Temporal sequence of cell transformation by E26 virus.

JS4⁺ cells were sorted by FACS and, after verifying that they were mostly single, they were infected with E26 virus and seeded in methylcellulose in 35-mm culture dishes that had been pretreated with collagen. After the cells had settled through the semisolid medium, starting at 18 hours after infection, micrographs were taken of the same field (marked on the bottom of the dish) at various intervals. The black asterisks in the frames taken at 18 and 24 hours indicate the cells from which the transformed colony visible in frames 66, 72, and 120 hours emerged. The red arrowhead in the 42-hour frame points to a cell that probably represents the first transformant in the nascent E26 colony. Blue arrowheads denote positions of normal erythroid colonies. Two erythroid “colonies” (seen on the upper and bottom left of the 18-hour frame) represent clusters formed during seeding. Two colonies in the 24-hour time frame appeared to form de novo resulted from cells that settled more slowly through the viscous medium after seeding of the cells in methylcellulose cultures. The same analysis was performed with another E26-transformed colony, with similar results. Original magnification for all panels, × 20.