Page 234 - Fundamentals of anatomy physiology

Chapter 6 Osseous Tissue and Bone Structure 221
Table 6–1 Hormones Involved in Bone Growth and Maintenance
Hormone Primary Source
Calcitriol Kidneys
Growth hormone Pituitary gland
Thyroxine Thyroid gland (follicle cells)
Sex hormones Ovaries (estrogens)
Testes (androgens)
Parathyroid hormone Parathyroid glands
Calcitonin Thyroid gland (C cells)
Minerals. Normal bone growth and maintenance cannot
take place without a constant dietary source of calcium
and phosphate salts. Lesser amounts of other minerals,
such as magnesium, fluoride, iron, and manganese, are
also required.
Calcitriol and vitamin D3. The hormone calcitriol, synthe-
sized in the kidneys, is essential for normal calcium and
phosphate ion absorption in the digestive tract. Calcitriol
is synthesized from a related steroid, cholecalciferol
(vitamin D3), which may be produced in the skin or
absorbed from the diet. p. 184
Vitamin C. Adequate levels of vitamin C must be present
in the diet. This vitamin, which is required for certain key
enzymatic reactions in collagen synthesis, also stimulates
osteoblast differentiation. One of the signs of vitamin C
deficiency—a condition called scurvy—is a loss of bone
mass and strength.
Vitamins A, K, and B12. Three other vitamins have signifi-
cant effects on bone structure. Vitamin A, which stimulates
osteoblast activity, is particularly important for normal
bone growth in children. Vitamins K and B12 are required
for the synthesis of proteins in normal bone.
Growth hormone and thyroxine. Growth hormone, produced by
the pituitary gland, and thyroxine, from the thyroid gland,
stimulate bone growth. Growth hormone stimulates pro-
tein synthesis and the rates of cell division and cell growth
throughout the body. Thyroxine stimulates cell metabo-
lism and increases the rate of osteoblast activity. In proper
balance, these hormones maintain normal activity at the
epiphyseal cartilages until puberty.
Sex hormones. At puberty, rising levels of sex hormones (es-
trogens in females and androgens in males) stimulate osteo-
blasts to produce bone faster than the rate at which epiphy-
seal cartilage expands. Over time, the epiphyseal cartilages
narrow and eventually close. The timing of epiphyseal
closure differs from bone to bone and from individual to
Effect on Skeletal System
Promotes calcium and phosphate ion absorption along the digestive tract
Stimulates osteoblast activity and the synthesis of bone matrix
With growth hormone, stimulates osteoblast activity and the synthesis of
bone matrix
Stimulate osteoblast activity and the synthesis of bone matrix; estrogens stimulate
epiphyseal closure earlier than androgens
Stimulates osteoclast (and osteoblast) activity; increases blood calcium
ion concentrations
Inhibits osteoclast activity; promotes calcium loss by kidneys; decreases blood
calcium ion concentrations
6
individual. The toes may complete ossification by age 11,
but parts of the pelvis or the wrist may continue to enlarge
until about age 25.
Differences in male and female sex hormones account
for significant variations in body size and proportions. Be-
cause estrogens cause faster epiphyseal closure than do an-
drogens, women are generally shorter than men at maturity.
Calcitonin and parathyroid hormone. Two other hormones—
calcitonin (kal-si-TO. -nin), from the thyroid gland, and para-
thyroid hormone, from the parathyroid gland—have impor-
tant effects on bone. They are involved in the homeostatic
control of calcium and phosphate levels in body fluids, as
we will see in the next section.
The major hormones affecting the growth and maintenance of
the skeletal system are summarized in Table 6–1.
The skeletal system is unique in that it persists after life,
providing hints to the gender, lifestyle, and environmental con-
ditions experienced by the individual. Not only do the bones
reflect the physical stresses placed on the body, but they also
provide clues about a person’s health and diet. By using the
appearance, strength, and composition of bone, forensic scien-
tists and physical anthropologists can detect features character-
istic of hormonal deficiencies. Combining the physical clues
from the skeleton with modern molecular techniques, such as
DNA fingerprinting, a wealth of information can be learned.
Checkpoint
1 4. Why would you expect the arm bones of a weight lifter
to be thicker and heavier than those of a jogger?
1 5. A child who enters puberty several years later than the
average age is generally taller than average as an adult.
Why?
1 6. A 7-year-old child has a pituitary gland tumor involving
the cells that secrete growth hormone (GH), resulting
in increased levels of GH. How will this condition
affect the child’s growth?
See the blue Answers tab at the back of the book.