• actin filament bundles;
  • LETS protein;
  • cytoskeleton;
  • chick embryo fibroblasts;
  • triton cytoskeleton;
  • nonmuscle actin


The Balb/3T3 and C3H/10T1/2 lines, noted for their marked postconfluence inhibition of proliferation and anchorage dependence, and frequently studied as nontumorigenic lines that are compared with tumorigenic sublines transformed with various agents, produced tumors within two to four months at low-cell dosage (3 × 104 cells) when implanted subcutaneously attached to 1 × 5 × 10 mm polycarbonate platelets. Platelets alone did not produce tumors. The cultured Balb/3T3 tumor cells showed loss of both postconfluence inhibition of proliferation and anchorage dependence. Tumors arising form attached Balb/3T3 cells in (BALB/c × C57B1/6)F1 hybrids were shown to be transplantable to BALB/c but not to C57B1/6 mice, proving that the tumors were derived form Balb/3T3 and not from host cells. The tumors exhibited unique transplantation rejection antigens that did not cross-react with each other. Scanning electronmicroscopy of Balb/3T3 cells and derive tumor cells on Teflon1 substrates (on which only the tumor cells and not the parent Balb/3T3 cells could grow) revealed that the two cell types were remarkably similar in appearance, except that the tumor cells were larger and showed many more microvilli that tended to concentrate over the nucleus. We conclude that Balb/3T3 cells and C3H/10T1/2 cells are preneoplastic and give rise to spontaneously transformed clones when implanted in vivo attached to a solid substrate.