This image depicts mouse embryonic fibroblasts (NIH/3T3 cell line) invading a hyaluronate gel after being cocultured in a 3D spheroid with mouse glioma cells (CT-2A cell line).  Importantly, since this spheroid is very small (<100 um in diameter), the activation profile of the emerging fibroblasts is quite scarce. We believe that this implies that spheroid size (and by proxy, cancer cell density) plays a critical role in the number and degree of activation of emergent fibroblasts.