Spreading ciliary arrest in a mussel gill epithelium: Characterization by quick fixation

Spreading ciliary arrest, induced by local laser microinjury, in freshwater mussel (e.g., Elliptio) gill lateral (L) cell cilia, has been characterized by quick fixation with osmium tetroxide, which permits the correlation of known features of the response with structural features of the gill epithelium. Quick fixation reliably preserves the state of the epithelium including the activity state of the L cilia at the moment of fixation. From a disrupted region, the stimulus that triggers arrest spreads outward along an undamaged filament preferentially from L cell to L cell for more than 300 μm to either side of the lesion. In physiological salt solutions transverse spread across the filament via heterologous cells is insufficient to elicit L ciliary arrest on the opposite side of the filament. The spread of arrest is dependent upon the structural integrity of the L epithelium, normally terminates at a boundary between adjacent L cells, and does not spread past a focal break. Arrest occurs asynchronously because cilia in different stroke positions respond to the stimulus with different time courses. The cilia stop in a uniform “hands up” position, i.e., pointing frontally. The arrest response is inhibited by reducing the concentration of extracellular Ca²⁺ (< 10⁻⁷ M) or by adding extracellular La³⁺ (1 mM) or K⁺ (15 mM). Recovery begins at the margin of a segment of arrested L cilia and spreads back toward the lesion at a constant initial velocity of ca. 60 μm/sec. About 300 μm from the lesion the recovery velocity rapidly falls to ca. 5 μm/sec. Recovery of ciliary beat precedes the recovery of metachronal coordination. Neither spread of the stimulus nor recovery require ciliary beat. The data support the hypothesis that the microinjury‐induced arrest is initiated by an injury potential that triggers a graded regenerative depolarization that is propagated electrotonically along the epithelium from L cell to L cell, triggering Ca²⁺ influx into the axoneme and consequent Ca²⁺‐induced L ciliary arrest as it spreads. A temporary non‐linear gradient of intracellular Ca²⁺ concentration is established along the injured L epithelial tract. As individual cells recover, they lower their intracellular Ca²⁺ concentration from pCa 5 to pCa 7 in about 10 seconds.