During gastrulation in sea urchins, cells at the tip of the archenteron extend filopodia that attach to the blastocoel wall and are thought to assist in the elongation of the archenteron. Upon completion of gastrulation, these cells migrate into the blastocoel. Time-lapse video records were made of preparations from which ectodermal cells have been removed, leaving the archenteron, mesenchyme cells and blastocoelar extracellular matrix (ECM) bounded by the basal lamina. In preparations of late gastrulae, cells at the tip of the archenteron extend filopodia that attach to the basal lamina and pull it inward, collapsing the preparation. This collapse does not occur in preparations made prior to the elongation phase and can be inhibited with cytochalasin B and azide, but not with colchicine. Migratory behavior increased in preparations treated with the laminin-derived peptide Tyr-Ile-Gly-Ser- Arg (YIGSR). Cells extend and retract filopodia, collapse the ECM and migrate out of the preparation. This behavior was not observed in preparations treated with whole laminin, fibronectin or Arg-Gly- Asp-Ser (RGDS) peptides. Cells in preparations treated with YIGSR extend significantly more processes than those incubated in RGDS, laminin, fibronectin or BSA. This effect is titratable between 10–3 and 10–6 M. Whole laminin has a significant inhibitory effect on the number of cell processes observed. Double labelling experiments with biotinylated laminin or biotinylated CDPGYIGSR and a mesenchyme-specific monoclonal antibody (Sp12) reveal that laminin and CDPGYIGSR label mesenchymal and non-mesenchymal cells. A CDPGYIGSR affinity column binds a 125I-labelled cell surface component, which elutes with YIGSR and has an Mr of about 80x103 on SDS-PAGE. We propose that cells at the tip of the archenteron attach to the basal lamina during archenteron elongation, and that domains of laminin containing YIGSR in the basal lamina of the target region stimulate migratory behavior in these cells.

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