Electron microscopy and x-ray microanalysis of calcium-binding sites of the plasma membrane of Beroe giant smooth muscle fibre.

When they are fixed with glutaraldehyde in the presence of calcium ions, the plasma membranes of Beroe giant smooth muscle fibres display micropapillae filled with an electron-dense deposit. After freeze-fracturing of fixed tissue, the micropapillae are still present, therefore their shape and size are determined during or before glutaraldehyde fixation, and are not due to rearrangement during subsequent steps of tissue processing; intramembranous particles are seen at the periphery rather than at the top of micropapillae. In conventional stained sections, the surface of most micropapillae is surrounded by fine fibrils; when the fuzzy coat is separated from the muscle cell by a clear space, this fibrillar material becomes conspicuous and links the micropapillae to the coat. After calcium-free (EGTA) fixation, the plasma membrane is completely free of electron-dense sites but "empty' micropapillae can be seen. X-ray microanalysis of single electron-dense deposits by wavelength-dispersive spectrometry reveals a high calcium content. A weak osmiophily is suspected, but does not seem to interfere with this analysis of calcium. The highest peak-to-background ratios for calcium were obtained using the smallest aperture of the Wehnelt of the analytical microscope. In the Discussion, the micropapillae are compared to similar structures described by other authors in a variety of cell types.