“All models are wrong, but some are useful,” says Sr. Product Manager Terry Riss, referencing statistician George Box.
In 2D culture, cells are typically plated in a flat monolayer. One-sided attachment induces a polarity that is unnatural for many cells. It also means that each cell is getting equal access to nutrients and oxygen, resulting in uniform growth and proliferation. Each cell is likely at the same metabolic state, which is rare in the body. Tumors, in contrast, show high levels of heterogeneity, with some cells actively proliferating, others quiescent, and some necrotic. This issue is less prevalent in 3D spheroids, where nutrient gradients emerge gradually as the diameter increases. As the spheroid grows, the cells in the core begin to experience reduced oxygen exposure. This results in a hypoxia gradient, which can lead inner layers to quiescence and even necrosis.
2D and 3D cultures also differ in the properties of their extracellular matrix (ECM). While ECM does form in 2D cultures, it fails to recapitulate the distribution and complexity of the tumor microenvironment seen in vivo. In 3D cultures, the cells are not only in close proximity to each other, but they develop a thicker, denser ECM that surrounds them and forms a barrier between the cells and the environment. “Attachment dependence is big for cancer cells,” says Drew. “The ECM sticks the cells together and makes it more like a true tumor mass.” The makeup and distribution of that ECM provides a more realistic representation of how cell:ECM interactions in solid tumors influence cell behaviors such as proliferation, differentiation and migration.
In addition to the nutrient gradients and ECM formation, 3D models can reflect the variety of cells surrounding a tumor in the body. Researchers can add immune cells and any other relevant cell types to the culture of cancer cells to get a better idea of how these interactions influence the biology of the tumor itself.
“Some cancers are really good at co-opting the activities of other cells,” says Drew. “I think that right now it’s really difficult for 2D models to recapitulate that tumor model, but in 3D, there can be multiple cell lineages in the sphere itself, which more or less simulates the actual tumor that’s present in a tissue.”