Tumor-derived exosomes are small membrane vesicles of endocytic origin that are released from the surface of tumor cells into the extracellular environment. They can contain proteins, RNA, microRNA, and DNA 1,2 , which are shuttled from a donor to recipient cell to modulate their behavior 3,4. In recent years, there has been an increased interest in understanding how exosomes modulate the microenvironment due to the discovery of their role in the pre-metastatic niche formation in lung 5. However, despite the very well known influence of the microenvironment in cells' behavior 6 , the mechanisms underlying tumor-derived exosomes regulation by the microenvironment remain largely unknown. Bioengineering methods that have transformed the stem cell research and regenerative medicine are just starting to enter the field of cancer research 7. Models of three-dimensional (3D) tumors are developed to mimic the in vivo situation 8. We and others are making progress in replicating the physiological landscape of human tumors using 3D in vitro tissue models that contain the essential components necessary for recapitulating in vivo conditions 9,10. We use novel biomaterials to accurately mimic biological properties of the tumor microenvironment, and tissue-engineering bioreactors to provide a controlled microenvironment to study tumor behavior 8. Here, we propose to discern the effect of the tumor microenvironment on exosomes regulation with the aid of Tissue Engineering using Ewing's sarcoma (ES) and neuroblastoma (NB) as clinical models. To this end, we have developed tissue-engineered microenvironments as 3-dimensional models to culture ES and NB. We have isolated and characterized tumor-derived exosomes from our biomimetic systems and compared to samples from patients. Our preliminary data confirm the role of the extracellular matrix on tumor-derived exosomes regulation and provide compelling evidences for a link between stiffness, 3-dimensionality and exosome properties. References