Scanning electron microscopic observation on cells grown in vitro. VI. aggregate formation in confrontation cultures of human diploid and tumor cells.

Two human cell lines, HeLa carcinoma cells and Wi38 lung fibroblasts have been used in confrontation experiments to study different adhesion processes including cellular motility, sorting out and invasion by SEM and statistics. Aggregates of one cell type were co-cultured with resuspended cells of the other type for up to 96 hours and vice versa. Homotypic aggregation kinetics as a control showed a saturation of the time-dependent increase in aggregate diameter and a mirror-like decrease in single cell number for both cell lines. In the heterotypic aggregation process, dissociated HeLa cells covered the surface of Wi38 aggregates without showing any particular distribution pattern. They partially penetrated the outer fibroblast layers and invasive actions were detected. Suspended Wi38 lung cells also adhered to the tumor cell aggregates. After a period of non-coordinated, random settlement (8 to 18 h or co-cultivation), the fibroblasts sorted out to form a network around the tumor aggregates in the form of sheets of cells arranged in parallel. The underlying HeLa cells then broke through the Wi38 layers, thus performing an inside-out invasion. Finally, all the aggregates were again covered by HeLa cells.