Neural induction and antero-posterior patterning in the amphibian embryo: past, present and future

Abstract

Neural induction and patterning in competent ectoderm occurs during gastrula and early neurula stages in response to signals from dorsal mesoderm. The earliest views of antero-posterior (A-P) patterning were modified beginning in the 1930s, as complexities concerning the timing of the pattern-forming process and potential sources of the patterning signals were revealed. In the 1950s and 1960s several different models for A-P patterning were proposed, all of which, however, bear a number of similarities, including a two-component system for generating A-P axial information in the embryo. Early attempts to identify neural-inducing molecules were largely unsuccessful due to technical limitations in biochemical analyses and concerns about assaying neural responses. The advent of modern molecular genetic technology has permitted more precise tests of a number of classic observations about the timing of A-P patterning and the sources of patterning signals. While some early observations have been confirmed, a number of new concepts have emerged in recent years, particularly concerning the source of patterning signals in the embryo. Striking progress has been made in identifying putative neural-inducing molecules, and recent experiments have begun to suggest how these might contribute to A-P patterning. While the successes in recent years have been revealing, many of the classic issues concerning neural induction and patterning remain essentially as they were when first defined many decades ago. The power of modern molecular genetics, however, should permit many of these issues to be significantly clarified in the decades to come.

Six1 promotes a placodal fate within the lateral neurogenic ectoderm by functioning as both a transcriptional activator and repressor

“…Wnts, FGFs) (Gould and Grainger, 1997;Gamse and Sive, 2000) influence where the PPE forms. To test this, endogenous Wnt signaling was reduced in the LNE of the intact embryo by co-expressing either a Wnt1 family antagonist (Frzb-1) or a dominant-negative form of Wnt8 (dnWnt8) with Noggin.…”

Towards a cellular and molecular understanding of neurulation

“…The details of neural induction are beyond the scope of this review (reviewed by Lemaire and Kodjabachian, 1996;Tanabe and Jessell, 1996;Gould and Grainger, 1997;Hemmati-Brivanlou and Melton, 1997;Sasai and Robertis, 1997;Weinstein and Hemmati-Brivanlou, 1999;Harland, 2000;Jessell and Sanes, 2000). However to summarize, recent studies have shown that neural induction actually involves suppression of an epidermal fate rather than induction of a neural fate, so that the default state of the naive ectoderm is neural, not epidermal as suggested by classical studies.…”

Anteroposterior Patterning in Xenopus Embryos: Egg Fragment Assay System Reveals a Synergy of Dorsalizing and Posteriorizing Embryonic Domains

“…Perhaps as striking as the induction of the surrounding host tissue adjacent to transplanted dorsal mesoderm was the near normal anteroposterior (A-P) axis present within the secondary embryo. This led to the idea that the dorsal mesoderm has the ability not only to induce neural tissue but also to organize its A-P pattern (Gould and Grainger, 1997). This A-P patterning has long been thought to be induced by the combined action of two signals from the SpemannMangold organizer (Nieuwkoop, 1952;Toivonen and Saxén, 1968;Sasai and De Robertis, 1997).…”