Mks form abundant podosomes on Horm collagen and fibrinogen matrix. Mks were spread on 100 μg/mL Horm collagen-coated surfaces for 3 hours and then fixed and stained for F-actin and (A) the Arp2/3 complex, (B) WASp, and (C) vinculin. (D) Mks spread on collagen from WASp^−/− mice were stained for F-actin and vinculin. The right column of pictures shows the merge of F-actin (red, phalloidin) and WASp, Arp2/3 complex, or vinculin (green). Red squares indicate magnified area of the cell. Red arrows in panel B indicate podosomes along what appears to be a collagen fiber. (E) Table shows the average plus or minus the standard error of the mean (SEM) for the spread Mk surface area in µm², the mean number of podosomes formed, podosomes formed per μm², and the size of podosomes for Mks spread for 3 hours on 100 μg/mL Horm collagen or 100 μg/mL fibrinogen-coated surfaces fixed and stained for F-actin and WASp. The podosome size was determined by measuring at least 100 podosomes of 5 different cells per experiment. Data are representative of 3 experiments. Asterisk indicates significant difference between collagen and fibrinogen data with a P value < .05. Pictures were taken with a confocal microscope using a 60× objective. Scale bars represent 10 μm.