Bone sarcomas are a clinically and molecularly heterogeneous group of malignancies characterized by varying degrees of mesenchymal differentiation. Despite advances in medical and surgical management, survival rates for high-grade tumors have remained static at 50% to 70%. Tumor stem cells have been recently implicated in the pathogenesis of other heterogeneous, highly malignant tumors. We demonstrate here the existence of a small subpopulation of self-renewing bone sarcoma cells that are capable of forming suspended spherical, clonal colonies, also called "sarcospheres," in anchorage-independent, serum-starved conditions. These bone sarcoma cells as well as tissue specimens express activated STAT3 and the marker genes of pluripotent embryonic stem (ES) cells, Oct 3/4 and Nanog. Expression levels of Oct 3/4 and Nanog are greater in sarcospheres than in adherent cultures. A subset of bone sarcoma cells displays several surface markers of mesenchymal stem cells (Stro-1, CD105, and CD44) as well as attributes of mesodermal, ectodermal, and endodermal differentiation. Although previously documented in brain and breast tumors, our results support the extension of the cancer stem cell hypothesis to include tumors of mesenchymal lineage. Furthermore, they suggest the participation of ES cell homeobox proteins in non-germ cell tumorigenesis.

Osteoarthritis in horses and in humans is

Hyaline articular cartilage, the load-bearing tissue of the joint, has very limited repair and regeneration capacities. The lack of efficient treatment modalities for large chondral defects has motivated attempts to engineer cartilage constructs in vitro by combining cells, scaffold materials and environmental factors, including growth factors, signaling molecules, and physical influences. Despite promising experimental approaches, however, none of the current cartilage repair strategies has generated long lasting hyaline cartilage replacement tissue that meets the functional demands placed upon this tissue in vivo. The reasons for this are diverse and can ultimately result in matrix degradation, differentiation or integration insufficiencies, or loss of the transplanted cells and tissues. This article aims to systematically review the different causes that lead to these impairments, including the lack of appropriate differentiation factors, hypertrophy, senescence, apoptosis, necrosis, inflammation, and mechanical stress. The current conceptual basis of the major biological obstacles for persistent cell-based regeneration of articular cartilage is discussed, as well as future trends to overcome these limitations. IntroductionStructure and function of articular cartilage Articular cartilage is a highly specialized tissue that protects the bones of diarthrodial joints from forces associated with load bearing and impact, and allows nearly frictionless motion between the articulating surfaces [1,2]. The extracellular matrix (ECM) of articular cartilage is distinct from that of other connective tissues, consisting of an intricate network containing predominantly fibrillar collagens and proteoglycans. The collagens, types II, IX and XI, form a fibrous framework that gives the tissue its shape, strength and tensile stiffness [3]. Collagen type VI is found pericellularly around chondrocytes [4], and collagen type X is found in calcifying cartilage [5]. Although collagen type I is the most prevalent collagen throughout the body, the primary constituent of the articular cartilage matrix is type II, comprising 80% to 90% of the collagen content [3]. The proteoglycans in articular cartilage in their most abundant form exist as large hydrophilic aggregates, which contain the fluid component and control its movement. The level of compaction of the proteoglycans within the collagen lattice will determine their level of hydration and, in turn, the stiffness of the articular cartilage. The synthesis, incorporation and degradation of ECM proteins are orchestrated by chondrocytes that populate the matrix at low density [3]. Because articular cartilage is avascular, nutrients for the chondrocytes are supplied from the capillaries of the synovium and must diffuse into the synovial fluid and then into the cartilage matrix. Coordinated synthesis and proteolytic breakdown of certain ECM components by chondrocytes enables certain components of the cartilage matrix to undergo turnover and maintenance [3]. Factors that impair cho...

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