Structure and function of the pore canals of the sea urchin madreporite

Abstract

The madreporite is one of the most enigmatic organs of the echinoderms. It connects the internal cavity of the water-vascular system to the external seawater through its many pores which are lined with ciliated epithelium. Its physiological function, even the nature of transport, if any, through its pores has been controversial since the 19th century. We report here that the pores of the echinoid madreporite are capable of changing their diameter in response to stimulation and that their cilia support bidirectional transport, drawing water into the water-vascular system while expelling larger particles. A reversible constriction of the pore was induced by acetylcholine (ACh). At > 10 -7 M, ACh reduced the pore diameter to about 70% in 2 mins and to 60% in 6 mins. Atropine (10 -4 M), but not d -tubocurarine (10 -4 M), blocked the response to ACh (10 -7 M). Adrenaline (10 -5 M) had no effect on the pore diameter. These results suggest that the size of the pore is under cholinergic control. Observations of isolated pore-canal tissue indicated that the changes in pore size are not accompanied by changes in volume of the cells surrounding the pores. Electron microscopy showed no muscle cells in or near the pore-canal tissue. In the apical region of the ciliated columnar epithelial cell lining the pore canal, there are fine filaments attached to the adherens junctions. This region of the cell was positively stained with rhodamine-phalloidin, suggesting that the filaments consist of F-actin. It is possible that the filaments are involved in the pore closure response. We studied the water flow through isolated single pore canals, using Indian ink, and found that the water flows inwards while particles larger than ca. 1 μM move in the opposite direction. When the pores became narrower by the action of ACh, complete blocking of the water flow was observed. It is possible that the madreporite controls the volume or the pressure of the fluid in the water-vascular system by both the ciliary-driven water flow and the pore closure response.