Withington, Sarah; (2000) Analysis of neural induction and patterning in amniote embryos. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

When grafted into a host embryo, Hensen's node, the chick gastrular organiser, is able to induce an ectopic second neural axis. The use of pan-neural and regionally specific neural genes, particularly forebrain markers such as BF-1 and GANF, shows that the chick node is able to induce a nervous system with complete anterior pattern, expressing all the markers tested. Evidence in mouse suggests that additional signalling information, established separately from the node and its derivatives, is required to generate complete anterior pattern. The ability of mouse node grafts to induce the same range of neural markers in chick hosts has therefore also been examined. In this assay, the mouse node is not able to induce expression of chick forebrain markers, and an anteriorly truncated second axis is formed. This work has also investigated the role of the foregut endoderm in patterning the anterior brain of the chick. This tissue is formed by cells that move through the node into the lower layer during gastrulation. Removal of the lower layer at stage 4 has no apparent effect. However, removal of foregut endoderm during the early head process stages (4+ to 5) causes reduction in forebrain pattern. By the 12 somite stage, most neuraxes lack telencephalon and eyes, and BF-1 and GANF expression domains are absent or severely reduced. This syndrome is preceded by a failure to establish normal FGF 8 expression in the anterior neural ridge signalling centre, at early somite stages. However, gene expression throughout axial mesoderm {BMP 7, chordin and shh) appears unaffected in all embryos. The homeobox gene hex and the chick Frzb homologue crescent are both expressed in the anterior definitive endoderm at the time when removal of this tissue results in forebrain defects. These results suggest that the definitive foregut endoderm contains information crucial for forebrain patterning in the chick embryo.

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