|Budget Amount *help
¥2,500,000 (Direct Cost : ¥2,500,000)
Fiscal Year 1997 : ¥1,100,000 (Direct Cost : ¥1,100,000)
Fiscal Year 1996 : ¥1,400,000 (Direct Cost : ¥1,400,000)
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.
これらの実験結果を踏まえて、ゼブラフィッシュにおける神経誘導モデルを提案した。このモデルでは、神経系の前後軸形成は、オ-ガナイザーを含めた中軸中胚葉から分泌される一般的な神経誘導因子(Chordinなど)と、胚盤周縁の中胚葉(non-axia1 mesoderm)からの後方化シグナルの組み合せで起こっている。従って、同じ神経誘導因子に対して、胚盤周縁に近いepiblastでは後方化シグナルを受けているために、後方の神経マーカー(hoxa-1など)を発現し、後方化シグナルが到達しない動物極周囲のepib1astでは前方の神経マーカー(otx2など)を発現することになる。このモデルは、従来の中軸中胚葉を中心とした神経誘導モデルに修正を迫るものである。 Less