Abstract
Development and patterning of neural tissue in the vertebrate embryo involves a set of molecules and processes whose relationships are not fully understood. Classical embryology revealed a remarkable phenomenon known as vertical signalling, a gastrulation stage mechanism that copies anterior-posterior positional information from mesoderm to prospective neural tissue. Vertical signalling mediates unambiguous copying of complex information from one tissue layer to another. In this study, we report an investigation of this process in recombinates of mesoderm and ectoderm from gastrulae of Xenopus laevis. Our results show that copying of positional information involves non cell autonomous autoregulation of particular Hox genes whose expression is copied from mesoderm to neurectoderm in the gastrula. Furthermore, this information sharing mechanism involves unconventional translocation of the homeoproteins themselves. This conserved primitive mechanism has been known for three decades but has only recently been put into any developmental context. It provides a simple, robust way to pattern the neurectoderm using the Hox pattern already present in the mesoderm during gastrulation. We suggest that this mechanism was selected during evolution to enable unambiguous copying of rather complex information from cell to cell and that it is a key part of the original ancestral mechanism mediating axial patterning by the highly conserved Hox genes.
Highlights
Determination of regional specificity along the anterior-posterior (A-P) axis of the vertebrate Xenopus laevis and of all other vertebrates begins during gastrulation
We have shown previously that zygotic Hox gene expression is first initiated in the non-organizer mesoderm (NOM) in the Xenopus mid gastrula (St. 10.5) [20]
This expression spreads during gastrulation to the overlying prospective neurectoderm that was induced from embryonic ectoderm by signals from the Spemann organizer (SO)
Summary
Determination of regional specificity along the anterior-posterior (A-P) axis of the vertebrate Xenopus laevis and of all other vertebrates begins during gastrulation. Known signalling pathways have been proposed to be involved in transformation: these include retinoids [11,12,13], FGFs [14] and Wnts [15] Elevated concentrations of these signalling molecules cause posteriorisation by inducing relatively posterior positional values in the neurectoderm and each of these factors has been proposed to act as a posterior to anterior gradient within the embryo [8,9,10,11,12,13]. The use of exogastrulae and other approaches revealed a second type of signal It appeared that posterior neural markers were expressed only at the border between ectoderm and mesoderm in exogastrulae, excluding the existence of very extensive planar signalling [16]. The nature of the molecules involved in vertical signalling remains unclear
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