Review The Tumor Microenvironment

Tumors are more than simply masses of equivalent and pro-liferating cancer cells. Rather, they are heterogeneous by nature, being composed of multiple distinct cell types that participate in tangled interactions with one another (Fig. 1). Those cells which form the tumor-associated stroma are active contributors to tumor development. Over the last decade, accepted opinion has evolved from reductionism—perceiving a tumor as nothing more than a collection of relatively equivalent cancer cells—to the recognition of tumors as organs with interdependent cells whose complexity is somehow comparable to, or even exceeds that of, normal tissues. In fact, the tumor microenvironment serves as the key support system of a cancer, becoming the source of the 3-dimensional organization and architecture of the stroma, as well as providing all the protumorigenic factors that facilitate the growth, invasion, angiogenesis, and even metastatic ability of th