The cell surface and intercellular junctions of transformed and non-transformed hybrid cells, obtained from fusion of murine mammary adenocarcinoma cells (TA3B) and normal rat embryofibroblasts (REF), were compared with those of the parental cells, using ruthenium red (RR) staining of cells fixed in situ. An RR-positive layer of variable thickness was found on the surface of all cell types. Measurements of the thickness of this layer on the free surface of the cell cultures deomonstrated a significant difference between transformed and non-transformed cells. The thickness distribution of the RR layer was similar on the surface of REF and non-transformed hybrid cells, but there was significant variation among all the transformed cell lines. Extracellular filamentous RR-positive material, usually in direct contact with the cell surface, was present in confluent cultures of REF and non-transformed hybrid cells, but was absent in the transformed cell lines. Gap junctions were few in both parental cell lines and rare or absent in transformed hybrids; a large increase in number and size of gap junctions was found in non-transformed hybrids. Abundance of long gap junctions was correlated with density-dependent inhibition of growth, since non-transformed hybrids grew in single layers whereas both normal and tumour parental cell types, and transformed hybrids grew in multilayers. Tight junctions were frequently encountered in TA3B and transformed hybrid cells but were not seen in REF cells and only occasionally seen in non-transformed hybrids. Intermediate-type junctions occurred in all cells, but desmosomes were found only rarely in TA3B cells and never in the other cell types.

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