Movie S1.

A 4 day old spheroid imaged for 55 days using a transmitted light microscope at 37°C with 5% CO2.. Note the initial growth of a cell cord which then grows on the spheroid surface to form a cell layer concentric with the core spheroid. The movie was acquired on an Incucyte live cell imaging system (Essen BioScience) in the transparent U-bottom shaped spheroid culture plate that was also used to culture spheroids.