Biology Reference
In-Depth Information
unmineralized organic part of bone). Osteoblasts mature into osteocytes, which become intri-
cately woven into the network of calcified bone matrix. Osteocytes maintain the ability to
communicate complex information along interconnecting pathways called canaliculi (as
many as 80 connections per osteocyte), providing information about mechanical forces in
order to make necessary modifications to the bone's shape (i.e. macrostructure) (
Pearson
and Lieberman, 2004
). In normal adult aging, the osteoblasts are typically more active at
the periosteum (external surface of the bone) and osteoclasts more actively resorb bone at
the endosteum (internal surface of medullary canal). Cell types and function are discussed
further in Trammell and Kroman (Chapter 13), this volume.
Bone strength depends on three characteristics: (1) material properties, (2) microstructure,
and (3) macrostructure (
Ruff, 1981
). The material properties include chemical composition
and bone density. The chemical composition of bone (percentage of collagen and hydroxyap-
atite) is the same in both trabecular and cortical bone in a single individual (Frankel and Nor-
din, 1980). Bone does, however, change in material properties throughout the life cycle, with
a greater percentage of collagen in a juvenile than in an elderly adult (
Beck et al., 1993
). The
diaphysis (shaft) of a long bone is predominantly cortical bone and the epiphyses (ends of
the long bone at the joint) are mostly trabecular bone. The bone macrostructure is a combina-
tion of external and internal geometric properties, as well as the trabecular orientation (
Ruff,
1981
).
Adult bone formation occurs via a process of Haversian remodeling compared to juvenile
bone modeling. The long bones of the skeleton in a subadult develop through a process called
endochondral ossification (i.e., bone formation that starts from a cartilage model). In remod-
eling, secondary osteons (those formed by the replacement of bone) will overlay the primary
lamellar bone (the original bone layers) (
Robling, 1998
). Bone remodeling in adults is mostly
subperiosteal expansion in which the bone shaft grows larger in diameter along with an
increase in the medullary canal, which is due to endosteal resorption by the osteoclasts.
There is some evidence of endosteal apposition in individuals, in which the bone grows
inward, decreasing the size of the medullary canal, but this is rare (
Ruff, et al., 1994; Pearson
and Lieberman, 2004; Moore et al., 2007
). When the bone is stressed, microfractures can
form in the gap junction (the space at the surface of each cell) between osteocytes to
help dissipate a force. If the stress is beyond a certain threshold, though not enough to
cause fracture, basic multicellular units (BMUs) will begin to increase bone apposition
(i.e., deposition), and decrease resorption to accommodate the increased load. BMUs are
made up of a combination of osteoblasts and osteoclasts that coordinate the apposition
and resorption. Conversely, when the load threshold has been brought to the lower end
of the spectrum for which the bone is well adapted, the remodeling is turned off (
Frost,
1997
). Microstructure of the bone is characterized by the modeling and remodeling of
Haversian systems and trabeculae, and is discussed at length in Trammell and Kroman
(Chapter 13), this volume.
BASIC BONE BIOMECHANICS
Bones can sense loads and self-regulate, via the process of mechanotransduction, in which
cells sense mechanical stimuli. The process is not completely clear, but it appears that the
