Ultrastructural Study of Bone Formation on Synthetic Hydroxyapatite in Osteoblast Cultures

Collagenase isolated rat calvaria cells forming a mineralized matrix in vitro were cultured in the presence of synthetic hydroxyapatite. Interactions between bone cells and hydroxyapatite biomaterial were followed by transmission electron microscopy. The appearance of a granular, collagen free, electron-dense layer at the periphery of the material was noted initially. Progressively, an amorphous, granular material formed and extended between the hydroxyapatite aggregates. Osteoblastic cells then synthesized an osteoid matrix which mineralized on the first formed granular collagen free layer, following a classical pattern of calcification . Demineralization of ultrathin sections confirmed the presence of this interface between the material and bone tissue formed in vitro

Hedgehog Signaling in Tumor Cells Facilitates Osteoblast-Enhanced Osteolytic Metastases

The remodeling process in bone yields numerous cytokines and chemokines that mediate crosstalk between osteoblasts and osteoclasts and also serve to attract and support metastatic tumor cells. The metastatic tumor cells disturb the equilibrium in bone that manifests as skeletal complications. The Hedgehog (Hh) pathway plays an important role in skeletogenesis. We hypothesized that the Hh pathway mediates an interaction between tumor cells and osteoblasts and influences osteoblast differentiation in response to tumor cells. We have determined that breast tumor cells have an activated Hh pathway characterized by upregulation of the ligand, IHH and transcription factor GLI1. Breast cancer cells interact with osteoblasts and cause an enhanced differentiation of pre-osteoblasts to osteoblasts that express increased levels of the osteoclastogenesis factors, RANKL and PTHrP. There is sustained expression of osteoclast-promoting factors, RANKL and PTHrP, even after the osteoblast differentiation ceases and apoptosis sets in. Moreover, tumor cells that are deficient in Hh signaling are compromised in their ability to induce osteoblast differentiation and consequently are inefficient in causing osteolysis. The stimulation of osteoblast differentiation sets the stage for osteoclast differentiation and overall promotes osteolysis. Thus, in the process of developing newer therapeutic strategies against breast cancer metastasis to bone it would worthwhile to keep in mind the role of the Hh pathway in osteoblast differentiation in an otherwise predominant osteolytic phenomenon

Synergistic activity of polarised osteoblasts inside condensations cause their differentiation

Condensation of pre-osteogenic, or pre-chondrogenic, cells is the first of a series of processes that initiate skeletal development. We present a validated, novel, three-dimensional agent-based model of in vitro intramembranous osteogenic condensation. The model, informed by system heterogeneity and relying on an interaction-reliant strategy, is shown to be sensitive to 'rules' capturing condensation growth and can be employed to track activity of individual cells to observe their macroscopic impact. It, therefore, makes available previously inaccessible data, offering new insights and providing a new context for exploring the emergence, as well as normal and abnormal development, of osteogenic structures. Of the several stages of condensation we investigate osteoblast 'burial' within the osteoid they deposit. The mechanisms underlying entrapment - required for osteoblasts to differentiate into osteocytes - remain a matter of conjecture with several hypotheses claiming to capture this important transition. Computational examination of this transition indicates that osteoblasts neither turn off nor slow down their matrix secreting genes - a widely held view; nor do they secrete matrix randomly. The model further reveals that osteoblasts display polarised behaviour to deposit osteoid. This is both an important addition to our understanding of condensation and an important validation of the model's utility

Pour une approche génétique et expérimentale de la croissance squelettique cranio-faciale : étude du rôle de l'homéogène divergent Msx1

Les agénésies dentaires et les fentes palatines sont associées à des mutations de l'homéogène Msx1, soulignant ainsi le rôle primordial de ce gène au cours du développement initial du squelette craniofacial. Il contrôle la prolifération et la différenciation terminale des cellules formant les tissus minéralisés cranio-faciaux. Récemment, un ARN antisens du gène Msx1 a été identifié ; il inhibe la synthèse de la protéine Msx1 dans ces mêmes cellules. Le but de cet article est d'examiner le rôle du gène Msx1 et sa régulation pendant les étapes tardives du développement du squelette cranio-facial. Les modalités de la croissance osseuse ainsi que les schémas d'expression des transcrits sens (qui codent pour la protéine), des transcrits antisens de Msx1 et de la protéine Msx1 ont été étudiés en post-natal chez la souris normale et chez la souris transgénique dont l'expression du gène Msx1 est supprimée. La protéine Msx1 s'exprime essentiellement dans les pré-ostéoblastes localisés dans certains sites squelettiques spécifiques comme la base de la mandibule. Ces mêmes sites présentent une diminution de croissance osseuse chez la souris transgénique. La comparaison des schémas d'expression de la protéine Msx1 et des transcrits sens/antisens de Msx1 suggère que la balance des transcrits sens/antisens de Msx1 contrôlerait la répartition des sites d'actions du gène Msx1 pendant la morphogenèse et la croissance du squelette cranio-facial. Chez l'humain, une étude clinique des patients présentant une mutation du gène MSX1 a été entreprise pour confirmer cette hypothèse, dans une perspective de diagnostic et de thérapeutique génétiques des anomalies de croissance cranio-faciale