It is the most common cause of eumycetoma in the Sudan. In clinical material, the grains in the tissue are black and numerous. In stained sections, the grain is rounded, oval or trilobed. It has a more compact cortex, which is dark brown in color due to pigment produced by the organism and has a lighter medulla. In some grains, the division into cortex and medulla is not evident. The grain filaments are usually embedded in a hard brown cement matrix.
Two main morphological types of grains are identified. The filamentous, which is the commonest type, consists of brown septate and branched hyphae that may be slightly more swollen towards the periphery. In the cortex, the filaments are arranged radially while in the medulla they tend to run multi-directionally. Round or oval cells, 7-15 um in diameter, are seen, particularly in the periphery.
The second type of grain is the vesicular one. It is less common than the filamentous and is composed of unusually large cells that look like vesicles. Both types of the grain can be found in the same lesion. Grains that are partially vesicular and partially filamentous are not uncommonly seen.
The surface of the grain may be scalloped giving the periphery a hob-nail appearance and a brightly eosinophilic layer of fibrin–like material sometimes covers the grain. This layer is of host origin, contains fibrin, immunoglobulins and complement. A similar material is seen around parasites as Hoepple phenomenon. Occasionally a thin layer of a haematoxophilic granular material is found on the surface of the grain. This material is Feulgen positive and is derived from the nuclei of disintegrating inflammatory cells surrounding the grains.