Organization of the adluminal and retractor cells in the coelomic lining from the tube foot of a phanerozonian starfish, Luidia foliolata

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Abstract
The coelomic lining in the tube feet of the phanerozonian starfish Luidia foliolata is a complex pseudostratified myoepithelium consisting largely of flagellated adluminal cells and myofilament-bearing retractor cells. The adluminal cells, joined by zonular intermediate and septate junctions, line the water–vascular canal. Basal processes of the adluminal cells penetrate the underlying layers of retractor cells to terminate as bulbous pedicels at the myoepithelial basal lamina. The longitudinally oriented retractor cells, linked by macular and fascial intermediate junctions, also direct processes obliquely to the basal lamina for anchorage. In keeping with its myoepithelial classification, the coelomic lining of the tube foot is devoid of connective tissue. The organization of the coelomic lining in this primitive species has not substantively improved our understanding of the events of excitation–contraction coupling. Both the coelomic lining and the podial connective tissue lack nerves, and there is no ultrastructural evidence of communicating (gap) junctions between the retractor cells. The sarcoplasmic reticulum of a retractor cell is a plexiform network of agranular cisternae lodged between the contractile apparatus and the sarcolemma. Peripheral couplings between the sarcoplasmic reticulum and the sarcolemma can be identified in ultrathin sections by the presence of electron-dense plaques in the sarcoplasmic gaps between the apposed membranes. In freeze-fracture replicas, the placement of these structural couplings is correlated with aggregations of large intramembranous particles over regions of cisternal confluence. Conceptual problems in understanding excitation–contraction coupling in a modern starfish are compounded in this primitive representative by the presence of many more layers of retractor cells and by a burrowing life-style which requires the tube feet to respond in a rapid, highly coordinated manner.

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