Electron microscopic studies on the myofibrils in the epithelial cells of the Bowman's capsule and of proximal tubules in rat renal cortex (first report).

Two kinds of filaments in parallel arrangement have been observed in the basal part of the cytoplasm in the epithelial cells of a Bowman's capsule and in the epithelial cells of a proximal tubule of a glycerinated rat kidney. The first was thin and linear in shape, 55-70 A in diameter and exhibited arrow-head structures upon treatment with heavy meromyosin (HMM). The other was rod shaped, with thick filaments, 110-130 A in diameter, 0.14-0.16 micrometer in length, few in number scattered among the thin filaments. From the non-glycerinated kidney, bundles of fibers were attached to some electron-dense regions inside the cell membrane. In highly magnified pictures, two kinds of filaments could be observed. One kind was thin, attached to dense regions, and the other thick, and not attached to dense regions. It is conceivable that these filaments correspond to myofibrils from their characteristic electron microscopical features. An electron microscopical analogy detected in both smooth muscle cells and renal cells leads us to speculate that these thin and thick filaments consisted of actin and myosin respectively.