Growth of Flk-1⁺ cell clusters supported by feeder layers. Sorted Flk-1⁺ cells were cultured at low cell density on a monolayer of each feeder cell line in the absence of exogenous factors. After 3 days of culture, immunostaining with MoAb for Flk-1 was performed as described in Materials and Methods. (A) NIH3T3 could not support their growth. (B) PA6 supported it poorly. (C and D) Notably, Flk-1⁺ cells grew to form the cord-like structure on Balb/c3T3 feeder layer (C), whereas they grew to form the sheet-like structure on OP9 feeder layer (D). Scale bar = 500 μm.