A sheet-like cluster of ECs generated from Flk-1⁺ cells cultured on OP9 feeder layer. Sorted Flk-1⁺ cells were cultured at low cell density on OP9 feeder layer in the absence of exogenous factors. After 3 days of culture, immunostaining with MoAb for Flk-1 (A) or VE-cadherin (B) was performed as described in Materials and Methods. As compared with the diffuse distribution of Flk-1 over the cell surface, VE-cadherin was concentrated at cell-cell junctions. (C) Uptake of DiI-Ac-LDL was observed by fluorescent microscopy using a rhodamine filter. Virtually all cells in this cluster showed uptake of DiI-Ac-LDL. Scale bar = 100 μm.