Effect of genetic inactivation of Pak1 on accumulation of cutaneous Nf1^+/− mast cells in response to local administration of SCF in vivo. SCF was delivered in vivo via a micro-osmotic pump on the middorsum at 10 μg/kg per day. Skin sections at the site of SCF administration were fixed and stained with hematoxylin and eosin to assess routine histology along with Giemsa to identify mast cells. (A) Cutaneous mast cells were quantitated in a blinded fashion by counting 2-mm² sections. (B) The percentage of degranulating mast cells present per 2-mm² section was calculated. Representative sections are displayed in panels C to F. Resting mast cells in panels C to F are marked with ■; degranulating mast cells in panels C to F are marked with an open arrow. Values in panels A and B represent the mean of 3 independent experiments each using 3 mice per genotype, and error bars represent SEM. *P < .05 compared with WT control. **P < .05 compared with Nf1^+/− control using Student unpaired t test. Images in panels C through F were obtained using a Nikon Eclipse 80i microscope (Tokyo, Japan) using a 10×/0.30 DIC L/N1 magnification and lens in an air medium with a QCapture 2.90.1 camera (QImaging, Surrey, BC).