The densities of positively charged, colloidal iron hydroxide particles binding to the surfaces of glutaraldehyde-fixed cultured Ehrlich ascites tumour cells, were determined on cells maintained in suspension and monolayer cultures. In both cases, there was a higher density of particles bound to the surfaces of microvilli than to the spaces between them. Prior treatment of the cells with ribonuclease did not significantly alter the proportions of particles binding to the 2 different surface regions, indicating that the ribonuclease-susceptible binding sites are distributed with similar mean densities over the microvilli and the regions between them. Prior treatment of the cells with neuraminidase reduced the densities of particles binding to microvilli to those of the intermicrovillous spaces, indicating that the original higher density over the microvilli is due to an increased number of sialic acid residues at their surfaces.
The findings are discussed in relationship to observed changes in the surface density of microvilli when these cells convert from suspension to monolayer cultures.