Surface Ultrastructure of the Gill Arch of the Killifish, Fundulus Heteroclitus, From Seawater and Freshwater, With Special Reference to the Morphology of Apical Crypts of Chloride Cells

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The surface ultrastructure of the gill arches of the killifish, Fundulus heteroclitus, adapted to seawater or freshwater, was found to be similar to that reported for other euryhaline teleosts. Two rows of gill filaments (about 42 filaments per row) extended posterolaterally, and two rows of gill rakers (about 10 rakers per row) extended anteromedially from each arch. Leaf‐like respiratory lamellae protruded along both sides of each filament, from its base to its apex. The distributions, sizes, and numbers of various surface cells and structures were also determined. All surfaces were covered by a mosaic of pavement cells, which measured about 7 × 4 μm and exhibited concentrically arranged surface ridges. Taste buds were especially prominent on the rakers and the pharyngeal surfaces of the first and second gill arches, but were often replaced by horny spines on the third and fourth gill arches. Apical crypts of chloride cells occurred mostly on the surfaces of the gill filaments adjacent to the afferent artery of the filament. In seawater adapted killifish, crypts resembled narrow, deep holes along the borders of adjacent pavement cells, had openings of about 2 μm2, and occurred at a frequency of about 1 per 70 μ2 of surface area. In freshwater fish, the crypts usually had larger openings (about 10 μ2), occurred less frequently (1 per 123 μ2), and exhibited many cellular projections in their interiors. Changes in crypt morphology may be related to the ion transport function of chloride cells.