MMP9 and neoplasm: To comprehensively reflect the interactions of the tumor microenvironment in vivo, inflammatory cells should also be integrated into the 3D tumor models in addition to tumor cells, endothelial cells and fibroblasts [33,41], e.g., tumor-associated macrophages are known to promote angiogenesis by secreting proangiogenic factors to activate endothelial cells [33,41,42], while neutrophils (differentiated to a tumor-supporting phenotype) are essential for angiogenesis, as they secrete MMP-9 into the tumor microenvironment.