Ultrastructural immunolocalization of the RS-like material in the holothurian nervous system. For an explanation of the difference in grain size (smaller grains in A, B inset, C and D and larger grains in B, E, and F), refer to the Methods section. A. Apical surface of a supporting glial cell of the ectoneural neuroepithelium covered with the RS-K10-positive secretion. Note also labelling of the electron-translucent vesicles in the apical cytoplasm of the cell. B. RS-K10-positive labelling of the apical surface of the epineural roof epithelium. The inset shows a labelled vacuole in the cytoplasm of a flattened glial cell. C, D. Apparent release of the immunoreactive material from secretory vacuoles into the lumen of the epineural canal by supporting glial cells (C) and cells of the roof epithelium (D), as revealed by immunogold reactions with the RS-K10 (C) and AFRU (D) antisera. E, F. Immunoreactive cell bodies (RS-K10-positive) within the ectoneural parenchyma. ec, epineural canal; if, bundle of intermediate filaments; v, vacuole. Scale bars = 1 μm in (A), (B), (F), 0.5 μm in (B inset), (C), (D), and (E).