NMU-deficient mice have mast cells, leukocytes, platelets and an intact early vascular response to arthritogenic antibodies. (A) Mast cells were enumerated in sections of skin obtained from control B6 (n = 3) and NMU-KO (n = 4) mice and stained with toluidine blue. Cells were counted in five ×40 HPF per mouse and averaged. Plotted values represent the number of mast cells per HPF in the indicated groups (mean ± SEM). (B) Complete blood counts from control B6 and NMU-KO mice were performed. Numbers shown are × 109/L ± SD, with four mice per group. (C) Serum from K/BxN mice was injected intravenously into NMU-KO mice (red) or littermate control mice (black). Vascular leak in the hindpaw was monitored by intravital confocal microscopic evaluation of extravasation of the intravascular tracer, Angiosense 680. Each line represents one mouse. B6: C57BL/6; HPF: high power field; KO: knockout; MFI; mean fluorescence intensity; NMU: neuromedin U; SD: standard deviation; SEM: standard error of the mean.