Meeting Abstract
P1.98 Saturday, Jan. 4 15:30 Evolution of ectoderm-mesoderm communication during skeletal patterning in echinoid larvae LYONS, D/C; MARTIK, M/L; KIMURA, J/O*; MCCLAY, D/R; Duke University dcl.duke@gmail.com
The life cycle of indirect-developing echinoids (e.g. sea urchins and sand dollars) includes a ciliated pluteus larval stage supported by a calcium carbonate endoskeleton. The development of this skeleton is a classic model system for studying cell differentiation, tissue patterning, and morphogenesis, and for studying the evolution of patterning as larval skeletons are an apomorphy for the group, and each species has a unique skeletal pattern. Using immunohistochemistry, live cell imaging, and inter-species chimeras, we compared larval skeleton formation between sea urchin and sand dollar. The sea urchin skeleton arises from primary mesenchyme cells (PMCs) that form a syncytium within which the skeleton is secreted, then branches, and elongates. The pattern of the skeleton is determined by a conversation between the PMCs and the immediately overlying ectoderm. During early gastrula stages, discrete bilateral patches of ectodermal cells produce VEGF and FGF signals that attract clusters of PMCs, which produce a triradiate spicule rudiment. Later, a re-deployment of VEGF and other unidentified signals emanating from the ectoderm instruct the PMCs to elaborate the triradiate by growth and branching, resulting in a skeleton with a highly reproducible pattern. In contrast, in the sand dollar, it appears that there has been a heterochronic shift relative to the urchin in which the ectodermal signals for triradiate and branching skeletogenesis occur coincident at gastrula stages. In the sand dollar, there exists an extra, more anterior half ring of PMCs on the dorsal side, which corresponds with the location of branching. This suggests that ectodermal signals that control the spatiotemporal pattern of underlying PMCs have diverged between these two echinoids. Such data provide the framework for ongoing studies in both species that address the details of these two patterning systems at the molecular level.
