Bones and Skeletal Tissues
Te organic components of bone include its cells (osteogenic
cells, osteoblasts, osteocytes, bone-lining cells, and osteoclasts)
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
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.
Te balance of bone tissue (65% by mass) consists of inorganic
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
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
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
Are crests, tubercles, and spines bony projections or
How does the structure of compact bone differ from that of
spongy bone when viewed with the naked eye?
Which membrane lines the internal canals and covers the
trabeculae of a bone?
Which component of bone—organic or inorganic—makes it
Which cell has a rufﬂed border and acts to break down bone
For answers, see Appendix H.
Compare and contrast intramembranous ossiﬁcation and
Describe the process of long bone growth that occurs at
the epiphyseal plates.
ĕ-sis) are synonyms
meaning the process of bone formation (
ginning). In embryos this process leads to the formation of the
bony skeleton. Later another form of ossiﬁcation known as
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
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.
tilage), a bone develops by replacing hyaline cartilage. Te
resulting bone is called a
, a bone develops from a ﬁ-
brous membrane and the bone is called a
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.
Except for the clavicles, essentially all bones below the base of
the skull form by
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
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
A bone collar forms around the diaphysis of the hyaline
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
Cartilage in the center of the diaphysis calciﬁes and then
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.