Abstract
The mechanisms imposing the Dorsal/Ventral (DV) polarity of the early sea urchin embryo consist of a combination of inherited maternal information and inductive interactions among blastomeres. Old and recent studies suggest that a key molecular landmark of DV polarization is the expression of nodal on the future ventral side, in apparent contrast with other metazoan embryos, where nodal is expressed dorsally. A subtle maternally-inherited redox anisotropy, plus some maternal factors such as SoxB1, Univin, and p38-MAPK have been identified as inputs driving the spatially asymmetric transcription of nodal. However, all the mentioned factors are broadly distributed in the embryo as early as nodal transcription occurs, suggesting that repression of the gene in non-ventral territories depends upon negative regulators. Among these, the Hbox12 homeodomain-containing repressor is expressed by prospective dorsal cells, where it acts as a dorsal-specific negative modulator of the p38-MAPK activity. This review provides an overview of the molecular mechanisms governing the establishment of DV polarity in sea urchins, focusing on events taking place in the early embryo. Altogether, these findings provide a framework for future studies aimed to unravel the inceptive mechanisms involved in the DV symmetry breaking.
Highlights
During development of bilaterian embryos symmetry breaking is imposed through establishing of distinct polarities, which are precursors of the larval axes
We have shown in P. lividus that the Hbox12 homeodomain repressor is expressed by prospective dorsal ectoderm cells, spatially facing and preceding the onset of nodal transcription, where it acts preventing the ectopic activation of nodal expression [51,52,53,54]
Once the expression of nodal is initiated by early inputs, the peculiar ventral localization of the nodal-expressing domain is probably consolidated by a reaction-diffusion system, direct evidence have to be provided in the sea urchin
Summary
During development of bilaterian embryos symmetry breaking is imposed through establishing of distinct polarities, which are precursors of the larval axes. In indirectly developing sea urchins, embryonic patterning along the dorsal/ventral (DV) axis, known as oral/aboral axis, has been extensively studied in various species. Nodal signaling provides a fundamental driver input for specifying DV asymmetries [17,18], differently with respect to other metazoan embryos, in the sea urchin Nodal operates on the ventral rather than on the dorsal side. DV polarity becomes morphologically recognizable from the onset of gastrulation, when the embryo begins to flatten on the ventral side, and two bilaterally symmetric thickenings of the ectoderm form on ventrolateral regions (Figure 1). One can detect many earlier signs of DV asymmetries and there are a number of early molecular indicators of these properties
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