Sequential Expression of Bone Matrix Proteins During Rat Calvaria Osteoblast Differentiation and Bone Nodule Formation In VitroJean R. Nefussia, Gabriel Bramia, Dominique Modrowskib, Martine Ob ufa, and
Nadine Foresta
a Laboratoire de Biologie-Odontologie, Université Paris VII, Paris, France b Institut National de la Santé et de la Recherche Médicale, Hôpital Lariboisière, Paris, France Correspondence to: Jean R. Nefussi, Laboratoire de Biologie-Odontologie, Inst. Biomédical des Cordeliers, 15-21, rue de lEcole de Médecine, F-75270 Paris Cedex 06, France. We investigated the expression of osteocalcin (OC), bone sialoprotein (BSP), osteonectin (ON), and alkaline phosphatase (ALP) during cell differentiation and bone nodule formation by fetal rat calvaria cells, using immunofluorescent and immunogold techniques at light and electron microscopic levels. Six hours after plating all proteins were expressed in calvaria cells. However, expression was not detected during the proliferation phase after plating. Cell morphological modifications were observed in osteoblastic cells expressing ALP, OC, and BSP, but not ON. During the matrix formation phase, all proteins were expressed with various intensities and OC was limited to differentiated osteoblastic cells. EM observations demonstrated that BSP was selectively associated with clusters of needle-like crystals, but not with collagen fibers, in mineralization foci and in the mineralized matrix. OC was localized intracellularly and in all the extracellular compartments, and was concentrated at the mineralization front. ON was distributed uniformly throughout the osteoid and mineralized matrix, which was intensely labeled. The results show that the expression of bone matrix proteins during differentiation of calvaria cells and nodule formation in vitro duplicate what is observed during osteogenesis in vivo. (J Histochem Cytochem 45:493-503, 1997) Key Words: osteogenic cell culture, differentiation and bone matrix formation, osteocalcin, osteonectin, bone sialoprotein
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