Chapter 6
Bones and Skeletal Tissues
177
6
Mineral and growth factor storage.
Bone is a reservoir for
minerals, most importantly calcium and phosphate. Te
stored minerals are released into the bloodstream in their
ionic form as needed for distribution to all parts of the body.
Indeed, “deposits” and “withdrawals” of minerals to and from
the bones go on almost continuously. Additionally, mineral-
ized bone matrix stores important growth factors.
Blood cell formation.
Most blood cell formation, or
hema-
topoiesis
(hem
0
ah-to-poi-e
9
sis), occurs in the red marrow
cavities of certain bones.
Triglyceride (fat) storage.
Fat, a source of energy for the
body, is stored in bone cavities.
Hormone production.
Bones produce osteocalcin, a hor-
mone which not only helps regulate bone formation, but also
protects against obesity, glucose intolerance, and diabetes
mellitus. (Osteocalcin is discussed further in Chapter 16.)
Check Your Understanding
7.
What is the functional relationship between skeletal muscles
and bones?
8.
What two types of substances are stored in bone matrix?
9.
Describe two functions of a bone’s marrow cavities.
For answers, see Appendix H.
Bone Structure
Describe the gross anatomy of a typical flat bone and a
long bone. Indicate the locations and functions of red
and yellow marrow, articular cartilage, periosteum, and
endosteum.
Indicate the functional importance of bone markings.
Describe the histology of compact and spongy bone.
Discuss the chemical composition of bone and the
advantages conferred by its organic and inorganic
components.
Because they contain different types of tissue, bones are organs.
(Recall that an organ contains several different tissues.) Although
bone (osseous) tissue dominates bones, they also contain nervous
tissue in their nerves, cartilage in their articular cartilages, fibrous
connective tissue lining their cavities, and muscle and epithelial
tissues in their blood vessels. We will consider bone structure at
three levels: gross, microscopic, and chemical.
Gross Anatomy
Bone Textures: Compact and Spongy Bone
Every bone has a dense outer layer that looks smooth and solid
to the naked eye. Tis external layer is
compact bone
(Fig-
ures 6.3 and 6.4). Internal to this is
spongy bone
(also called
trabecular bone
), a honeycomb of small needle-like or flat pieces
called
trabeculae
(trah-bek
9
u-le; “little beams”). In living bones
the open spaces between trabeculae are filled with red or yellow
bone marrow.
Structure of Short, Irregular, and Flat Bones
Short, irregular, and flat bones share a simple design: Tey all
consist of thin plates of spongy bone covered by compact bone.
Tese plates are covered outside and inside by connective tissue
membranes, respectively the periosteum and endosteum (de-
scribed on pp. 178–179). However, these bones are not cylindri-
cal and so they have no sha± or epiphyses. Tey contain bone
marrow (between their trabeculae), but no well-defined mar-
row cavity. Where they form movable joints with their neigh-
bors, hyaline cartilage covers their surfaces.
Figure 6.3
shows a typical flat bone of the skull. In flat bones,
the spongy bone is called the
diploë
(dip
9
lo-e; “folded”) and the
whole arrangement resembles a stiffened sandwich.
Structure of a Typical Long Bone
With few exceptions, all long bones have the same general struc-
ture: a sha±, bone ends, and membranes
(Figure 6.4)
.
Compact
bone
Trabeculae of
spongy bone
Spongy
bone
(diploë)
Figure 6.3
Flat bones consist of a layer of spongy bone
sandwiched between two thin layers of compact bone.
(Photomicrograph at bottom, 25
3
)
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