of the extracellular matrix during a process called cal-
cification (
kal-si-fi-KA-shun
). The mineral salts crys-
tallize and the tissue hardens.
A bone’s hardness is determined by the degree of cal-
cification, but its flexibility is determined by the collagen
fibers. These fibers, along with other organic molecules,
provide
tensile strength
(resistance to being stretched). Be-
sides osteoblasts, three other cell types help maintain
bone tissue: osteoclasts, osteocytes, and osteogenic cells
(see Figure 5.2).
Bone is not solid but contains numerous spaces for
blood vessels and storage areas for marrow. Depending on
the size and distribution of the spaces, bone can be clas-
sified as compact bone or spongy bone (see Figure 5.2).
Features of compact bone include the following:
Compact bone is strong and dense, provides protection
and support, and resists the stresses produced by
weight and movement.
Compact bone is found beneath the periosteum of all
bones and makes up the bulk of long bones.
Compact bone is made of cylindrical units called
osteons (
OS-te-ons
).
Each osteon consists of concentric lamellae (
la-MEL-e
),
concentric layers that surround a central canal, or
haversian canal, containing blood and lymph vessels.
Between
the
lamellae
are
spaces
called
lacunae
(
la-KOO-ne
), which contain osteocytes, and smaller
channels called canaliculi (
kan'-a-LIK-u-li
) that radiate
out from the lacunae. These canaliculi allow nutrients
and wastes to be passed more easily from one osteocyte
to another within the osteon.
In contrast to compact bone, spongy bone is lightweight.
Spongy bone, also known as cancellous bone, contains ir-
regular lattices of thin bone columns called trabeculae
(
tra-BEK-u-le
). Trabeculae form a supportive framework
that is firm but not exceedingly strong. This tissue must
be covered by compact bone or cartilage because it could
be damaged easily if exposed. The spaces between the
trabeculae of some bones are filled with red bone mar-
row; in such cases, the functions of the trabeculae are to
support and protect the red bone marrow. Spongy bone is
found mostly in short, flat, and irregular bones. In long
bones, spongy tissue forms the majority of the epiphyses
and is also found around the inner rim of the diaphysis.
Now that we have examined the microscopic structure
of bones, let’s see how they form.
Bone Is Formed During Ossification
and Maintained by Remodeling
The process of bone formation, called ossification (
os'-i-
fi-KÀ-shun
), occurs in four situations:
Initial formation of bones in the embryo and fetus
Bone growth during infancy, childhood, and adolescence
prior to adulthood
Bone remodeling, which occurs as old bone tissue is
replaced with new bone tissue throughout life
Repair of broken bones (fractures) throughout life
Bones form initially in the embryo by two processes. In
the first process, called intramembranous ossification
(
in'-tra-MEM-bra-nus
), bone forms
directly from mesenchyme. Intra-
membranous ossification occurs in
the flat bones of the skull, man-
dible, and clavicle. In the second
process, called endochondral ossification (
en'-do-KON-
dral
), bone forms within and replaces cartilage (Figure 5.3
on the next page). Intramembranous ossification is the
simpler of these two processes.
During
infancy,
childhood,
and
adolescence,
long
bones grow in both length and thickness:
Growth in length:
• Within the epiphyseal plate (cartilage) are chondrocytes
that divide and form additional cartilage.
• New chondrocytes form on the epiphyseal side, while
the cartilage on the diaphyseal side is replaced by bone.
• The thickness of the epiphyseal plate remains the
same, but the bone lengthens.
• Cartilage
growth stops
at
adulthood,
and bone
replaces
the
remaining
cartilage
to
form
the
epiphyseal line.
Growth in thickness:
• As the bone lengthens, it also thickens.
• Cells in the perichondrium differentiate into osteoblasts,
which secrete extracellular matrix that calcifies.
• Osteoblasts differentiate into osteocytes as new
lamellae are formed.
• Osteoclasts break down the inner surface of the
medullary cavity but at a slower rate than the bone
forms on the outer surface. So the medullary cavity
grows in diameter as the bone thickens.
mesenchyme
(MEZ-en-kim) Em-
bryonic connective
tissue.
The Structure of Bone Controls Function and Growth
117
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