ABSTRACT
We have recently shown that acetylcholine induces a reversible constriction of the external openings of the echinoid madreporite. In this research, we studied the mechanism of this pore-closure response using Hemicentrotus pulcherrimus. Observations made on the isolated soft tissue lining the madreporic pores (“pore tubes”) suggested that the tube does not change its volume during the pore-closure response. To see if any contractile structures are involved in the response, we studied the structure of the madreporite by transmission electron microscopy. The pore canal of the madreporite is mostly composed of slender ciliated epithelial cells. No muscle cell was found within or around the pore-canal tissue. The epithelial cells are connected to each other by adherens junctions and septate junctions. Filaments were found linking the adherens junctions and encircling the apical region of the epithelial cell. The regions where filaments were found were positively stained with rhodamine-phalloidin. This result suggests that the filaments consist of F-actin. It is possible that the pore-closure response is due to an apical constriction of the pore-canal cells in which actin filaments are involved.
