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

Ultrastructural observations on structures associated with the 'haplosporosome'-like electron-dense bodies (EDBs) of the primary cell of the extrasporogonic stage of PKX are described. Observations made include formation of EDBs by the trans-Golgi network, an additional membrane associated with EDB structure, confronting cisternae, engulfment and presence of EDBs in multivesicular bodies, fusion of EDBs with the plasmalemma, degeneration of EDBs in disintegrating primary cells and endocytosis of PKX cytoplasm by adherent macrophages. Immunogold localisation of a PKX-specific monoclonal antibody (MAb A3) suggests that the EDBs contain periodate-sensitive carbohydrates on their membranes. Tissues prepared for immunogold electron microscopy further suggest that some contain a lipid-rich core. An interpretation is made on their possible function and their relationship with the haplosporosomes and sporoplasmasomes found in members of the Haplosporidea and Myxosporea respectively.