Bone defects represent a medical and socioeconomic challenge. Different types of biomaterials are applied for reconstructive indications and receive rising interest. However, autologous bone grafts are still considered as the gold standard for reconstruction of extended bone defects. The generation of bioartificial bone tissues may help to overcome the problems related to donor site morbidity and size limitations. Tissue engineering is, according to its historic definition, an "interdisciplinary field that applies the principles of engineering and the life sciences toward the development of biological substitutes that restore, maintain, or improve tissue function". It is based on the understanding of tissue formation and regeneration and aims to rather grow new functional tissues than to build new spare parts. While reconstruction of small to moderate sized bone defects using engineered bone tissues is technically feasible, and some of the currently developed concepts may represent alternatives to autologous bone grafts for certain clinical conditions, the reconstruction of largevolume defects remains challenging. Therefore vascularization concepts gain on interest and the combination of tissue engineering approaches with flap prefabrication techniques may eventually allow application of bone-tissue substitutes grown in vivo with the advantage of minimal donor site morbidity as compared to conventional vascularized bone grafts. The scope of this review is the introduction of basic principles and different components of engineered bioartificial bone tissues with a strong focus on clinical applications in reconstructive surgery. Concepts for the induction of axial vascularization in engineered bone tissues as well as potential clinical applications are discussed in detail.

Bone regeneration is a complex process requiring highly orchestrated interactions between different cells and signals to form new mineralized tissue. Blood vessels serve as a structural template, around which bone development takes place, and also bring together the key elements for bone homeostasis into the osteogenic microenvironment, including minerals, growth factors and osteogenic progenitor cells. Vascular endothelial growth factor (VEGF) is the master regulator of vascular growth and it is required for effective coupling of angiogenesis and osteogenesis during both skeletal development and postnatal bone repair. Here, we will review the current state of knowledge on the molecular cross-talk between angiogenesis and osteogenesis. In particular, we will focus on the role of VEGF in coupling these two processes and how VEGF dose can control the outcome, addressing in particular: (1) the direct influence of VEGF on osteogenic differentiation of mesenchymal progenitors; (2) the angiocrine functions of endothelium to regulate osteoprogenitors; (3) the role of immune cells, e.g., myeloid cells and osteoclast precursors, recruited by VEGF to the osteogenic microenvironment. Finally, we will discuss emerging strategies, based on the current biological understanding, to ensure rapid vascularization and efficient bone formation in regenerative medicine.

Objective. To test the hypothesis that engineered cartilage can provide a mechanically functional template capable of undergoing orderly remodeling during the repair of large osteochondral defects in adult rabbits, as assessed by quantitative structural and functional methods.Methods. Engineered cartilage generated in vitro from chondrocytes cultured on a biodegradable scaffold was sutured to a subchondral support and the resulting composite press-fitted into a 7-mm long, 5-mm wide, 5-mm deep osteochondral defect in a rabbit knee joint. Defects left empty (group 1) or treated with cell-free composites (group 2) served as controls for defects treated with composites of engineered cartilage and the support, without or with adsorbed bone marrow (groups 3 and 4, respectively).Results. Engineered cartilage withstood physiologic loading and remodeled over 6 months into osteochondral tissue with characteristic architectural features and physiologic Young's moduli. Composites integrated well with host bone in 90% of cases but did not integrate well with host cartilage. Structurally, 6-month repairs in groups 3 and 4 were superior to those in group 2 with respect to histologic score, cartilage thickness, and thickness uniformity, but were inferior to those in unoperated control tissue. At 6 months, Young's moduli in groups 2, 3, and 4 (0.68, 0.80, and 0.79 MPa, respectively) approached that in unoperated control tissue (0.84 MPa), whereas the corresponding modulus in group 1 (0.37 MPa) was significantly lower.Conclusion. Composites of tissue-engineered cartilage and a subchondral support promote the orderly remodeling of large osteochondral defects in adult rabbits.Various techniques for the repair of fullthickness articular cartilage lesions are under investigation, reflecting the well-known problem that damaged adult articular cartilage has limited healing capacity (1,2). Large osteochondral defects are associated with mechanical instability and are accepted indications for surgical intervention to prevent development of degenerative joint disease (3,4). Ideally, a large osteochondral defect should be repaired with a graft that can provide mechanical stability and allow early postoperative function under physiologic loading conditions. These requirements potentially can be met using engineered cartilage (5).In the present study, we tested the hypothesis that engineered cartilage can provide a mechanically functional template capable of undergoing orderly remodeling and permit the repair of large osteochondral defects in adult rabbits, as assessed by quantitative structural and functional methods. Engineered cartilage generated in vitro from chondrocytes cultured on a biodegradable scaffold (6) was sutured to a subchondral support and the resulting composite, with or without adsorbed bone marrow, was press-fit into a surgically created femoropatellar groove defect (Figure 1). Defects that were left empty, defects that were treated with cell-free composites of the scaffold and the support, and native tissues from unopera...

Treatment of large defects requires the harvest of fresh living bone from the iliac crest. Harvest of this limited supply of bone is accompanied by extreme pain and morbidity. This has prompted the exploration of other alternatives to generate new bone using traditional principles of tissue engineering, wherein harvested cells are combined with porous scaffolds and stimulated with exogenous mitogens and morphogens in vitro and͞or in vivo. We now show that large volumes of bone can be engineered in a predictable manner, without the need for cell transplantation and growth factor administration. The crux of the approach lies in the deliberate creation and manipulation of an artificial space (bioreactor) between the tibia and the periosteum, a mesenchymal layer rich in pluripotent cells, in such a way that the body's healing mechanism is leveraged in the engineering of neotissue. Using the ''in vivo bioreactor'' in New Zealand White rabbits, we have engineered bone that is biomechanically identical to native bone. The neobone formation followed predominantly an intramembraneous path, with woven bone matrix subsequently maturing into fully mineralized compact bone exhibiting all of the histological markers and mechanical properties of native bone. We harvested the bone after 6 weeks and transplanted it into contralateral tibial defects, resulting in complete integration after 6 weeks with no apparent morbidity at the donor site. Furthermore, in a proof-of-principle study, we have shown that by inhibiting angiogenesis and promoting a more hypoxic environment within the ''in vivo bioreactor space,'' cartilage formation can be exclusively promoted.cartilage ͉ tissue engineering ͉ hard tissue ͉ vascularized organs