Physiological and morphological studies on the madreporite of echinoderms

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The madreporite is an organ unique to the echinoderms. It connects the water-vascular system to the external sea water through ciliated pores. The physiological function of the madreporite, even the direction of water movement, if any, through the madreporite, has not yet been elucidated. To investigate the function of the madreporitel, we studied the external circular openings of the madreporite with electron microscopy and also the effect of chemical stimulation on the pore size of the madreporite. When fresh, unfixed and uncoated madreporites of Hemicentrotus pulcherrimus were observed with a low-vacuum scanning electron microscope (Akashi WS- 250, "WET-SEM"), its circular openings showed a wide variation in size while the pores in the surrounding ossicles were almost constant in size. This suggests that each pore can change its diameter. To see if the pore size changes in response to pharmacological stimulation, we studied the effect of chemicals on the isolated aboral piece of madreporite. Acetylcholine at > 10^<-9> M induced a reversible pore constriction. A high concentration of K^+ also induced a similar response, but adrenaline was without effect.These findings suggest that the size of each pore is under a cholinergic nervous control. Morphometrical analyses of the pores fixed in both open and closed states suggested that both the outer and the inner diameters of the pore canal decrease during the pore closure. Although the existence of a sphincter-like structure might have been expected, no such structure has not yet been found among the epidermal cells covering the outer surface of the madreporite or lining the pore canals by transmission electron microscopy. We are now developing a new procedure to isolate the epidermal cell layer from the madreporite which is treated with collagenase. This new preparation appears to be useful for further morphological and physiological studies of the madreporite.