## 4 Conclusions  
This preliminary animal study was restricted to a single rat, because we found incomplete osseous bridging in the other animals. This single rat became the basis of a thorough imaging study combining \(\upmu\)CT before and after de-calcification with histology. Non-rigid registration and the extraction of common volumes allowed for segmenting anatomical features, which led to the hypothesis that the combination of the three modalities, i.e., laboratory-based \(\upmu\)CT before decalcification, synchrotron radiation-based tomography after decalcification and histology with H&E stain, could be used to discriminate between different types of collagen. This hypothesis should be tested by means of dedicated histological analysis, preferably using a protocol with higher chances of osseous bridging.  
The results of this study are in line with the hypothesis that injecting hMSC at the beginning of the distraction phase is effective. We could confirm that at least some implanted hMSC survived and differentiated into human osteocytes, which contributed to new bone formation. In order to quantify the impact of injecting hMSC at the beginning of the distraction phase, an animal study with a sufficiently large number of rats should be performed. The methodology presented herein forms a sound basis for such a study.  
Figure 12: Histological analyses of the injected stem cells (hMSC). Histological sections allowed for identifying implanted hMSC by using in situ hybridization of the repetitive human specific Alu sequences, see images in the top row. Human cells possess a blue color clearly seen in the enlarged view on the right. Immunostaining for HLA-1 (bottom row) shows human osteocytes, indicated by the red color. They are part of the newly formed bone.  
**Author Contributions:** Conceptualization, C.J., C.K. and B.M.; Formal analysis, G.R., G.R.S. and C.T.; Funding acquisition, S.E.H., C.K. and B.M.; Methodology, G.R.S., S.E.H., C.K. and B.M.; Project administration, C.K.; Resources, C.J., C.K. and B.M.; Software, G.R., C.T. and G.S.; Validation, G.R.S.; Visualization, G.R., G.R.S., C.T., S.E.H., F.B., G.S. and A.S.; Writing--original draft, B.M.; Writing--review and editing, G.R., G.R.S., C.T., G.S. and B.M. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was partially funded by a research fellowship of the PhD program in Biomedical Engineering of the University of Basel (G. S.). Beamtime at DESY (Hamburg, Germany) was granted through the project proposals I-20110780 EC and I-20110555 EC.  
**Institutional Review Board Statement:** The study was conducted according to guidelines set out in the Declaration of Helsinki and approved by the local Ethical Committee (Ethikkommission beider Basel, Ref. 78/07) and by the Veterinary Office of Basel (ethical approval code 2295, 15 August 2008).  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Data presented in this study are available on request from the corresponding author.  
**Acknowledgments:** The authors would like to thank [PERSON], [PERSON] and [PERSON] for their support. The work of [PERSON] in preparing the stem cells and [PERSON] in preparing the H&E-stained slide is gratefully acknowledged.  
**Conflicts of Interest:** The authors declare no conflict of interest.