Fig. 1. HUVECs form tubular structures with lumens when cultured within fibrin gels. (A) HUVECs were cultured within fibrin gels in standard serum-containing medium and stimulated with VEGF (25 ng/ml) and FGF-2 (25 ng/ml). The cells were incubated at 37°C and 5% (v/v) CO₂ for 3 days, after which time the cells were photographed (bar, 100 µm). (B) HUVECs were treated as above then fixed in glutaraldehyde, embedded in LR resin, sectioned and analysed by transmission electron microscopy. n, nucleus; 1, lumen. Bar, 1 µm. (C) HUVECs were cultured as above, then fixed in glutaraldehyde, embedded in LR resin, serially sectioned, stained with toluidine blue and analysed by light microscopy. i-ix correspond to the order of serial semi-thin sections (bar, 10 µm).