Parasitic forms of a myxosporean in the kidney of the arctic lamprey, Lampetra japonica: an ultrastructural study

We found frequent and unique parasitism by an unidentified myxosporean in the kidney of the arctic lamprey, Lampetra japonica, living in Japan. Trophozoites (pseudoplasmodia with or without sporoblasts) existed predominantly in the lumina of proximal urinary tubules, but were rarely found in any other regions of the kidney. Since no mature spores were produced in the trophozoites, exact identification of the species was impossible. Two parasitic forms were recognized in proximal urinary tubules: one adhering to the epithelial cells of renal tubules, and the other free-floating in the lumina of tubules. Ultrastructurally, the attaching trophozoites developed microvilli-like projections towards the apical surface of epithelial cells and vigorously interdigitated with microvilli of the brush border. In contrast, the whole surface of the floating trophozoite was smooth without any cell projections. The developed projections in the former type of trophozoite may contribute to their firm attachment to the epithelial cells and/or to absorption of nutrients via the epithelial cells. Against the myxosporean infection, the lamprey as the host exhibited a local immune reaction by disposition of numerous lymphocytes and macrophages into the epithelium of urinary tubules.