IL-7 inhibits bone formation in mice in vivo. Mice were injected intraperitoneally with IL-7 (10 μg/kg body weight) or vehicle (PBS plus 0.1% BSA) for 30 days and pulsed with calcein 7 and 3 days prior to sacrifice. Tibiae were plastic-embedded, sectioned, and examined by fluorescence microscopy. (a) Representative calcein fluorescence bands from four different mice are shown for each group. Bands (1 mm in length) were photographed at comparable anatomic positions at the mid-diaphysis of each bone at ×100 magnification. (b) Fluorescence intensity of the entire area of each of the four individual panels shown in a, for both vehicle- and IL-7–treated groups, was quantitated by densitometry using NIH Image. Data represent the average OD of four panels per group ± SEM.