Fig. 1. A close-up look at the different characteristics and analytes having a biological significance within the TME, which can be precisely examined by ratiometric fluorescence methods. The heterogeneity of the TME is mainly due to the complex ecosystem created by the interactions among tumor, stromal and immune cells, all of which are immersed in a dense and dysregulated ECM. Poor blood flow and crowded glycolytic tumor cells form niches characterized by reduced oxygenation, pH acidity, reduced nutrient loading, collection of anti-inflammatory cytokines and chemokine, and storage of metabolic by-products, such as lactate.