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Summary Mesodermal tissues produce various inductive signals essential for morphogenesis of endodermal organs. However, little is known about how the spatial relationship between the mesodermal signal-producing cells and their target endodermal organs is established during morphogenesis. Here, we report that a mutation in the zebrafish myosin phosphatase targeting subunit 1 (mypt1) gene causes abnormal bundling of actin filaments and disorganization of lateral plate mesoderm (LPM) and endoderm cells. As a result, the coordination between mesoderm and endoderm cell movements is disrupted. Consequently, the two stripes of Bmp2a-expressing cells in the LPM fail to align in a V-shaped pocket sandwiching the liver primordium. Mispositioning Bmp2a-producing cells with respect to the liver primordium leads to a reduction in hepatoblast proliferation and final abortion of hepatoblasts by apoptosis, causing the liverless phenotype. Our results demonstrate that Mypt1 mediates coordination between mesoderm and endoderm cell movements in order to carefully position the liver primordium such that it receives a Bmp signal that is essential for liver formation in zebrafish.

Hematopoiesis is a complex process which gives rise to all blood lineages in the course of an organism's lifespan. However, the underlying molecular mechanism governing this process is not fully understood. Here we report the isolation and detailed study of a newly identified zebrafish ugly duckling (Udu) mutant allele, Udu sq1 . We show that loss-of-function mutation in the udu gene disrupts primitive erythroid cell proliferation and differentiation in a cell-autonomous manner, resulting in red blood cell (RBC) hypoplasia. Positional cloning reveals that the Udu gene encodes a novel factor that contains 2 paired amphipathic ␣-helix-like IntroductionHematopoiesis in vertebrate occurs in 2 waves, primitive and definitive. [1][2][3] In the mouse, the primitive or embryonic wave of hematopoiesis occurs around embryonic day 7.5 in the yolk-sac blood island, and produces primitive erythrocytes and macrophages. 4,5 This primitive wave of hematopoiesis lasts for a transient period of a few days and is subsequently replaced by the definitive program. Murine definitive hematopoiesis is believed to originate from a distinctive region known as the aorta-gonad-mesonephros at embryonic days 7.5 to 8. 6,7 These definitive hematopoietic precursors, presumably the definitive hematopoietic stem cells, then migrate to the fetal liver, where they undergo rapid proliferation and differentiation, and finally colonize the bone marrow for adult hematopoiesis. In contrast to primitive hematopoiesis, the definitive hematopoietic program gives rise to all the mature blood cell types and remains active throughout the lifetime of the organism.In zebrafish, hematopoiesis also comprises both primitive and definitive programs and produces mature blood cell types similar to those found in mammals. 8,9 Zebrafish primitive erythropoiesis begins at the 4-somite stage as a pair of bilateral stripes in the posterior lateral mesoderm. 10 These stripes extend anteriorly and posteriorly, and then converge in the midline at the 20-somite stage to form the intermediate cell mass (ICM), where erythroid precursors further develop and enter the circulation by 24 to 26 hours after fertilization (hpf). 10,11 On the other hand, primitive myelopoiesis originates from the rostral blood island in the anterior lateral mesoderm at around the 10-somite stage and produces mainly macrophages and possibly some neutrophils. [12][13][14][15] Recent studies demonstrate that zebrafish definitive hematopoiesis initiates in the ventral wall of dorsal aorta between 26 and 48 hpf, 16,17 and then establishes the adult hematopoietic organ in the kidney by 5 days after fertilization (dpf). 11 The zebrafish mutant, ugly duckling (Udu tu24 ), was first isolated from the 1996 Tuebingen large-scale screen as a mutant defective in morphogenesis during gastrulation and tail formation. 18 In this article, we report the isolation and detailed study of a new udu allele, udu sq1 , as a mutant with a defect in blood cell development. Cell-cycle, cytologic, and transplantation analyses sho...