NANOINDENTATION OF BONE - Nanoindentation with Biological Applications - page 183

research have been devoted to studying the communication among these
cells, significant gaps remain in our knowledge of the specific molecular
and biochemical mechanisms underlying their behavior.
The principal role of each cell type is generally understood and is
covered in detail elsewhere. 2 In brief, osteoclasts are giant (20-100
micrometers in diameter), multinucleated cells that resorb, or remove,
bone by solubilizing the mineral and releasing the matrix proteins to the
surrounding interstitial fluid. Osteoblasts are small (≈15-30 micrometers
in diameter), single nucleus bone-forming cells. They lay down the
unmineralized osteoid, an organic extracellular matrix (ECM) onto
which these cells subsequently regulate the deposition of mineral through
a cellular mediated and/or thermodynamically driven process. 3,4 The
osteoblasts line a bone-forming surface that moves through space over
time, leaving a region of continual mineralizing bone behind. Osteocytes,
the most abundant cell type in bone, are osteoblasts that remain behind
the mineralizing front and are 'trapped' within the mineralized osteoid.
Information, such as chemical signals and mechanosensation, is thought
to be communicated through a porous, fluid-filled network of canalicular
processes that connect osteocytes to one another. 5 In addition to other
critical roles, osteocytes are thought to regulate local mineralization
levels by maintaining extracellular ion concentrations. Through their
collective actions and interactions, these three cell types determine and
regulate the material properties of bone.
1.3 . Bone as a material
Bone consists of an organic, mineral, and water phase where each
constituent contributes to the mechanical properties at multiple length
scales. 6 Fully mineralized, healthy bone is composed of approximately
by volume 50% mineral, 20% water, and 30% organic ( Fig. 7-2 ) . 7,8
While the macro-scale structure and composition of bone has been
widely studied and is fully understood, its nano-scale structure is much
more elusive.
At the microscopic scale, bone is formed of distinct types, woven or
lamellar, that each contributes to bone's material heterogeneity and