We study the spreading of spheroidal aggregates of cells, expressing a tunable level of E-cadherin molecules, on glass substrates decorated with mixed fibronectin and polyethylene glycol. We observe the contact area by optical interferometry and the profile by side-view microscopy. We find a universal law of aggregate spreading at short times, which we interpret through an analogy with the spreading of viscoelastic droplets. At long times, we observe either partial wetting or complete wetting, with a precursor film of cells spreading around the aggregate with two possible states. In strongly cohesive aggregates this film is a cellular monolayer in the liquid state, whereas in weakly cohesive aggregates, cells escape from the aggregate, forming a 2D gas. The escape of isolated cells is a physical mechanism that appears also to be present in the progression of a noninvasive tumor into a metastatic malignant carcinoma, known as the epithelial-mesenchymal transition.collective migration | cell adhesion | tissue viscoelasticity | tumor invasion T issue spreading is a fundamental process in embryonic development (1-3), wound healing (4), and cancer invasion and propagation. A tumor is not malignant if it remains cohesive. Understanding how noninvasive tumor cells become metastatic is the most prominent challenge in current cancer research. The first step of cancer propagation (invasion) is characterized by a loss of cell adhesion associated to an increase in cell motility, followed by an entry into blood circulation (intravasation), an escape into a new tissue (extravasation), and the proliferation leading to a secondary tumor (5). The loss of cell adhesion, characteristic of aggressive metastatic cancer, is analogous to that of the epithelial-mesenchymal transition (EMT) during embryonic development (6, 7), which is a key process during gastrulation (8) or neural crest development (9). A repression of E-cadherin expression (involved in the formation of adherens junctions between cells mediated by homophilic ligation in the presence of calcium) has been reported for cells undergoing an EMT transition (10). Here we study the role of E-cadherin expression in the wetting behavior of tissues. We use as a model system cellular aggregates of variable cohesivity, spreading on glass substrates of variable adhesivity.Spherical cellular aggregates are useful in vitro systems to study the properties of tissues. The characterization of tissue mechanics through viscosity has been debated since the pioneering work of Steinberg. He demonstrated that embryonic tissues behave like liquids and are characterized by a well-defined surface tension (11). Mixing cells of two tissues, he observed cell sorting: The tissue with the lower surface tension surrounds the tissue with a higher tension (12-14). If two aggregates are brought in contact, they coalesce to form a single, larger spheroid. The fusion of two aggregates (15, 16) leads to the determination of the capillary velocity V Ã ¼ γ∕η, where γ is the surface tension and η is the viscos...