Improved visualization of apical vesicles in chloride cells of fish gills using an osmium quick‐fix technique

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Abstract
Quick‐fixation with buffered osmium is a well established technique for fixing rapid forms of cell motility, e.g., metachronal ciliary beat. One proposed mechanism of salt transport in the gills of saltwater fish is a vesicular shuttle in the apical tubulovesicular system of gill chloride cells. We decided to use the osmium quick‐fix technique to see if rapid fixation causes modification of observed gill ultrastructure. Gill chloride cells from two species of fish, Fundulus heteroclitus and Poecilia reticulata, were examined ultrastructurally under two conditions of fixation: (1) Conventional fixation in glutaraldehyde followed by osmium or (2) quick‐fixation for 20 seconds in osmium tetroxide buffered with Na cacodylate followed by conventional fixation. Following osmium quick‐fixation, the ultrastructure of the apical tubulovesicular system is substantially different, such that more vesicles are seen in the cortical region and membrane fusion events between vesicles and the apical membrane are much more prevalent. The appearance of other portions of the chloride cell, e.g., mitochondria, tubular reticulum, cell junctions, was identical under the two conditions of fixation. These results suggest that a rapid vesicular shuttle is present in the chloride cell apex. A model indicating how such a shuttle might be instrumental in transepithelial ion transport is presented and evaluated. We currently lack sufficient information to accept or reject any of the current models of ion transport. It is anticipated that the osmium quick‐fix technique will serve as a useful tool in further studies of the morphology, movements, and function of apical vesicles in fish chloride cells.