Interaction between intracellular vacuoles and the cell surface analysed by finite aperture theory interference reflection microscopy.

Using finite aperture theory we have shown that localized very dark areas in the interference reflection images of Dictyostelium discoideum amoebae are due to the close intracellular approach of vesicles and tubular elements of the contractile vacuole system to the plasma membrane adjacent to the substratum. Vesicles interacting in this way become locally deformed to the planar contour of the substratum and are separated from the cell surface membrane by a constant approximately less than 0.1 micron of cytoplasm. Lamellar processes formed by these cells on very adhesive surfaces have identical dimensions. This minimal thickness may be a mechanical consequence of a contractile mechanism which pulls membranes together.