Bone Cell Mechanoresponsiveness - Skeletal Aging and Osteoporosis: Biomechanics and Mechanobiology - page 182

bone loss in a tail-limb suspension model [ 96 ]. 1 As with MSCs, osteocyte number
decreases with age [ 62 ], and, quite interestingly, osteocyte number is different in
males and females [ 103 ].
7 Conclusions
There is in vivo evidence both for, and against, diminished skeletal adaptation to
load with age. If in fact there is diminished mechanotransduction with age, there
appears to be little evidence that this is due to intrinsic deficits in the ability of an
osteogenic cell to respond to loading. While aged cells demonstrate decreased
responsiveness to agents like PTH or IGF, the limited available data suggest that
there is little intrinsic influence of aging or disease state upon the ability of in vitro
osteoblastic cells to perceive or respond to mechanical stresses.
Acknowledgments This work was supported by NIH NIAMS R03 AR57547 (DCG) and NIH
NIAMS R01 AR45989, NIAMS R21 AR45156, and New York Stem Cell Grant N06G-210
(CRJ). The authors are grateful to Dr. C.E. Yellowley for suggestions to Fig. 1 , and to Dr. R.Y.
Kwon for helpful discussion.
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1 Interestingly, transgenic mice that lost bone under tail suspension did not require osteocytes to
regain bone mass, suggesting that osteocytes are indispensible for bone loss due to unloading,
whereas reloading-induced increases in bone mass are osteocyte-independent. These data indicate
that, in the absence of osteocytes, osteoblasts are sufficiently mechanosensitive and mechano-
responsive to initiate skeletal adaptation.