Fig. 6

Functional consequences of conditional KO of Gli1 signaling. a, b Typical x-ray images of Nse-BMP4;Gli1-creERT^+/− (a) and Nse-BMP4;Gli1-creERT^−/− (b) mice after injury. Note that injury-induced HO was dramatically inhibited in Nse-BMP4;Gli1-creERT^−/− mice. c, d pSMAD1/5 staining of lesional tissues of Nse-BMP4;Gli1-creERT^+/− (c) and Nse-BMP4;Gli1-creERT^−/− mice (d). Note that overall BMP signaling was not obviously different between these two groups, but, unlike that of Nse-BMP4;Gli1-creERT^+/−, in lesional tissues of BMP4;Gli1-creERT^−/− mice, the BMP signaling gradient was not observed. e–i H&E staining showed that, in BMP4;Gli1-creERT^+/− mice (e–g), all stages of the typical HO process, i.e., early fibroproliferative (e), chondrogenesis (f), and mature HO (g), were robustly observable. However, in Nse-BMP4;Gli1-creERT^−/− mice, even though the whole HO process was dramatically inhibited, not all stages were equally affected (h, i). Note that chondrogenesis was less affected, while the fibroproliferative region was severely underdeveloped and could only be found around the cartilage core, and the most mature structure found in Nse-BMP4;Gli1-creERT^−/− mice was underdeveloped HO without well-developed trabeculae or bone marrow (i). Intriguingly, based on morphology, at least in the chondrogenesis stage, the MSC niche was still observable (only mildly affected). j, k In comparison to Nse-BMP4;Gli1-creERT^+/−; Zsgreen (j), pre-existing Gli1-creERT labeled stem/progenitor cells in Nse-BMP4;Gli1-creERT^−/−; Zsgreen mice could survive and contribute to the MSC niche in the absence of ongoing Gli1 signaling (k). c, d, j, and k are on the same scale, bar = 100 μm; f–i are on the same scale, bar = 100 μm