SUMMARYIt is still controversial whether cranial placodes and neural crest cells arise from a common precursor at the neural plate border or whether placodes arise from non-neural ectoderm and neural crest from neural ectoderm. Using tissue grafting in embryos of Xenopus laevis, we show here that the competence for induction of neural plate, neural plate border and neural crest markers is confined to neural ectoderm, whereas competence for induction of panplacodal markers is confined to non-neural ectoderm. This differential distribution of competence is established during gastrulation paralleling the dorsal restriction of neural competence. We further show that Dlx3 and GATA2 are required cell-autonomously for panplacodal and epidermal marker expression in the non-neural ectoderm, while ectopic expression of Dlx3 or GATA2 in the neural plate suppresses neural plate, border and crest markers. Overexpression of Dlx3 (but not GATA2) in the neural plate is sufficient to induce different non-neural markers in a signaling-dependent manner, with epidermal markers being induced in the presence, and panplacodal markers in the absence, of BMP signaling. Taken together, these findings demonstrate a non-neural versus neural origin of placodes and neural crest, respectively, strongly implicate Dlx3 in the regulation of non-neural competence, and show that GATA2 contributes to non-neural competence but is not sufficient to promote it ectopically.

Our data characterize SW1353 cells as a cell line with only a very limited potential to mimic PHCs, though SW1353 cells can be of value to study the induction of protease expression within cells, a phenomenon also seen in chondrocytes.

Cranial placodes are local thickenings of the vertebrate head ectoderm that contribute to the paired sense organs (olfactory epithelium, lens, inner ear, lateral line), cranial ganglia and the adenohypophysis. Here we use tissue grafting and dye injections to generated fate maps of the dorsal cranial part of the non-neural ectoderm for Xenopus embryos between neural plate and early tailbud stages. We show that all placodes arise from a crescent-shaped area located around the anterior neural plate, the pre-placodal ectoderm. In agreement with proposed roles of Six1 and Pax genes in the specification of a panplacodal primordium and different placodal areas, respectively, we show that Six1 is expressed uniformly throughout most of the pre-placodal ectoderm, while Pax6, Pax3, Pax8 and Pax2 each are confined to specific subregions encompassing the precursors of different subsets of placodes. However, the precursors of the vagal epibranchial and posterior lateral line placodes, which arise from the posteriormost pre-placodal ectoderm, upregulate Six1 and Pax8/Pax2 only at tailbud stages. Whereas our fate map suggests that regions of origin for different placodes overlap extensively with each other and with other ectodermal fates at neural plate stages, analysis of co-labeled placodes reveals that the actual degree of overlap is much smaller. Time lapse imaging of the pre-placodal ectoderm at single cell resolution demonstrates that no directed, large-scale cell rearrangements occur, when the pre-placodal region segregates into distinct placodes at subsequent stages. Our results indicate that individuation of placodes from the pre-placodal ectoderm does not involve large-scale cell sorting in Xenopus.

The expression of zebrafish hoxb3a and hoxb4a has been found to be mediated through five transcripts, hoxb3a transcripts I-III and hoxb4a transcripts I-II, driven by four promoters. A "master" promoter, located about 2 kb downstream of hoxb5a, controls transcription of a pre-mRNA comprising exon sequences of both genes. This unique gene structure is proposed to provide a novel mechanism to ensure overlapping, tissue-specific expression of both genes in the posterior hindbrain and spinal cord. Transgenic approaches were used to analyze the functions of zebrafish hoxb3a/hoxb4a promoters and enhancer sequences containing regions of homology that were previously identified by comparative genomics. Two neural enhancers were shown to establish specific anterior expression borders within the hindbrain and mediate expression in defined neuronal populations derived from hindbrain rhombomeres (r) 5 to 8, suggesting a late role of the genes in neuronal cell lineage specification. Species comparison showed that the zebrafish hoxb3a r5 and r6 enhancer corresponded to a sequence within the mouse HoxA cluster controlling activity of Hoxa3 in r5 and r6, whereas a homologous region within the HoxB cluster activated Hoxb3 expression but limited to r5. We conclude that the similarity of hoxb3a/Hoxa3 regulatory mechanisms reflect the shared descent of both genes from a single ancestral paralog group 3 gene.

Cell migration has been studied extensively in vitro, however, fewer studies address this subject in vivo. Our aim is to determine in vivo, the role of the actin regulatory protein, Ena. To enable this we are studying Drosophila hemocytes in the developing embryo. Hemocytes are the primary immune cells of Drosophila and during development they follow stereotyped pathways to distribute throughout the embryo. Using the Gal4UAS system to express GFP specifically in hemocytes allows us to visualize the migrating hemocytes using the confocal microscope. This system is also used to overexpress Ena or interfere with its function. Here we show that Ena localizes to the

Noggin1 is a famous embryonic neural inducer that can sequester TGF-beta cytokines of Bone Morphogenetic Protein family, thereby antagonizing Smad1-dependent signaling pathway activity. During early embryogenesis noggin1 executes two major biological functions: conversion of embryonic mesoderm into skeletal muscles (dorsalization) and of ectoderm into neural tissue (neuralization). Anterior neural tissue is involved in the formation of head structures and particularly in the development of rostral forebrain (RF), which is important evolutionary innovation of Vertebrates responsible for their higher cognitive functions.Besides ''classical" noggin1, two other groups of noggin proteins, noggin2 and noggin4, were recently identified in Vertebrates. Different expression patterns of noggin1, noggin2 and noggin4 in early embryonic development and their amino acid sequence distinctions suggest that these factors may execute different biological functions.Noggin2 is specifically expressed in Xenopus embryos in the RF primordium where it presumably duplicates antagonizing effect of noggin1 on BMP signaling. Now we report that in addition to latter function, and in contrast to noggin1, noggin2 can also antagonize another TGF-beta factor, activin, which, in its turn, influences on the activity of Smad2-dependent signaling pathway.Down-regulation of noggin2 causes severe abnormalities of forehead development, indicating its crucial role in this process.In case of ectopical expression of noggin2, its ability to inhibit both BMP and activin signaling pathways can result in development of secondary forehead structures. Consistently, we show that inhibition of activin signaling by noggin2 in cells of the anterior neural plate is essential for the forebrain development in normal embryogenesis.In chick, although the polarity of the early embryo is specified by the time of egg-laying, the embryo is highly regulative: when a blastoderm-stage embryo (about 20,000 cells) is cut in half, both halves can develop an embryonic axis (Bertocchini et al., 2004; Lutz, 1949; Spratt and Haas, 1960). This implies that normal embryos possess inhibitory mechanisms that prevent formation of multiple axes (Bertocchini and Stern, 2002; Bertocchini et al., 2004). While many genes are expressed posteriorly at these early stages and are involved in axis formation, only one to date, the transcription factor Gata2, shows a stronger expression anteriorly, decreasing towards the posterior region (Sheng and Stern, 1999).Here, we investigate its role in specification of embryonic polarity. Morpholino mediated knock-down of Gata2 causes upregulation of posterior markers along the circumference of the embryo, or ectopic sites of axis initiation; these embryos then develop an ectopic axis or show a displacement of the endogenous axis. This suggests that Gata2 plays a role in positioning the embryonic axis, and that it somehow inhibits axis formation anteriorly. At very early stages, Gata2 and Vg1 (a member of the TGFb family of signalling molecules) are expre...

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