Figure 1.

Schematic of a 12 day old spheroid (cells are not drawn to scale). Nutrients and oxygen are most abundant at the outside of the spheroid and decrease in the center. The pH changes from 7 to 5 in the center of the spheroid due to the accumulation of waste. Three populations of cells develop in response to the radially symmetric chemical gradients—the inner necrotic core, the middle quiescent layer, and the outer proliferating rim—although the boundaries between the cellular populations also represent a subtle gradient. The total spheroid diameter at 12 days of growth is ~1 mm and contains ~1 million cells.¹⁹