Chapter 6
Bones and Skeletal Tissues
181
6
Te manner in which canaliculi are formed is interesting.
When bone is forming, the osteoblasts secreting bone matrix
surround blood vessels and maintain contact with one another
and local osteocytes by tentacle-like projections containing gap
junctions. Ten, as the newly secreted matrix hardens and the
maturing cells become trapped within it, a system of tiny canals—
the
canaliculi
filled with tissue fluid and containing the osteocyte
extensions—is formed. Te canaliculi tie all the osteocytes in a
mature osteon together, allowing them to communicate and per-
mitting nutrients and wastes to be relayed from one osteocyte to
the next throughout the osteon. Although bone matrix is hard
and impermeable to nutrients, its canaliculi and cell-to-cell relays
(via gap junctions) allow bone cells to be well nourished.
Interstitial and Circumferential Lamellae
Not all the lamellae
in compact bone are part of complete osteons. Lying between
intact osteons are incomplete lamellae called
interstitial lamel-
lae
(in
0
ter-stish
9
al) (Figure 6.7c, right photomicrograph). Tey
either fill the gaps between forming osteons or are remnants of
osteons that have been cut through by bone remodeling (dis-
cussed later).
Circumferential lamellae
, located just deep to
the periosteum and just superficial to the endosteum, extend
around the entire circumference of the diaphysis (Figure 6.7a)
and effectively resist twisting of the long bone.
Spongy Bone
In contrast to compact bone, spongy bone looks like a
poorly organized, even haphazard, tissue (see Figure 6.4 and
Figure 6.3b). However, the trabeculae in spongy bone align
precisely along lines of stress and help the bone resist stress.
Tese tiny bone struts are as carefully positioned as the ca-
bles on a suspension bridge.
Bone Lining Cells
Bone lining cells
are flat cells found on bone
surfaces where bone remodeling is not going on. Like osteocytes,
they are thought to help maintain the matrix. Bone lining cells on
the external bone surface are also called
periosteal cells
, whereas
those lining internal surfaces are called
endosteal cells
.
Osteoclasts
Derived from the same hematopoietic stem cells
that differentiate into macrophages,
osteoclasts
are giant multi-
nucleate cells located at sites of bone resorption. When actively
resorbing (breaking down) bone, the osteoclasts rest in a shal-
low depression called a
resorption bay
and exhibit a distinctive
ruffled border
which directly contacts the bone. Te deep plasma
membrane infoldings of the ruffled border tremendously in-
crease the surface area for enzymatically degrading the bones
and seal off that area from the surrounding matrix.
Compact Bone
Although compact bone looks solid, a microscope reveals that
it is riddled with passageways that serve as conduits for nerves
and blood vessels (see Figure 6.7).
Osteon (Haversian System)
Te structural unit of compact
bone is called either the
osteon
(os
9
te-on) or the
Haversian
system
(ha-ver
9
zhen). Each osteon is an elongated cylinder ori-
ented parallel to the long axis of the bone. Functionally, osteons
are tiny weight-bearing pillars.
As shown in the “exploded” view in
Figure 6.6
, an osteon is
a group of hollow tubes of bone matrix, one placed outside the
next like the growth rings of a tree trunk. Each matrix tube is
a
lamella
(lah-mel
9
ah; “little plate”), and for this reason com-
pact bone is o±en called
lamellar bone
. Although all of the
collagen fibers in a particular lamella run in a single direction,
the collagen fibers in adjacent lamellae always run in different
directions. Tis alternating pattern is beautifully designed to
withstand torsion stresses—the adjacent lamellae reinforce one
another to resist twisting. You can think of the osteon’s design as
a “twister resister.”
Collagen fibers are not the only part of bone lamellae that are
beautifully ordered. Te tiny crystals of bone salts align between
the collagen fibrils and thus also alternate their direction in ad-
jacent lamellae.
Canals and Canaliculi
Running through the core of each
osteon is the
central canal
, or
Haversian canal
, containing
small blood vessels and nerve fibers that serve the osteon’s
cells. Canals of a second type called
perforating canals
, or
Volkmann’s canals
(folk
9
mahnz), lie at right angles to the long
axis of the bone and connect the blood and nerve supply of the
periosteum to those in the central canals and the medullary
cavity
(Figure 6.7a)
. Unlike the central canals of osteons, the
perforating canals are not surrounded by concentric lamellae,
but like all other internal bone cavities, these canals are lined
with endosteum.
Spider-shaped osteocytes (Figures 6.5c and 6.7b) occupy
la-
cunae
(
lac
5
hollow;
una
5
little) at the junctions of the lamel-
lae. Hairlike canals called
canaliculi
(kan
0
ah-lik
9
u-li) connect
the lacunae to each other and to the central canal.
Structures
in the
central
canal
Artery with
capillaries
Vein
Nerve fiber
Lamellae
Collagen
fibers
run in
different
directions
Twisting
force
Figure 6.6
A single osteon.
The osteon is drawn as if pulled out
like a telescope to illustrate the individual lamellae.
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