Catch connective tissue is collagenous connective tissue that changes its mechanical properties rapidly. Calcium has been assumed to play a key role in the catch mechanism because calcium concentration profoundly affects the mechanical properties. We have found a calcium-storage cell, the vacuole cell, in the dermis of the sea cucumberStichopus chloronotusa wellstudied catch connective tissue. The processes of this cell are packed with vacuoles measuring 0.1–1.5 μm in diameter. The ultrastructure was compared in a relaxed state and in a catch (stiff) state induced by high-potassium artificial sea water. When fixed with pyroantimonate, the vacuoles contained precipitates in the relaxed state but not in the catch state. Analysis with an energy-dispersive X-ray microanalyzer suggested that the precipitate contains calcium. The release of calcium ions from the vacuole cell is proposed to induce connective tissue cat
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