In the absence of FXIIIa activity, RBCs are extruded from the clot during clot contraction. (A-B) Clot formation and contraction were triggered by recalcification (10 mM, final) and addition of TF (1 pM, final) to whole blood spiked with octadecyl rhodamine B-labeled RBCs, Alexa Fluor 488-labeled fibrinogen, and ε-ACA (to inhibit fibrinolysis). Clot contraction was visualized on a Zeiss 710 NLO confocal laser scanning microscope with an incubation chamber and temperature stage (Carl Zeiss, Thornwood, NY), and monitored at 40× magnification. Representative frames of contracting clots formed in the (A) absence and (B) presence of T101. Times (in seconds) are indicated in each panel. Scale bar, 50 μm. (C) Clot formation and contraction was triggered by addition of thrombin (1 U/mL [10 nM], final) and recalcification (10 mM, final) to whole blood. Clot contraction was visualized at 10× with digital zoom on a Nikon Eclipse TE2000-U inverted microscope (Nikon Instruments, Melville, NY). Frames depict RBC extrusion from a contracting clot. Black arrowheads highlight a deforming RBC as it exits the clot. Numbers indicate successive frames. Scale bar, 10 μm.