Cellular basis of gastrulation in the sand dollar Scaphechinus mirabilis

The processes of gastrulation in the sand dollar Scaphechinus mirabilis are quite different from those in regular echinoids. In this study, we explore d the cellular basis of gastrulation in this species with several methods. Cell-tracing experiments revealed that the prospective endodermal cells wer e convoluted throughout the invagination processes. Histological observatio n showed that the ectodermal layer remained thickened, and the vegetal cell s retained an elongated shape until the last step of invagination. Further, most of the vegetal ectodermal cells were skewed or distorted. Wedge-shape d cells were common in the vegetal ectoderm, especially at the subequatoria l region. In these embryos, unlike the embryos of regular echinoids, second ary mesenchyme cells did not seem to exert the force to pull up the archent eron toward the inner surface of the apical plate. In fact, the archenteron cells were not stretched along the axis of elongation and were in close co ntact with each other. Here we found that gastrulation was completely block ed when the embryos were attached to a glass dish coated with poly-L-lysine , in which the movement of the ectodermal layer was inhibited. These result s suggest that a force generated by the thickened ectoderm, rather than rea rrangement of the archenteron cells, may play a key role in the archenteron elongation in S. mirabilis embryos.