Horizontal and vertical pathways in neural induction

The regional pattern of the developing nervous system is established early in development. Cues for regionalization of the neuroectoderm might originate from two sources: a vertical signal from the underlying mesoderm, or a signal transmitted horizontally within the neuroectoderm. Recently, these questions have been examined in the Xenapus embryo using molecular markers that are expressed at different positions along the anteroposterior neuraxis. Anterior and posterior mesoderm differ in their ability to induce expression of these genes, emphasizing the role of mesoderm in regional patterning. Another recent study confirmed the ability of the mesoderm to confer regional identity. Nevertheless, other molecular data are explicable only in terms of a contribution of a spreading influence within the plane of the neuroepithelium