Ultrastructural features of osmotic shock in mussel gill cilia

Terminal gill cilia of the freshwater mussel Unio stop abruptly within 5 min (shock-stop) when distal gill filaments are placed in hypertonic (>217 ± mOsm) salt, urea or sucrose solutions. Electron microscopy of stopped cilia reveals that the axonemal complexes are obscured by an electron-dense matrix, although microtubule protofilaments are visible after fixation with tannic acid-glutaraldehyde. We conclude tubulin was not depolymerized because colchicine and vinblastine, inhibitors of tubulin polymerization, did not prevent stopped cilia from resuming beating in water. The cytosol of stopped cilia which resumed beating in the same solution or in water is less dense, and axonemal complexes have reappeared. Solutions of increasing tonicity retard or stop cilia, possibly due to aggregation of protein-rich cytosol around microtubules in the ciliary stalk because dense cytosol of stopped cilia is digested by pepsin and trypsin.