The initial phase of gastrulation in sea urchins is accompanied by the formation of bottle cells.

The morphogenetic processes responsible for the initial phase of gastrulation in sea urchins have yet to be satisfactorily defined. Using conventional and confocal microscopy we have analyzed the buckling of the vegetal plate to form the archenteron in embryos of Strongylocentrotus purpuratus. The cells of the vegetal plate elongate and a ring of 34 to 36 bottle cells forms within the vegetal plate during invagination. Rhodamine phalloidin staining reveals a reorganization of the actin cytoskeleton associated with these changes in cell shape. During buckling, the ring of bottle cells within the vegetal plate fluoresce intensely at their apical surface and in the narrow neck region. Ionophore A23187 induces precocious buckling and the formation of the ring of bottle cells. The calcium channel blocker verapamil and the calmodulin inhibitor trifluoperazine reversibly inhibit buckling and the formation of the ring of bottle cells. Treatment with antibodies to the apical lamina, which interferes with the initial stage of gastrulation, blocks the appearance of the vegetal plate phalloidin staining. Measurements of the dimensions of cells and an analysis of shape changes suggest that the formation of bottle cells reduces the surface area of the vegetal plate by more than 50%. We propose that actin-mediated changes in cell shape within the vegetal plate are responsible for producing forces which cause buckling of the vegetal plate during the initial phase of gastrulation.