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Gravity-Induced Hyperproliferation in Primary Osteoblast Cultures is not Dependent on the Fraction of the Proliferative Cell Population.  J. Wilcox1 , M. Da Costa1, W. Vercoutere1, R. K. Globus1,2 and E. A.C. Almeida1,2  1 NASA Ames Research Center, Moffett Field, CA, 2University of California San Francisco.

   Gravity-generated forces are an important factor contributing to mechanostimulation in many cell types. Bone-forming osteoblasts in particular respond quickly to decreases in loading, contributing to loss of bone density in bed rest patients and astronauts.  Using hypergravity conditions as a model for mechanical loading in primary rat calvarial osteoblast cultures, we have previously determined that this stimulus promotes increased cell proliferation in a matrix and integrin-dependent fashion. Specifically, at 25g, and over 24h, we observe up to two-fold increase in cell number compared to osteoblasts grown at 1g.  In this study we sought to elucidate the cellular mechanisms by which gravity mechanostimulation promotes increased primary osteoblast proliferation.  Using BrdU (bromodeoxyuridine) incorporation over 24h to measure the percentage of replicating cells, we found no significant differences between 25g and 1g, (respectively 88.03%, ± 5.63% and 86.17% ± 4.55%).  This result indicates that a similar fraction of replicative primary osteoblast populations are present under both g-levels, and suggests the possibility that the observed cell number increases may result from faster progression through the cell cycle at 25g.   Additional studies are now underway to measure the length of the cell cycle under both 1g and 25g.

Source of support: NASA 00-OBPR-01-066