Observations on the electron-dense bodies of the PKX parasite, agent of proliferative kidney disease in salmonids

Ultrastructural observations on structures associated with the 'haplosporos ome'-like electron-dense bodies (EDBs) of the primary cell of the extraspor ogonic stage of PKX are described. Observations made include formation of E DBs by the trans-Golgi network, an additional membrane associated with EDB structure, confronting cisternae, engulfment and presence of EDBs in multiv esicular bodies, fusion of EDBs with the plasmalemma, degeneration of EDBs in disintegrating primary cells and endocytosis of PKX cytoplasm by adheren t macrophages. Immunogold localisation of a PKX-specific monoclonal antibod y (MAb A3) suggests that the EDBs contain periodate-sensitive carbohydrates on their membranes. Tissues prepared for immunogold electron microscopy fu rther suggest that some contain a lipid-rich core. An interpretation is mad e on their possible function and their relationship with the haplosporosome s and sporoplasmasomes found in members of the Haplosporidea and Myxosporea respectively.