Ultrastructural relationship between cell and substrate coats in serum-free and serum-supplemented cultures

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The substrate and cell coats in a nontumor human diploid cell line, a mouse sarcoma line, and its serum-free subline were examined using surface replicas and ruthenium red (RR) staining of substrates and cells fixed and sectioned in situ. Both methods showed that a continuous substrate coat was present on dishes which had supported both serum-free and serum-supplemented cultures, and also on dishes which had contained medium with serum but no cells. In all the replicas both from cell cultures and from cell-free preparations, the substrate and cell coats were similar in surface ultrastructure. After RR staining, two components were found in the substrate coat of the cultures, a dense zone next to the substrate and a layer of flocculent material covering it. Both components were thicker in cultures maintained with serum. An extra electron-dense zone was distinguished with plastic but not Melinex substrates. A similar flocculent layer was also present on the cell surface, and cell and substrate coats made contact at “adhesion” points formed by protrusions of the underside of the cell. Groups of intracytoplasmic filaments were frequently present at these points. In the human cell line RR-positive strands were stretched between the underside of the cell and the substrate coat. These strands were not seen in the tumor cell cultures.