Fig. 2.

Stromal dynamics in the pre-metastatic and metastatic niches. Using the lung as an example, the normal lung contains mostly alveolar epithelium with high penetration of blood vessels and little stromal content. Systemic changes are elicited by the primary tumor prior to dissemination and formation of overt metastatic lesions including the influx of specific immune cells such as immature monocytes, activation of fibroblasts, and increased deposition of ECM components such as collagen and fibronectin. Such features are progressively exacerbated during the formation of tumors at the secondary site and essential to continued growth