![]() ![]() In addition, several studies have established that specific cell populations in the neural plate itself influence the fate of their neighbours: a recent report identified the anteriormost row of neurectoderm cells as a source of signals with forebrain patterning properties ( Houart et al., 1998). The prechordal plate and surrounding mesendoderm which come underneath the anterior neurectoderm are other well-known sources of inducing molecules with later patterning activities. Evidence is accumulating that anterior specification relies on intrinsic properties of the neurectoderm (in fish, Koshida et al., 1998), as well as on a competence to respond to inducing signals originating from several organising centres, such as the embryonic shield in fish ( Shih and Fraser, 1996 Grinblat et al., 1998). It was first proposed that the neurectoderm is initially specified as anterior and then oriented to more posterior fates along the anteroposterior (AP) axis ( Nieuwkoop, 1950 Toivonen and Saxen, 1968). Complex cellular interactions progressively lead to the definition of domains within the neural plate, which become evident by specific patterns of gene expression. Our data demonstrate that this change can be mimicked by ectopic engrailed expression alone.Ī major event in the establishment of the vertebrate body plan is the specification and patterning of the neurectoderm. A competence of the diencephalon to change to a midbrain fate has been demonstrated in isthmic graft experiments. Therefore, we propose that Ol-eng2 induces de novo formation of an isthmus-like structure, which correlates with the development of ectopic midbrain structures, including optic tectum. Furthermore, the endogenous Ol-eng2, Pax2, Wnt1 and Fgf8, which are essential components of the MHB genetic cascade, were ectopically expressed in this region. In contrast, expressions of markers of the diencephalon were strongly repressed in injected embryos. No change in the patterns of mesencephalic and telencephalic markers were observed. We found that the ectopic expression of Ol-eng2 predominantly results in an altered development of the anterior brain, including an inhibition of optic vesicle formation. We have monitored the effects of its widespread expression following mRNA injections in 1- and 2-cell medaka and Xenopus embryos. Here we report the isolation of Ol-eng2, a medaka fish ( Oryzias latipes) engrailed gene. In contrast, little is known about the effect of an overexpression of engrailed genes in vertebrate development. Knock-out experiments in mice demonstrated that these genes are essential for the development of this structure and of its derivatives. In vertebrates, the engrailed genes are expressed at early neurula stage in a narrow stripe encompassing the midbrain-hindbrain boundary (MHB), a region from which a peculiar structure, the isthmus, is formed. ![]()
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