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The zebrafish is an essential and widely used vertebrate model organism in many scientific researches, for example in developmental biology. This paper is mainly talking about specification of the zebrafish nervous system by nonaxial signals. There are two different signals, neutralizing and posteriorizing, which are regulating in the neurectoderm of the amphibian gastrula. However, in zebrafish, that signals more likely come from tissues. Nonaxial or axial mesendoderm caused different results. Thus, the signals from the organizer and the germring to pattern the neural axis might help the specification of the zebrafish.
In neural fate map, there are three different progenitors in the forebrain, hindbrain and midbrain, which are located in the embryonic shield. Meanwhile, the progenitors in forebrain are far from the germling but oppositely in hindbrain. This neural fate map, it allowed us to detect the signals that might differentially pattern the neuraxins. To investigate if they have more posterior neural fates, they did some transplantation of progenitors. It showed that the signals that can help cells to adjust the hindbrain fate are normally active in vivo at 6 hours, and that signals for hindbrain may are not exclusive from the shield. Germring tissue may be a source of such a posteriorizing signal.
They also did some researches about the pattering by nonaxial germring tissue by transplanted the shield and the germring. If the germring were the source of a patterning signal, some of grafts might use forebrain cells to adopt more posterior fates. Then they used the fluorescent labels in the forebrain, which can show the morphological changes clearly. By this way, there is a drawback that it does not know the cell population such as somatic mesoderm, endoderm, or both is use for these transformations. To solved this problem, they also analyzed the expression of krox2o in the forebrain, krox20 can easily recognized the molecular changes in the forebrain after germring transplant. It showed that there is no any expression of Krox20 in the forebrain of most embryos with a shield graft, but only a few Krox20-positive cells near the graft.
They found basic fibroblast growth factor (bFGF) as candidate transformer signals in Xenopus that might are relative to zebrafish. These studies implied that the negative FGF receptor in the early cleavage stages zebrafish causes a loss of posterior structures, which means bFGF is a candidate transformer signal in zebrafish. To know if bFGF could be like the transforming effect of germring tissue transplants, they took bFGF beads into the gastrula and arranged it as two groups (Control and bFGF). In control group, the beads without bFGF showed no specific effect on forebrain morphology or gene expression, whereas the beads with bFGF causes severe brain deformation. Therefore, bFGF can involve in the germring activity with other factors.
Last but not least, there is a modified two-signal model in zebrafish, and two physically separated signals in the zebrafish gastrula might showed in neuronal fate map. The first one is an activator signal which induces neural tissue in anterior area in that map, and the second on is a transform signal that regulated neural tissue in axial area. Both of two signals might be either instructive or permissive. According to the observation, we knew that FGFs may not be only part in the neural transforming activity in vivo, there are also extra key factors might be responsible for the neural system.
This paper determined the importance of candidate transformer molecules in zebrafish nervous system by nonaxial signals, and given the advanced methods to investigate how these signals work by a spatial and temporal context. Furthermore, this paper took fluorescent labels, the expression analysis of krox20 as well as basic fibroblast growth factor to provide the evidences for transformer action in vivo. The manipulation showed in this paper can be used to further research and mechanisms in the zebrafish nervous system.
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