We imaged 'papilla cells' in the skin pores of ampullary organs in electroreceptors on the rostrum of freshwater paddlefish (Polyodon spathula), by lectin labeling and fluorescence stack imaging of frozen sections. >50,000 ampullary organs, each 0.3-0.5 mm deep, are embedded in special skin covering the rostrum; their skin pores offer invasion routes into the body interior. We imaged numerous small papillas, of lengths 6-31 microns, lining the luminal surfaces of the narrowed neck and skin pore, at the entrance to each ampullary organ. Lectins WGA or SBA labeled their surfaces. Each papilla led to an oblong base containing a nucleus, embedded in the cuboidal epithelium of the neck wall; the papillas were anucleate. Our immunolabeling of junctions between papilla cells for ZO1, TJP2, occludin, tricellulin, phalloidin, or collagen-1 was consistent with zona occludens. We confirmed the two-part morphology of papilla cells by acute dissociation of individual neck epithelial cells (using two methods), which showed a conical papilla, angled (on some) relative to a nucleated base. We propose that papilla cells may be defensive in function, strategically located at the skin pore and neck to deter entry of pathogens into ampullary organs, including microbes or small parasites.
Keywords: Ampullary organ; Ducted cutaneous gland; Electrosensory receptor; Epithelial topology; Innate defense; Papilla.
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