Oligomerization of the αC domains facilitates endothelial cell spreading. A total of 2 × 10⁴ HUVECs in serum-free DMEM were plated at 37°C for indicated periods of time on plastic wells coated with 20 μg/mL αC monomers (A), αC(FXIII) oligomers (B), or αC(tTG) oligomers (C). At different time points of spreading, cells were fixed with 3.7% paraformaldehyde, stained with Coomassie blue, destained, and photographed. Shown are representative photographs of cells 90 minutes after plating on the substrates. Bar = 50 μm. (D) Time-dependent increase in cell spreading on αC monomers () and oligomers (▪, αC(FXIII); ▴, αC(tTG)). The average areas were determined for 120 sparsely plated cells on each substrate. Results are the means ± standard deviations of 2 independent experiments performed in triplicate.