HUMAN BIOLOGY

the SKeLetaL SyStem 89

What is bone?

• Bone is a form of connective tissue that consists of living cells
  and a nonliving matrix that is hardened by the mineral calcium.


• Bones contain two types of bone tissue—dense compact bone
  and lacy but strong spongy bone.


• Bones grow, become strong, and are repaired through the
  process called bone remodeling.


• Bone remodeling is important in homeostasis because it plays
  a major role in maintaining the proper balance of calcium in the
  blood.

taKe-hoMe Message

bone Connective tissue

that functions in move‑

ment and locomotion,

protection of other organs,

mineral storage, and (in

some bones) blood cell

production.

bone remodeling Recy‑

cling of calcium as it is

deposited and withdrawn

from bones.

compact bone Denser,

solid‑looking type of bone

tissue, which forms in slen‑

der, circular layers.

epiphysis End of a long

bone.

osteoblast Type of bone

cell that forms the matrix of

bone tissue, which eventu‑

ally becomes mineralized.

osteoclast Type of bone

cell that breaks down the

matrix of bone tissue.

osteocyte Name for a

bone‑forming cell (osteo‑

blast) after the matrix

around it has become min‑

eralized and the cell stops

forming bone matrix.

osteon Each set of thin,

circular layers that forms in

compact bone. The layers

encircle a channel for blood

vessels and nerves.

spongy bone Lacy, more

open type of bone tissue. In

long bones it occurs inside

the bone shaft.

bone, osteoclasts release more cal-

cium than usual from bone matrix.

Osteoblasts then use the calcium to

repair the injured bone tissue.

A child’s body requires lots

of calcium to meet the combined

demands of bone growth and other

needs for the calcium stored in

bones. Along with dietary calcium,

remodeling helps meet the demand.

For example, the diameter of a grow-

ing child’s thighbones increases as

osteoblasts form bone at the surface

of each shaft. At the same time, how-

ever, osteoclasts break down a small

amount of bone tissue inside the

shaft. Thus the child’s thighbones

become thicker and stronger to sup-

port the increasing body weight, but

they don’t get too heavy.

Bone remodeling also plays a key

role in maintaining homeostasis of

the blood level of calcium. Neither

our nervous system nor our muscles

can function properly unless the

blood level of calcium stays within

a narrow range. When the level falls

below this range, a hormone called

PTH stimulates osteoclasts to break

down bone and release calcium

to the blood. If the level rises too

high, another hormone, calcitonin,

stimulates osteoblasts to deposit cal-

cium in bone tissue. Notice that this

control mechanism is an example

of negative feedback. You will read more about it in

Chapter 15, when we take a closer look at hormones.

Human growth hormone (GH) prevents the plates from
calcifying, so the bone can lengthen. When growth stops,
usually when people reach their late teens or early twenties,
bone replaces the cartilage plates.

bone tissue is constantly “remodeled”

Calcium is constantly entering and leaving our bones.
Calcium is deposited when osteoblasts form bone,
and it is withdrawn when “bone breaker” cells called
osteoclasts break down the matrix of bone tissue. This
ongoing calcium recycling is called bone remodeling,
and it has several important functions.
Regularly breaking down “old” bone and replacing it
with fresh tissue helps keep bone resilient, so it is less likely
to become brittle and break. When a bone is subjected to
mechanical stress, such as load-bearing exercise, the remod-
eling process is adjusted so that more bone is deposited
than removed. That is why the bones of regular exercisers
are denser and stronger than the bones of couch potatoes.
On the other hand, when the body must heal a broken

Figure 5.2 A long bone forms on a cartilage model. First,
osteoblasts begin to function in a cartilage model in the
embryo. The bone-forming cells are active first in the shaft,
then at the knobby ends. In time, cartilage is left only in the
epiphyses at the ends of the shaft.

Cartilage model of

future bone in embryo

When organs form in embryo,

blood vessel invades

model; osteoblasts

start producing bone

tissue; marrow

cavity forms

Remodeling and

growth continue

in newborn;

secondary bone-

forming centers

appear at knobby

ends of bone

Mature bone of adult

Forming

bone collar

epiphyses © Cengage Learning

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