We review the characteristics of osteoblast differentiation and bone matrix synthesis. Bone in air breathing vertebrates is a specialized tissue that developmentally replaces simpler solid tissues, usually cartilage. Bone is a living organ bounded by a layer of osteoblasts that, because of transport and compartmentalization requirements, produce bone matrix exclusively as an organized tight epithelium. With matrix growth, osteoblasts are reorganized and incorporated into the matrix as living cells, osteocytes, which communicate with each other and surface epithelium by cell processes within canaliculi in the matrix. The osteoblasts secrete the organic matrix, which are dense collagen layers that alternate parallel and orthogonal to the axis of stress loading. Into this matrix is deposited extremely dense hydroxyapatite-based mineral driven by both active and passive transport and pH control. As the matrix matures, hydroxyapatite microcrystals are organized into a sophisticated composite in the collagen layer by nucleation in the protein lattice. Recent studies on differentiating osteoblast precursors revealed a sophisticated proton export network driving mineralization, a gene expression program organized with the compartmentalization of the osteoblast epithelium that produces the mature bone matrix composite, despite varying serum calcium and phosphate. Key issues not well defined include how new osteoblasts are incorporated in the epithelial layer, replacing those incorporated in the accumulating matrix. Development of bone in vitro is the subject of numerous projects using various matrices and mesenchymal stem cell-derived preparations in bioreactors. These preparations reflect the structure of bone to variable extents, and include cells at many different stages of differentiation. Major challenges are production of bone matrix approaching the in vivo density and support for trabecular bone formation. In vitro differentiation is limited by the organization and density of osteoblasts and by endogenous and exogenous inhibitors.

Objective-Uterine leiomyoma produce an extracellular matrix (ECM) that is abnormal in its volume, content, and structure. Alterations in ECM can modify mechanical stress on cells, leading to activation of Rho-dependent signaling. Here we sought to determine whether the altered ECM produced by leiomyoma was accompanied by an altered state of mechanical homeostasis.Study Design-Measurement of the mechanical response in paired leiomyoma and myometrium, immunogold, confocal microscopy, and immunohistochemical analyses.Results-Leiomyoma were significantly stiffer than matched myometrium. The increased stiffness was associated with a moderate increase in total sulfated glycosaminoglycan content and a slight increase in hydroxyproline. Levels of the Rho-GEF, AKAP13, were increased and subcellular localization was altered in leiomyoma. Phosphorylation of p38MAPK was greater in leiomyoma extracts. Conclusions-Leiomyoma

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This international consensus derived from leaders in the field will assist clinicians with debridement, curettage and bone marrow stimulation as a treatment strategy for osteochondral lesions of the talus.

Objective/Hypothesis: To determine if angiogenic growth factors including vascular endothelial growth factor (VEGF) and platelet‐derived endothelial cell growth factor (PD‐ECGF) are expressed in human paragangliomas. Study Design: A histopathologic and molecular examination of paraganglioma specimens obtained from surgical cases or retrieved from the Pathology Department of the Massachusetts Eye and Ear Infirmary. Methods: Fresh tumor or archival, paraffin‐embedded paraganglioma specimens were analyzed by immunohistochemistry, Western blotting, and ELISA. Results: Positive immunohistochemical staining for VEGF was observed in five of nine surgical specimens and in six of eight archival specimens (11/17, or 65%). PD‐ECGF immunoreactivity was detected in four of five surgical specimens and six of eight archival specimens (10/13, or 77%). The presence of PD‐ECGF was confirmed by Western blot assay and ELISA confirmed the presence of VEGF in tumor extract. Conclusions: Both VEGF and PD‐ECGF are expressed in paragangliomas and may contribute to the extreme vascularity of these tumors.