Comparison of the Densities of Clonogenic Cells From EMT6 Fibrosarcoma Monolayer Cultures, Multicell Spheroids, and Solid Tumors in Ficoll Density Gradients2

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Abstract
The densities of viable EMT6 cells grown in vitro as monolayer exponential and plateau-phase cells or as multicell spheroids or in vivo as a solid tumor in BALB/c mice were determined with the use of isopyknic centrifugation in linear Ficoll gradients. Exponential cells banded at a density of 1.069 gl ml, whereas plateau-phase cells appeared at 1.073 g/ml. Cells grown as spheroids were more dense than the monolayer cells and were recovered mainly at 1.081. Solid-tumor cells, separated under the same conditions, banded at 1.080. Narrowing the range of the Ficoll gradient failed to resolve more than one band of cells in the solid-tumor separation. This provides evidence that the density of cells obtained from the spheroids is greater than that of the monolayer cells but agrees well with the density of the solid-tumor cells. Ficoll was demonstrated to be nontoxic to the cells, and plating efficiency assays showed similar cell viabilities between noncentrifuged and centrifuged cells. The plating efficiency of the peak fraction of exponential cells after centrifugation was 68%, and that of plateau-phase cells was 50%. Corresponding figures for the multicell spheroid and solid-tumor cells were 62 and 28%, respectively. The recovery of cells after centrifugation in the Ficoll gradients ranged from 62 to 83%. The effects of cell load and centrifugation time on the density distributions of the EMT6 cells were also investigated.

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