NIRF visualization of abdominal large lymphatic vessel draining the urinary bladder. A constant feature observed following i.v. administration of Cy5.5-conjugates was the labeling of abdominal lymphatic sacs (blue arrows in Figures 5G, 5I, and 5J). In order to investigate whether NIRF would permit visualization of these collecting lymphatic vessels and lymph sacs, high molecular weight dextran (500,000 MW)-Cy5.5 was injected in the pelvic floor underneath the urinary bladder, in the same region of lymph sacs of anesthetized mice, and serial images were immediately captured. Figures 10 A-D are negative NIRF images acquired along with time and indicate the movement of Cy5.5 in major collecting lymphatic vessels. It was noted that a series of lymph sacs were observed squirting the fluorescent content rhythmically and upwards (black and white arrows indicate the direction of the flow in two major collecting lymphatic vessels). A quick time movie, Figure 10 E illustrates the upward movement of Cy5.5 within collecting lymphatic vessels. Figures 10 F and 10 G are a high magnification of an individual frame of the movie illustrated in Figure 11 and additional file 1.