  Tumor invasion, the process by which tumor cells break away from their
primary tumor and gain access to vascular systems, is an important step in
cancer metastasis. Most current 3D tumor invasion assays consisted of single
tumor cells embedded within an extracellular matrix (ECM). These assays taught
us much of what we know today on how key biophysical (e.g. ECM stiffness) and
biochemical (e.g. cytokine gradients) parameters within the tumor
microenvironment guided and regulated tumor invasion. One limitation of the
single tumor cell invasion assay was that it did not account for cell-cell
adhesion within the tumor. In this article, we developed a micrometer scale 3D
co-culture spheroid invasion assay that was compatible with microscopic
imaging. Micrometer scale co-culture spheroids (1:1 ratio of metastatic breast
cancer MDA-MB-231 and non-tumorigenic epithelial MCF-10A cells) were made using
an array of microwells, and then were embedded within a collagen matrix in a
microfluidic platform. Real time imaging of tumor spheroid invasion revealed
that the spatial distribution of the two cell types within the tumor spheroid
critically regulated tumor invasion. This work linked tumor architecture with
tumor invasion and highlighted the importance of the biophysical cues within
the bulk of the tumor in tumor invasion.
