Swim bladder gas gland cells produce surfactant: in vivo and in culture

Electron microscopical examination of gas gland cells of the physostome Eur opean eel (Anguilla anguilla) and of the physoclist perch (Perca fluviatili s) revealed the presence of significant numbers of lamellar bodies, which a re known to be involved in surfactant secretion. In the perch, in which the gas gland is a compact structure and gas gland cells are connected to the swim bladder lumen via small canals, lamellar bodies were also found in fla ttened cells forming the swim bladder epithelium. Flat epithelial cells are absent in the eel swim bladder, in which the whole epithelium consists of cuboidal gas gland cells. In both species, Western blot analysis using spec ific antibodies to human surfactant protein A (SP-A) showed a cross-reactio n with swim bladder tissue homogenate proteins of similar to 65 kDa and in the eel occasionally of similar to 120 kDa, probably representing SP-A-like proteins in a dimeric and a tetrameric state. An additional band was obser ved at similar to 45 kDa. Western blots using antibodies to rat SP-D again resulted in a single band at similar to 45 kDa in both species, suggesting that there might be a cross-reaction of the antibody to human SP-A with an SP-D-like protein of the swim bladder tissue. To localize the surfactant pr otein, eel gas gland cells were cultured on permeable supports. Under these conditions, the gas gland cells regain their characteristic polarity. Elec tron microscopy confirmed the presence of lamellar bodies in cultured cells , and occasionally, exocytotic events were observed. Immunohistochemical st aining using an antibody to human SP-A demonstrated the presence of surfact ant protein only in luminal membranes and in adjacent lateral membranes. On ly occasionally, evidence was found for the presence of surfactant protein in lamellar bodies.