MDA-231 and HUVEC tumor spheroid-induced angiogenesis. (a–d) Growth of the tumor spheroid mixture consisting of MDA-231 cells and HUVECs. (a) A schematic illustration of the experimental design. (b) A representative bright-field experimental image of the tumor spheroids and HLF-laden ECM in the microfluidic chip device supplemented with EGM-2. (c,d) Representative confocal images of the MDA-231 + HUVEC spheroids at day 2 (c) and day 5 (d), respectively. (e–h) Representative confocal images of MDA-231 + HUVEC tumor spheroid-induced angiogenesis at day 0 (e), day 2 (f), day 6 (g), and day 10 (h), respectively. (e,f) Enlarged cross fluorescence confocal images of tumor spheroid-vasculature areas highlighted in (h) as original fluorescence (i) and reversed pseudo-color (j).