Figure 1.

Tumor formation and its complex microenvironment. Epigenetic mutations in normal stem cells or precursor/progenitor cells lead to generation of CSCs, which play crucial roles in tumor stemness, initiation, maintenance, and metastasis. Tumor niche contains not only tumor cells and CSCs but also cancer-associated immune cells (T-cells, macrophages (MΦ), dendritic cells (DC)), cancer-associated fibroblasts (CAF), and other cells. In addition, MSCs are recruited to tumor by chemoattractant factors secreted from all those cells (CXCL16, SDF-1, CCL-25, and IL-6, TNF-α, and IL-1β), playing crucial roles in tumor progression.