Throughout life, bone is constantly made and destroyed
in an ongoing process called bone remodeling. At various
stages of life, osteoblasts and osteoclasts regulate bone forma-
tion and bone destruction. During some stages, such as child-
hood, formation exceeds destruction, allowing for growth of
the skeleton. For a short period of time after ossification pro-
cesses cease, formation and destruction are fairly equivalent.
As a person ages, destruction often exceeds formation, lead-
ing to bone loss. Rates of remodeling vary depending on the
location within the body. The remodeling process allows for
mineral homeostasis, as well as repair of damaged bone tis-
sue. Remodeling is affected by many factors:
Hormones:
Parathyroid hormone (PTH)
—Stimulates osteoclasts
Calcitonin
—Stimulates osteoblasts
Human growth hormone (hGH)
—Stimulates cartilage
and bone growth
Insulin-like growth factor (IGF)
—Stimulates cartilage
and bone growth
Sex hormones
—Sex-related differences in skeletal growth
Minerals—Availability of calcium, magnesium, and
phosphorus
Vitamins A, C, and D
Activity level—Active versus sedentary lifestyle
Diet
Bone remodeling plays an active role in calcium homeo-
stasis because well over 90% of the calcium in the body is
located in the bone extracellular matrix. A certain level of
ionized calcium (Ca2+) in the blood (1.3 mM) is necessary
for proper functioning of nerve and muscle cells. Parathy-
roid glands detect decreased Ca2+
in the blood and secrete
PTH,
which acts on bone (stimulating
osteoclasts to resorb matrix), on
kidneys (to prevent calcium loss
into the urine), and indirectly on
intestines (to help absorb calcium
from food following
the
forma-
tion of calcitriol), bringing about
an increase in blood levels of Ca2+
(Figure 5.4).
The parafollicular cells of the
thyroid gland produce the hormone calcitonin, which is also
involved in calcium homeostasis. The parafollicular cells
monitor blood calcium levels, releasing calcitonin if the blood
calcium increases above normal. This hormone stimulates
osteoblasts to increase formation of bone extracellular ma-
trix and the kidneys to eliminate excess calcium, thus lower-
ing the calcium to more normal levels.
Parathyroid
glands
(par'-a-
THT-royd) Four small
endocrine glands
embedded in the
posterior surfaces of
the lateral lobes of the
thyroid gland.
calcitriol
The active
form of vitamin D.
Calcium h o m eo stasis • Figure 5.4
When blood Ca2+ level falls, PTH stimulates osteoclasts to resorb
bone and the kidneys to retain calcium and make calcitriol, which
acts on the intestines to absorb Ca2+. These actions restore Ca2+
to normal levels.
Effectors
Osteoclasts
increase
bone
resorption
Kidneys
retain Ca2
in blood
and
produce
calcitriol
Return to homeostasis
when response brings
blood Ca2+ level back
to normal
Increase in blood
Ca2+ level
I
I
I
I
The Structure of Bone Controls Function and Growth
119
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