Pathology of the mucous coat of trout skin during an erosive bacterial dermatitis: a technical advance in mucous coat stabilization for ultrastructural examination

A fixation regime which combined cryopreservation, freeze drying and vapour fixation with osmium tetroxide, was found to preserve the mucous coat of trout skin for ambient temperature scanning electron microscopy. The regime was used to study changes to the mucous coat of trout skin during a spontaneous outbreak of "columnaris" disease--a common dermatitis of commercial salmonids associated with the bacterial pathogen Cytophaga columnaris. Infected and damaged regions of skin were covered by a mucous coat which differed from that which covered adjacent unaffected areas. In unaffected areas, the mucous coat topography was smooth and relatively featureless. In contrast, the mucous coat which covered damaged areas was fissured, cratered and contained exfoliated epithelial cells. Nevertheless, this study showed that even sites of extensive dermal ulceration, in which mucous cells had been destroyed, retained a partial mucous coat. This suggests that mucus, after secretion, flows over the skin surface, rather than functioning only near the site of production. Because of the various protective functions attributed to the mucous coat, its partial presence over areas of skin damage would contribute to defence against secondary pathogens and to the prevention of excess movement of ions and water at these sites. The technical development in mucous stabilization, described in this paper, will provide a means for examining morphological changes to the mucous coat of fish skin in response to a range of stimuli in future studies.