3-somite stage overlaps with that of pax2.1 and pax8 embryo shown in lateral view), blood cells from the RBI are found (AJD and LIZ, unpublished results), encoding two scattered across the anterior yolk (blue spots), while blood cells homeobox genes required for pronephros formation from the ICM have converged at the trunk midline (blue stripe). (Majumdar et al., 2000; Bouchard et al., 2002). Similar For clarity, only mesoderm is shown for early and late gastrula- stage embryos. The 5-somite stage embryo is shown removed from observations have been found in the dorsolateral plate the yolk and flattened (flatmount). Black arrows indicate gastrula- mesoderm of Xenopus laevis embryos (Walmsley et al., tion movements at the marginal zone. A ¼ anterior, P ¼ posterior, 2002), suggesting that ICM precursors are derived from V ¼ ventral, D ¼ dorsal mesoderm that has initially activated a kidney gene program. The significance of this finding is unclear because by the 5-somite stage, the posterior mesoderm is clearly subdivided along the medial–lateral axis into 1999) and suggest that dorsal–ventral patterning of nonoverlapping bilateral stripes of scl þ ICM precursors the posterior mesoderm (equating to medial–lateral and pax2.1 þ kidney progenitors (Figure 3e). cloche cell fates at the end of gastrulation) occurs along mutants fail to express scl at this time and display the animal–vegetal axis. This hypothesis is still com- normal kidney stripe patterning (Figure 3f), suggesting patible with the BMP gradient model, although it that scl is not required to downregulate pax2.1 does require that the gradient is reoriented along transcription. The transient