Calcium hydroxyapatite ceramics (CHA) are nontoxic materials, provoke little reaction from tissues, and by virtue of these properties represent a good starting point for creating bone substitutes. Although several porous CHAs have been used clinically, there have been few reports that CHA is fully replaced by newly formed bone, which may be due to its structure and the limited connectivity between pores. We recently developed a fully interconnected porous CHA (IP-CHA) by adopting a "foam-gel" technique. Structural analysis by scanning electron microscopy revealed that IP-CHA had spherical pores of uniform size that were interconnected by window-like holes. The surface of the wall structure was smooth, and hydroxyapatite particles were bound tightly to one another. Most of the interpore connections of IP-CHA ranged from 10 to 80 microm in diameter (average, 40 microm). When the cylindrical IP-CHA (diameter, 6 mm; height, 15 mm) was implanted into a rabbit femoral condyle, bone, and bone marrow with abundant vessels formed deep in the pores through the interpore connections. Within a period of 6 weeks, new bone had formed and penetrated to a distance of 3 mm from the surface of the IP-CHA implant. Furthermore, a compression test at 9 weeks revealed that the implanted IP-CHA steadily increased in strength to more than double the value of the initial test. These results indicate that the IP-CHA may have clinical utility as a superior bone substitute.

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Cartilage functions at a lower oxygen tension than most other tissues. To determine the role of oxygen tension in chondrocyte differentiation and function, we investigated the influence of oxygen tension in the pluripotent mesenchymal cell line C3H10T1/2 and 14.5E mice embryo forelimb organ culture. 10T1/2 cells and embryo forelimbs were cultured under normoxia (20% O 2 ) or hypoxia (5% O 2 ) in the presence of recombinant human bone morphogenetic protein 2. To elucidate the mechanism by which oxygen tension influences chondrocyte differentiation, the Smad pathway was examined using Smad6 overexpression adenovirus and Smad6 transgenic mice embryo forelimbs. The p38 MAPK pathway was examined using dominant-negative MKK3 and FR167653, a specific p38 MAPK inhibitor. The transcriptional activities of Sox9 and Runx2 were also investigated. Hypoxia promoted bone morphogenetic protein 2-induced glycosaminoglycan production and suppressed alkaline phosphatase activity and mineralization of C3H10T1/2. Thus, hypoxia promoted chondrocytic commitment rather than osteoblastic differentiation. In the mice embryo forelimb organ culture, hypoxia increased cartilaginous matrix synthesis. These effects were primarily mediated by p38 MAPK activation, independent of Sox9. Hypoxia inhibited Col10a1 (type X collagen ␣1) expression via downregulation of Runx2 activity by Smad suppression and histone deacetylase 4 activation. In conclusion, hypoxia promotes chondrocytic differentiation and cartilage matrix synthesis and suppresses terminal chondrocyte differentiation. These hypoxiainduced phenomena may act on chondrocytes to enhance and preserve their phenotype and function during chondrocyte differentiation and endochondral ossification.A number of pathophysiological findings suggest that a correlation exists between hypoxia and chondrogenesis. For example, articular cartilage is an avascular tissue that functions at an oxygen tension that is lower than that of most other tissues. Articular cartilage derives both its nutrition and oxygen supply by diffusion from the synovial fluid and the subchondral bone. It has been estimated that articular chondrocytes in the deepest layers may have access to no more than 1-6% O 2 (1-6). Furthermore, although the majority of mammalian cells derive their energy by using oxygen for mitochondrial oxidative phosphorylation (7), few mitochondria are present in articular chondrocytes (8). Carbohydrate breakdown in articular cartilage is dominated by the conversion of glucose to lactate via the Embden-Meyerhof-Parnas pathway (9 -11) that consumes no O 2 . Similarly, during the endochondral ossification processes that occur in the growth plate, chondromodulin-1, an endogenous inhibitor of neovascularization, is highly expressed by chondrocytes. Of note, most of the growth plate is avascular (12). Recently, in an in vivo experiment, it was found that hypoxia-inducible factor 1, which appears to be one of the major regulators of the hypoxic response, is essential for chondrocyte growth arrest and survival (1...