HUMAN BIOLOGY

88 Chapter 5

bone: Mineralized Connective tissue

matrix around osteoblasts hardens when salts of the min-

eral calcium are deposited in it. Each osteoblast is trapped

in a space, or lacuna, in the matrix (lacuna means hole). At

this point their bone-forming function ends and they are

called osteo cytes (osteo 5 bone; cyte 5 cell).

The minerals and collagen in bone tissue make it hard,

but it is the collagen that gives our bones the strength to

withstand the mechanical stresses associated with activi-

ties such as standing, lifting, and tugging.

there are two kinds of bone tissue

Bones contain two kinds of tissue, compact bone and

spongy bone. Figure 5.1 shows where these tissues are in a

long bone such as the femur (thighbone). As its name sug-

gests, compact bone is a dense tissue that looks solid and

smooth. In a long bone, it forms the bone’s shaft and the

outer part of its two ends. A cavity inside the shaft contains

bone marrow.

Compact bone tissue forms in thin, circular layers

around small central canals. Each set of layers is called an

osteon (os-tee-ahn; sometimes called a Haversian system).

The canals connect with each other and serve as channels

for blood vessels and nerves that transport substances to

and from osteocytes (Figure 5.1A, right, and Figure 5.1B).

Osteocytes also extend slender cell processes into narrow

channels called canaliculi that run between lacunae. These

“little canals” allow nutrients to move through the hard

matrix from osteocyte to osteocyte. Wastes can be removed

the same way.

The bone tissue inside a long bone’s shaft and at its ends

looks like a sponge. Tiny, flattened struts are fused together

to make up this spongy bone tissue, which looks lacy and

delicate but actually is quite firm and strong.

a bone develops on a cartilage model

An early embryo has a rubbery skeleton that consists of

cartilage and membranes. Yet, after only about 2 months

of life in the womb, this flexible framework is transformed

into a bony skeleton. Once again, we can look at the devel-

opment of a long bone as an example.

As you can see at the top of Figure 5.2, a cartilage

“model” provides the pattern for each long bone. Once

the outer membrane is in place on the model, the bone-

forming osteoblasts become active and a bony “collar”

forms around the cartilage shaft. Then the cartilage inside

the shaft calcifies, and blood vessels, nerves, and elements

including osteoblasts begin to infiltrate the forming bone.

Soon, the marrow cavity forms and osteoblasts produce the

matrix that will become mineralized with calcium.

Each end of a long bone is called an epiphysis (e-pif-uh-

sis). As long as a person is growing, each epiphysis is sepa-

rated from the bone shaft by an epiphyseal plate of cartilage.

n Bone tissue is a form of connective tissue hardened by the

mineral calcium.

n Link to Connective tissues 4.2

Bone is a connective tissue, so it is a blend of living cells
and a matrix that contains fibers. Bones are covered by a
sturdy two-layer membrane called the periosteum (mean-
ing “around the bone”). The membrane’s outer layer is
dense connective tissue, and the inner layer contains bone
cells called osteoblasts (“bone formers”). As bone devel-
ops, the osteo blasts secrete collagen and some elastin, as
well as carbohydrates and other proteins. With time, this

Figure 5.1 Animated! Bones contain both compact and
spongy bone tissue. A Spongy and compact bone tissue in
a femur (thighbone). B The canal in the center of each osteon
contains blood vessels and nerves. The blood vessel carries
substances to and from osteocytes, the living bone cells in
small spaces in the bone tissue. Narrow tunnels called
canaliculi connect neighboring spaces. (© Cengage Learning)

blood

vessel

space occupied

by living bone cell

A

B

compact

bone tissue

spongy

bone

tissue

spongy

bone

tissue

compact

bone tissue

blood vessel

outer layer

of dense

connective tissue

osteon

(Haversian system)

© Ed Reschke/Peter Arnold

5.1

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