**4. Influence of angiogenesis during bone regeneration**

The statements and examples of the preceding sections underlined clearly the impact of angiogenic processes on any tissue regeneration.

One well investigated area of research in this context in the regeneration of bone, e.g. in terms of skeletal development or fracture repair. The vasculatures´ job is to bring oxygen and nutrients to the metabolically active areas, but also to provide the bone with precursors or inflammatory cells. As far as the cytokines are concerned, there are many factors that act as key protagonists in angiogenesis as well as in bone regeneration and remodelling: VEGF, especially its isoforms VEGF120, 164 and 188 play a significant role. But there are other relevant players: bFGF, TGFβ, HIF are among the most potential ones.

VEGF in its isoforms with the corresponding receptors have emerged as the decisive coupling factors between epi- and metaphyseal vascularisation and cartilage develop‐ ment and enchondral ossification. A block of VEGFR-1 and -2 with selective antibodies leads to a reduced VEGF signalling and consecutively to a reduced intramembraneous bone formation in distraction osteogenesis; VEGF in this setting is produced by local inflamma‐ tory cells (Jacobsen, Al-Aql et al. 2005).

The angiopoietins Ang-1 and Ang-2, hepatocyte growth factor HGF, platelet-derived growth factor PDGF, the IGF family and the neurotrophins NGFs also have angiogenic properties.

The effect of HIF 1-alpha as stimulator of bone regeneration also has been observed: in a mouse model with increased HIF activity the animals showed significantly higher bone mass. The stimulated HIF activity led to enhanced intramembraneous bone regeneration in a mouse distraction model (Wang et al., 2007, Portal-Núñez, Lozano et al., 2012, Hankenson, Dishowitz et al., 2011)

In fracture vascularisation and repair VEGF function is required: here the matrix-bound forms of VEGF are activated by matrix metalloproteinases, enzymes that fulfil many functions during bone and matrix degradation and remodelling.

MMP9, expressed in osteo- and chondroclasts during fracture repair, initiates cartilage resorption. This degradation process releases matrix-bound VEGF from the cartilage matrix and thus stimulates the vascularisation. This callus degradation in addition provides the base for bony fracture repair in contrast to persisting cartilage non-union (Colnot, Thompson 2003).

MMP 13 activates VEGF relase independently: whereas MMP9 depends on osteo- can chondroclast functionality, MMP13 is expressed by hypertrophic osteo- and chondroblasts. Lack of MMP13 interferes with the proteoglycan degradation leading to a reduced permea‐ bility of the cartilage matrix for recruited inflammatory cells and sprouting blood vessels. The result is delayed callus resorption and altered vascular invasion (Behonick, Xing et al., 2007).
