Chapter 6
Bones and Skeletal Tissues
183
6
Organic Components
Te organic components of bone include its cells (osteogenic
cells, osteoblasts, osteocytes, bone-lining cells, and osteoclasts)
and
osteoid
(os
9
te-oid), the organic part of the matrix. Osteoid,
which makes up approximately one-third of the matrix, includes
ground substance (composed of proteoglycans and glycopro-
teins) and collagen fibers, both of which are made and secreted
by osteoblasts. Tese organic substances, particularly collagen,
contribute both to a bone’s structure and to the flexibility and
tensile strength that allow it to resist stretch and twisting.
Bone’s resilience is thought to come from
sacrificial bonds
in
or between collagen molecules. Tese bonds stretch and break
easily on impact, dissipating energy to prevent the force from
rising to a fracture value. In the absence of continued or addi-
tional trauma, most of the sacrificial bonds re-form.
Inorganic Components
Te balance of bone tissue (65% by mass) consists of inorganic
hydroxyapatites
(hi-drok
0
se-ap
9
ah-tītz), or
mineral salts
, largely
calcium phosphates present as tiny, tightly packed, needle-
like crystals in and around collagen fibers in the extracellular
matrix. Te crystals account for the most notable characteris-
tic of bone—its exceptional hardness, which allows it to resist
compression.
Te proper combination of organic and inorganic matrix
elements makes bone exceedingly durable and strong without
being brittle. Healthy bone is half as strong as steel in resisting
compression and fully as strong as steel in resisting tension.
Because of the mineral salts they contain, bones last long af-
ter death and provide an enduring “monument.” In fact, skeletal
remains many centuries old can still reveal the shapes and sizes
of ancient peoples, the kinds of work they did, and many of the
ailments they suffered, such as arthritis.
Growth arrest lines
, hori-
zontal lines on long bones, provide visible proof of illness when the
body uses nutrients to fight disease and the bones stop growing.
Check Your Understanding
10.
Are crests, tubercles, and spines bony projections or
depressions?
11.
How does the structure of compact bone differ from that of
spongy bone when viewed with the naked eye?
12.
Which membrane lines the internal canals and covers the
trabeculae of a bone?
13.
Which component of bone—organic or inorganic—makes it
hard?
14.
Which cell has a ruffled border and acts to break down bone
matrix?
For answers, see Appendix H.
Bone Development
Compare and contrast intramembranous ossification and
endochondral ossification.
Describe the process of long bone growth that occurs at
the epiphyseal plates.
Ossification
and
osteogenesis
(os
0
te-o-jen
9
ĕ-sis) are synonyms
meaning the process of bone formation (
os
5
bone,
genesis
5
be-
ginning). In embryos this process leads to the formation of the
bony skeleton. Later another form of ossification known as
bone
growth
goes on until early adulthood as the body increases in size.
Bones are capable of growing thicker throughout life. However,
ossification in adults serves mainly for bone
remodeling
and repair.
Formation of the Bony Skeleton
Before week 8, the skeleton of a human embryo is constructed
entirely from fibrous membranes and hyaline cartilage. Bone
tissue begins to develop at about this time and eventually re-
places most of the existing fibrous or cartilage structures.
In
endochondral ossification
(
endo
5
within,
chondro
5
car-
tilage), a bone develops by replacing hyaline cartilage. Te
resulting bone is called a
cartilage
, or
endochondral
,
bone
.
In
intramembranous ossification
, a bone develops from a fi-
brous membrane and the bone is called a
membrane bone
.
Te beauty of using flexible structures (membranes and carti-
lages) to fashion the embryonic skeleton is that they can accom-
modate mitosis. Were the early skeleton composed of calcified
bone tissue from the outset, growth would be much more difficult.
Endochondral Ossification
Except for the clavicles, essentially all bones below the base of
the skull form by
endochondral ossification
(en
0
do-kon
9
dral).
Beginning late in the second month of development, this pro-
cess uses hyaline cartilage “bones” formed earlier as models,
or patterns, for bone construction. It is more complex than in-
tramembranous ossification because the hyaline cartilage must
be broken down as ossification proceeds.
For example, the formation of a long bone typically be-
gins in the center of the hyaline cartilage sha± at a region
called the
primary ossification center
. First, blood vessels
infiltrate the perichondrium covering the hyaline cartilage
“bone,” converting it to a vascularized periosteum. As a re-
sult of this change in nutrition, the underlying mesenchymal
cells specialize into osteoblasts. Te stage is now set for os-
sification to begin
(Figure 6.8)
:
1
A bone collar forms around the diaphysis of the hyaline
cartilage model.
Osteoblasts of the newly converted perios-
teum secrete osteoid against the hyaline cartilage diaphysis,
encasing it in a cuff or collar of bone called the
periosteal
bone collar.
2
Cartilage in the center of the diaphysis calcifies and then
develops cavities.
As the bone collar forms, chondrocytes
within the sha± hypertrophy (enlarge) and signal the sur-
rounding cartilage matrix to calcify. Ten, because calcified
cartilage matrix is impermeable to diffusing nutrients, the
chondrocytes die and the matrix begins to deteriorate. Tis
deterioration opens up cavities, but the bone collar stabi-
lizes the hyaline cartilage model. Elsewhere, the cartilage
remains healthy and continues to grow briskly, causing the
cartilage model to elongate.
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