Kinetics of clonogenic melanoma cell proliferation and the limits on growth within a bilayer agar system

Accurate descriptions of the kinetics of cell growth in semi‐solid agar clonogenic systems have been difficult because the number of cells in colonies of different sizes is largely unknown. We stained and removed tumor cell colonies from agar, directly counted their cells, and established equations to quantitate the number of cells within colonies of different sizes. We used these equations to quantitate, in terms of cell number and volume, the total amount and kinetics of clonogenic cell proliferation from biopsies of human melanoma and cell lines of several different tumor types. Daily observations of cells in agar and serial photography indicated a 0‐ to 4‐day delay in the onset of proliferation in agar followed by rapid growth and then abrupt cessation of proliferation. We quantified the extent of proliferation of cells from melanoma biopsies of seven patients and 11 cell lines after they were allowed to proliferate in agar until they stopped. Approximately 10% of cells divided one to five times while only 0.01% divided six to nine times. The total number of cells within the colonies at the end of growth was different while the total volume of cells within the colonies per plate was similar; approximately 109 μm3 cellular volume per plate represents an upper limit for proliferation within the closed, nonrefed bilayer agar system. Previous replating studies using the same biopsy cells have shown that clonogenic melanoma cells can self‐renew and have more proliferative capacity than that expressed during primary colony formation. Thus, the clonogenic assay only measured initial proliferative capacities. Furthermore, variable delays in the onset of proliferation may contribute to the heterogenity of colony size within clonogenic assays.