Koshida S, Shinya M, Mizuno T, Kuroiwa A, Takeda H
Division of Biological Science, Graduate School of Science, Nagoya University, Chikusa-ku, Nagoya, Japan.
Development. 1998 May;125(10):1957-66. doi: 10.1242/dev.125.10.1957.
Analyses 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, irrespective 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.
利用两栖类胚胎进行的分析表明，中枢神经系统的诱导和前后模式形成是由组织者及源自组织者的轴向中胚层产生的信号启动的。然而，我们在此表明，斑马鱼中枢神经系统的初始前后模式取决于外胚层细胞的不同感受态，而非由源自组织者的信号强加形成。这种前后信息存在于整个外胚层中那些通常会产生神经和非神经衍生物的外胚层细胞中。由于这种信息的存在，移植到胚胎腹侧的组织者组织会诱导神经组织的形成，但诱导产生的神经组织的前后身份取决于该诱导组织在外胚层中的位置。因此，otx2（一种前部神经标记物）仅在胚胎的前部区域被诱导产生，而与移植的位置无关。同样，hoxa - 1（一种后部神经标记物）仅在后部区域被诱导产生。此外，腹侧每个异位表达域的边界始终与胚胎背侧内源性表达的纬度相当。外胚层的前后特化独立于胚胎的背腹特化，因为在腹化胚胎中诱导产生的神经组织也显示出前后极性。细胞移植和RNA注射实验表明，非轴向边缘中胚层和FGF信号对于外胚层的前后特化是必需的。然而，对FGF信号的需求是间接的，因为对FGF反应能力受损的细胞仍然能够对前后位置信息做出反应。
