Colloidal iron binding to the surface of HeLa cells, spreading in monolayer culture

We have used the colloidal iron (CI) binding technique, adapted for transmission electron microscopy, for semiquantitative evaluation of the negative charge density at the surface of HeLa cells in monolayer culture. The surface area increases when HeLa cells spread on the substrate. This increase brings about a decrease in the thickness of the CI rim, indicating a decrease in negative surface charge density. This phenomenon implicates lowering of the electrostatic repulsion, and explains the formation of intercellular contacts at the level of spread parts of the cell. Because of lack of penetration, CI particles are absent in regions of cose apposition between cells and between cells and substrates. Absence of CI binding in broader intercellular or cell-substrate spaces was explained through masking of the anionic groups.