The timely formation of new blood vessels is a critical process during
embryonic and fetal development [7,8]. In the adult, angiogenesis
occurs during physiological processes like wound healing, the menstrual
cycle, and pregnancy but also in specific diseases such as intraocular
neovascular disorders and tumorigenesis. As opposed to soft-tissue
healing, bone can regenerate itself without the formation of fibrous
scar tissue and hereby maintains its physiological and mechanical
characteristics. Normal fracture healing in adults occurs through
intramembranous or endochondral bone formation, closely mimicking
skeletal development in the embryo [9]. Intramembranous ossification
is characterized by direct formation of bone by committed
osteoprogenitor cells and mesenchymal stem cells (MSC) from the
periosteum. During endochondral ossification, MSC differentiateinto chondrocytes, which in turn produce a cartilaginous matrix.
Consecutively, this matrix undergoes calcification and is eventually
replaced by bone [10]. The scar-less regeneration of fractured bones
through the endochondral pathwaymight be attributed to the avascular
nature of the cartilage template. In fact, chondrocytes are metabolically
well adapted to survive in poorly oxygenated regions and still produce
the extracellular matrix needed for mineral deposition, and hereby
likely contribute to optimal fracture healing.
Despite the remarkable regenerative capacity of bone tissue, fracture
healing fails in about 10% of the cases leading to delayed union or nonunion.
Adequate vascularization has been shown to be critical for
successful bone healing, next to the presence of osteoprogenitor cells
andmechanical stabilization. Indeed, inappropriate blood vessel supply
is a major cause of delayed union or non-union during fracture healing
[11]. More precisely, when fracture is associated with large vascular
injuries, the rate of impaired healing is as high as 46%, exceeding by
far the global 10% non-union rate [2]. In addition, inhibition of angiogenesis
during fracture repair in animalmodels resulted in the formation of
fibrous scar tissue, resembling human atrophic non-union [12].
Therefore, treatment modalities that promote tissue vascularization
possibly provide a central strategy to accelerate the healing response
and tissue regeneration.