Newly formed blood vessels do not penetrate islets transplanted to MMP-9^−/− mice. (A-D) Left panels: Confocal z-projections of image stacks (stack height 80 μm) showing transplanted wild-type islets (blue, Celltracker), vasculature (red, CD31-mAb), and Gr-1⁺ leukocytes (green) in cremaster muscles of wild-type (A-B) and MMP-9^−/− (C-D) mice 4 days after transplantation. Bars represent 50 μm. Right panels: Single optical slices 40 μm down into the grafts illustrating the lack of intra-islet vessels in the MMP-9^−/− mice, whereas the wild-type contains many newly formed capillaries. Grafts are outlined by dashed lines. Bars represent 50 μm. (E) Quantification of intra-islet vascular density in CD31-mAb–stained sections of grafts in muscle at days 3-5 after transplantation (n = 4 per day and group). *P < .05.