Ultrastructural and immunogenic changes in the formation of mycetoma grains.

The differences in the fine structure and antigenic determinants of mycetoma fungi in the mycelial phase in vitro and in grains in vivo facilitate an interpretation of grain formation. Aggregates of hyphal elements with multiple and thickened walls was a feature of the fungi in vivo. Associated with hyphal wall material, numerous polysaccharide microfibrils were detected in grains of Madurella mycetomatis. These were not seen associated with hyphal elements in vitro and it is suggested that these structures may be concerned with the aggregation of fungal elements in the formation of grains. Antibodies directed against the fungi in vitro were shown, by indirect immunogold labelling, to bind at identical sites in fungal material grown in vitro as in mycetoma grains. However the grain matrix was not labelled, suggesting that part of the structure formed in vivo is composed of modified antigen or is host derived.