Formation of sea urchin primary mesenchyme: cell shape changes are independent of epithelial detachment

Primary mesenchyme cells in sea urchin embryos emerge from the epithelium of the blastula wall and move into the blastocoel in a process referred to as ingression. Ingressing cells are bottle-shaped with their basal end protruding into the blastocoel and their apical end narrowed into a thin strand. Soon after becoming bottle-shaped, the ingressing cells move into the blastocoel and form the primary mesenchyme which ultimately assembles the skeleton of the larva. Primary mesenchyme formation requires zygotic gene expression but the pathways that lead from gene expression to ingression are not known. In this study cordycepin, an inhibitor of mRNA adenylation, and thus translation, is used to determine when ingression becomes sensitive to disruption of gene expression. When added to embryos 4 h before the onset of ingression, cordycepin inhibits the release of mesenchyme cells from the epithelium. Interestingly, though, it does not inhibit the shape changes that mark the onset of ingression. If added 2 h before the onset of ingression, cordycepin does not interfere with the formation of primary mesenchyme. These results indicate that both cell shape changes and the detachment process require zygotic transcripts and that the shape changes can occur independent of detachment.