Figure 2.

Similarities between the 3D spheroid model and a tumor in vivo. The spheroid received nutrients from the medium and the tumor through its vascularization, although they are subject to concentration gradients, which limit efficient diffusion. This creates a microenvironment, leading to various conditions of cells within the tumors and spheroids. The necrotic core exists due to the lack of available nutrients and the accumulation of waste, whereas the increased metabolic activity at the periphery is due to the more efficient diffusion of nutrients and waste. In each condition, the cells exist in a 3D conformation with interactions with the extracellular matrix. 3D, three-dimensional.