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Lymphangiogenesis is the formation of lymphatic vessels from pre-existing vessels, a dynamic process that requires cell migration. Regardless of location, lymphatic endothelial cell (LEC) progenitors probe their surroundings while migrating to form the lymphatic network. Lymphatic development regulation depends on the transcription factor MAFB in different species. Zebrafish Mafba, expressed in LEC progenitors, is essential for their migration in the trunk. However, the transcriptional mechanism that orchestrate LEC migration in different lymphatic endothelial beds remains elusive. Here, we uncover topographically different requirements of the two paralogues, Mafba and Mafbb, for lymphatic cell migration. Both mafba and mafbb are necessary for facial lymphatic development, but mafbb is dispensable for trunk lymphatic development. On the molecular level, we demonstrate a regulatory network where Vegfc-Vegfd-SoxF-Mafba-Mafbb are essential in the facial lymphangiogenesis. We identify that mafba and mafbb fine-tune the directionality of LEC migration and vessel morphogenesis that is ultimately necessary for lymphatic function.

Our vasculature plays diverse and critical roles in homeostasis and disease. In recent decades, the use of zebrafish has driven our understanding of vascular development into new areas, identifying new genes and mechanisms controlling vessel formation and allowing unprecedented observation of the cellular and molecular events that shape the developing vasculature. Here, we highlight key mechanisms controlling formation of the zebrafish vasculature and investigate how knowledge from this highly tractable model system has informed our understanding of vascular disease in humans.

Despite its essential roles in development and disease, the lymphatic vascular system has been less studied than the blood vascular network. In recent years, significant advances have been made in understanding the mechanisms that regulate lymphatic vessel formation, both during development and in pathological conditions. Remarkably, lymphatic endothelial cells are specified as a subpopulation of pre-existing venous endothelial cells. Here, we summarize the current knowledge of the transcription factor pathways responsible for lymphatic specification and we also focus on the factors that promote or restrict this event.

Summary: Lymphatic vascular development establishes embryonic and adult tissue fluid balance and is integral in disease. In diverse vertebrate organs, lymphatic vessels display organotypic function and develop in an organ-specific manner. In all settings, developmental lymphangiogenesis is considered driven by vascular endothelial growth factor (VEGF) receptor-3 (VEGFR3), whereas a role for VEGFR2 remains to be fully explored. Here, we define the zebrafish Vegf/Vegfr code in receptor binding studies. We find that while Vegfd directs craniofacial lymphangiogenesis, it binds Kdr (a VEGFR2 homolog) but surprisingly, unlike in mammals, does not bind Flt4 (VEGFR3). Epistatic analyses and characterization of a kdr mutant confirm receptor-binding analyses, demonstrating that Kdr is indispensible for rostral craniofacial lymphangiogenesis, but not caudal trunk lymphangiogenesis, in which Flt4 is central. We further demonstrate an unexpected yet essential role for Kdr in inducing lymphatic endothelial cell fate. This work reveals evolutionary divergence in the Vegf/Vegfr code that uncovers spatially restricted mechanisms of developmental lymphangiogenesis. : Lymphatic vessels display organotypic function and develop in an organ-specific manner. Vogrin et al. find that the zebrafish Kdr receptor is indispensible for craniofacial, but not trunk, lymphangiogenesis whereas Flt4 is essential for the latter. Thus, vascular endothelial growth factor (VEGF) receptor signaling pathways are differentially employed in different tissues to drive developmental lymphangiogenesis. Keywords: developmental biology, Kdr, lymphangiogenesis, lymphatic, organotypic, vascular biology, VEGF, VEGF receptor, Vegfd, zebrafish

Summary In the zebrafish embryo, primitive hematopoiesis initiates in two spatially distinct regions. Rostrally, the cells of the anterior lateral plate mesoderm (ALPM) give rise exclusively to cells of the myeloid lineage in a pu.1 -dependent manner [1–5]. Caudally, in the posterior lateral plate mesoderm (PLPM), the expression of gata1 defines a precursor pool that gives rise predominantly to the embryonic erythrocytes [6]. The transcription factor scl acts upstream of both gata1 and pu.1 in these precursor pools, activating a series of conserved transcription factors that cell-autonomously specify either myeloid or erythroid fates [1, 4, 7, 8]. However, the mechanisms underlying the spatial separation of the hematopoietic precursor pools and the induction of differential gene expression within these pools are not well understood. We show here that the Bmp receptor lost-a-fin/alk8 is required for rostral pu.1 expression and myelopoiesis, identifying an early genetic event that distinguishes between the induction of anterior and posterior hematopoiesis. Introducing a constitutively active version of the Alk8 receptor led to increased pu.1 expression, but the role of alk8 was independent of the scl- dependent cell-fate pathway. Furthermore, the role of Alk8 in myelopoiesis was genetically separable from its earlier role in dorsal-ventral embryonic patterning.

The pharyngeal arches give rise to multiple organs critical for diverse processes, including the thymus, thyroid and parathyroids. Several molecular regulators of thymus and thyroid organogenesis are strikingly conserved between mammals and zebrafish. However, land animals have parathyroids whereas fish have gills. The murine transcription factor Glial cells missing 2 (Gcm2) is expressed specifically in the parathyroid primordium in the endodermal epithelium of the third pharyngeal pouch, and in both mice and humans is required for normal development of parathyroid glands. The molecular regulation of fish gill organogenesis remains to be described. We report the expression of gcm2 in the zebrafish pharyngeal epithelium and a requirement for Hox group 3 paralogs for gcm2 expression. Strikingly, zebrafish gcm2 is expressed in the ectodermal portion of the pharyngeal epithelium and is required for the development of the gill filament buds, precursors of fish-specific gill filaments. This study identifies yet another role for a GCM gene in embryonic development and indicates a role for gcm2 during the evolution of divergent pharyngeal morphologies.

The Netrin receptor Deleted in colon cancer (Dcc) has been shown to play a pivotal role in the guidance of nascent axons towards the ventral midline in the developing nervous systems of both vertebrates and invertebrates. In contrast, the function during embryogenesis of a second Dcc-like Netrin receptor Neogenin has not yet been defined. We used antisense morpholino oligonucleotides to knockdown Neogenin activity in zebrafish embryos and demonstrate that Neogenin plays an important role in neural tube formation and somitogenesis. In Neogenin knockdown embryos, cavitation within the neural rod failed to occur, producing a neural tube lacking a lumen. Somite formation was also defective, implicating Neogenin in the migration events underlying convergent extension during gastrulation. These observations suggest a role for Neogenin in determining cell polarity or migrational directionality of both neuroectodermal and mesodermal cells during early embryonic development. (C) 2004 Elsevier Inc. All rights reserved.