Abstract
We review a recently discovered developmental mechanism. Anterior-posterior positional information for the vertebrate trunk is generated by sequential interactions between a timer in the early nonorganizer mesoderm (NOM) and the Spemann organizer (SO). The timer is characterized by temporally collinear activation of a series of Hox genes in the early ventral and lateral mesoderm (i.e., the NOM) of the Xenopus gastrula. This early Hox gene expression is transient, unless it is stabilized by signals from the SO. The NOM and the SO undergo timed interactions due to morphogenetic movements during gastrulation, which lead to the formation of an anterior-posterior axial pattern and stable Hox gene expression. When separated from each other, neither the NOM nor the SO is able to induce anterior-posterior pattern formation of the trunk. We present a model describing that the NOM acquires transiently stable hox codes and spatial collinearity, and that morphogenetic movements then continually bring new cells from the NOM within the range of SO signals that cause transfer of the mesodermal pattern to a stable pattern in neurectoderm and, thereby, create patterned axial structures. In doing so, the age of the NOM, but not the age of the SO, defines positional values along the anterior-posterior axis. We postulate that the temporal information from the NOM is linked to mesodermal Hox expression. The role of the SO for trunk patterning turns out to be the induction of neural tissue as prerequisite for neural hox patterning. Apparently, development of a stable anterior-posterior pattern requires neural hox patterning. We believe that this mechanism represents a developmental principle.
Highlights
While still at school, most of us are deeply impressed by the underlying principles that so beautifully explain why the chemical elements are ordered as they are in the periodic table and may wonder, with the influential innovator Brian Goodwin[1], whether there might be powerful principles that account for the awe-inspiring diversity of body forms in the living realm
Whereas in Drosophila this anterior-posterior patterning mechanism acts during a static phase of development, anterior-posterior patterning in vertebrates is established during gastrulation, a highly dynamic period with a series of complex morphogenetic movements
The sections that follow discuss a novel dynamic mechanism that patterns the trunk region of the vertebrate main body axis. This mechanism has three aspects: temporally collinear expression of Hox genes in gastrula nonorganizer mesoderm (NOM); interaction between the NOM and the Spemann organizer (SO); vertical signaling from gastrula NOM to neurectoderm
Summary
Most of us are deeply impressed by the underlying principles that so beautifully explain why the chemical elements are ordered as they are in the periodic table and may wonder, with the influential innovator Brian Goodwin[1], whether there might be powerful principles that account for the awe-inspiring diversity of body forms in the living realm. The morphogen can be an intracellular protein, e.g., a transcription factor, but is more usually a low-molecular-weight signal molecule This idea has been proposed and evidence has been presented in a large number of cases, there are only a couple of cases where the evidence is unambiguously convincing. The sections that follow discuss a novel dynamic mechanism that patterns the trunk region of the vertebrate main body axis. This mechanism has three aspects: temporally collinear expression of Hox genes in gastrula nonorganizer mesoderm (NOM); interaction between the NOM and the Spemann organizer (SO); vertical signaling from gastrula NOM to neurectoderm
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