Incorporation of mitochondria-stained MSCs in GFP-expressing EC capillaries in matrigel. (A) Phase contrast (left) and fluorescence (right) images show day-old, GFP-expressing, RLMEC-derived capillaries. Bar = 500 μm. (B) Confocal microscopy of a capillary shows the lumen (double-headed arrows) in z-axis images. Bar = 50 μm. (C) Phase contrast images taken at 1-hour intervals show migration of a single MSC (red dot) toward capillaries (bracket). Bar = 20 μm. (D) High magnification image shows Mito Tracker Deep Red (MTDR)–loaded MSC (single arrow) intercalated between ECs (double arrows) in a single, day-old capillary. The nucleus is Hoechst 33342-stained (blue). Bar = 25 μm. (E) Confocal microscopy of a single EC from a capillary network shows GFP fluorescence (EC-GFP) and MTDR fluorescence of MSC mitochondria (MSC-MITO). The merged image shows in different planes the incorporation of MSC mitochondria (red) in EC (green). Images were taken 1 day after adding MTDR-stained MSC to day-old capillaries. Bar = 10 μm. All imaging data replicated at least 6 times. (F) Images are by phase contrast (top) and fluorescence (bottom) microscopy on days indicated after addition of mitochondria-stained MSC (red) to day-old capillaries (green) at EC:MSC ratio of 1:1. Bar = 500 μm. (G) Group data show responses to addition of the indicated ratio of ECs to MSC, CHO cells, or RNLFB (FB) to day-old capillaries. Capillary density in capillaries to which no cells were added (CT) was 7 (± 2) capillaries per low-power (4×) field. In one group (INS), MSCs were grown in an insert, which was placed adjacent to the matrigel culture. Data are mean plus or minus SD. *P < .05, significantly different from CT.