Biology Reference
In-Depth Information
Bone Remodeling
Bone is in a constant state of turnover. Bone remodeling involves the continuous process of
resorption and deposition. Remodeling is the replacement of primary bone with secondary
bone. Basic multicellular units (or BMUs) are complex arrangements of cells responsible
for remodeling (
Frost, 1964; Martin and Burr, 1989; Junqueira et al., 1998
). Unlike bone
modeling, bone remodeling occurs at a specific site and in a consecutive order
d
activation,
resorption, and formation (ARF). During the processes of bone remodeling, the osteoclasts
and osteoblasts work sequentially to resorb and deposit new bone in response to normal
turnover, biomechanical factors, hormonal changes,
increases or decreases in activity
patterns, or from injury or trauma.
Bone remodeling can involve the removal or resorption of older bone and the deposition of
newer bone to help alleviate biomechanical stress, damage, or increased strain.
Martin and
Burr (1989)
describe “normal” bone remodeling occurring in six successive phases. The first,
activation, involves the procurement of differentiated cells from available stem cell popula-
tions. In the second phase, bone resorption, the newly derived osteoclasts begin to break
down bone. The third phase, reversal, is the transitional period between the resorptive
and formative stages. Osteoclastic activity converts at varying rates to osteoblastic activity.
During this time secondary osteons are formed and surrounded by cement reversal lines
(
Frost, 1964; Lacroix, 1971; Recker, 1983
). These lines are mineral-deficient layers of the
bone matrix that separate secondary osteons from the lamellar bone.
Bone formation is the fourth phase. Osteoblasts deposit osteoid and lay down concentric
lamellae leaving a centralized Haversian canal to serve as a pathway for blood vessels and
nerves. The fifth phase, mineralization, involves the calcification of the secreted osteoid. Over
60% of the osteoid becomes mineralized within the first 24 hours of its secretion (
Martin and
Burr, 1989
). The final phase is quiescence. During this sixth step, osteoclasts have vanished
and most of the osteoblasts have become osteocytes or have disappeared (
Martin and Burr, 1989
).
The total amount of time required to move through these six phases is called “sigma.”
Frost (1963)
coined this termwith “Sigma R” referring to resorption time and “Sigma F” refer-
ring to refilling time. Refilling time refers to bone formation. In cortical bone, it would take an
estimated 120 days (30 days for resorption and 90 days for formation) to move from the initial
phase and resorption cavity to a completed secondary osteon (
Frost, 1963; Martin and Burr,
1989
).
These six consecutive phases can occur in different locations in the bony matrix and at
varying magnitudes and rates. An increase or decrease in the lag time between subsequent
phases can affect bone structure and integrity.
Bone remodeling forms secondary bone and secondary osteons. The secondary osteon is
characterized by lamellar bone surrounding a central Haversian canal with a clear cement or
reversal line marking its boundary. Secondary osteons replacing primary bone are solely the
result of remodeling. Mature compact bone is comprised of secondary osteons (
Figure 13.3
).
Being able to distinguish between primary and secondary osteons is vital to histological
research. As mentioned, the primary osteon is initial bone tissue laid down on an existing
bone surface (immature woven bone) and surrounded by concentric rings of lamellar
bone. These concentric rings allow for the protection of the central blood vessel that passes
through each osteon. If these primary osteons do not have concentric lamellae, they are called
