Regulation of region-specific neural genes in zebrafish early embryos

利用两栖动物胚胎的分析表明，中枢神经系统的诱导和前后模式是由组织者产生的信号和组织者衍生的中轴中胚层产生的。然而，我们在这里表明，斑马鱼中枢神经系统的初始前后模式取决于外胚层的分化能力，而不是由组织者衍生的信号强加的。这一前后信息存在于外胚层细胞的整个上胚层中，外胚层细胞通常同时产生神经和非神经衍生物。由于这一信息，组织者组织被移植到胚胎的腹侧形成神经组织，但被诱导的神经组织的前后特性取决于诱导组织在上胚层中的位置。因此，OTX2是一种前神经标记物，仅在胚胎的前部区域诱导，与移植物的位置无关。类似地，Hoxa-1，一种后脑神经标记物，仅在后脑区域诱导。此外，每个异位表达结构域在胚胎腹侧的边界始终处于与胚胎背侧内源表达的纬度相等的纬度。上胚层的前后规格与胚胎的背腹规格无关，因为腹化胚胎中诱导的神经组织也表现出前后极性。细胞移植和RNA注射实验表明，非轴向边缘中胚层和成纤维细胞生长因子信号是上胚层前后特化所必需的。然而，对成纤维细胞生长因子信号的要求是间接的，因为反应成纤维细胞生长因子的能力受损的细胞仍然可以对前后位置信息做出反应。

Analyzes using amphibian embryos proposed that induction and anteroposterior patterning of the central nervous system is initiated by signals that are produced by the organizer and organizer-derived axial mesoderm. However, we show here that the initial anteroposterior pattern of the zebrafish central nervous system depends on the differential competence of the epiblast, and is not imposed by organizer derived signals. This anteroposterior information is present throughout the epiblast in ectodermal cells that normally give rise both to neural and non-neural derivatives. Because of this information, organizer tissues transplanted to the ventral side of the embryo induce neural tissue but the anteroposterior identity of the induced neural tissue is dependent upon the position of the induced tissue within the epiblast. Thus, otx2, an anterior neural marker, was only ever induced in anterior regions of the embryo, irrespictive of the position of the grafts. Similarly, hoxa-1, a posterior neural marker was induced only in the posterior regions. Furthermore, the boundary of each ectopic expression domain on the ventral side was always at an equivalent latitude to that of the endogenous expression of the dorsal side of the embryo. The anteroposterior specification of the epiblast is independent of the dorsoventral specification of the embryo because neural tissues induced in the ventralized embryos also showed anteroposterior polarity. Cell transplantation and RNA injection experiments showed that non-axial marginal mesoderm and FGF signalling is required for anteroposterior specification of the epiblast. However, the requirement for FGF signalling is indirect in that cells with compromised ability to respond to FGF can still respond to anteroposterior positional information.