C h a p t e r 4 2
Structure and Function of the Skeletal System
1073
With adequate exposure to sunlight, the amount of
vitamin D that can be produced by the skin is usually
sufficient to meet physiologic requirements. Modern
conditions of dress, lifestyle, and recommendations
regarding sun screens and avoidance of sun exposure to
reduce skin cancer risk may prevent a large proportion
of the population from producing adequate amounts
of vitamin D. Elderly persons who are housebound or
institutionalized frequently have low vitamin D levels.
The deficiency often goes undetected until there are
problems such as pseudofractures or electrolyte imbal-
ances. Seasonal variations in vitamin D levels probably
reflect changes in sunlight exposure.
The most potent of the vitamin D metabolites is
1,25-(OH)
2
D
3
. This metabolite increases intestinal
absorption of calcium and promotes the actions of
PTH on resorption of calcium and phosphate from
bone. Bone resorption by the osteoclasts is increased
and bone formation by the osteoblasts is decreased;
there is also an increase in acid phosphatase and a
decrease in alkaline phosphatase. Intestinal absorption
and bone resorption increase the amount of calcium
and phosphorus available to the mineralizing surface
of the bone. The role of 24,25-(OH)
2
D
3
is less clear.
There is evidence that 24,25-(OH)
2
D
3
in conjunction
with 1,25-(OH)
2
D
3
may be involved in normal bone
mineralization.
The regulation of vitamin D activity is influenced by
several hormones. PTH and prolactin stimulate 1,25-
(OH)
2
D
3
production by the kidney. States of hyper-
parathyroidism are associated with increased levels of
1,25-(OH)
2
D
3
, and hypoparathyroidism leads to lowered
levels of this metabolite. Prolactin may have an ancillary
role in regulating vitamin D metabolism during preg-
nancy and lactation. Calcitonin inhibits 1,25-(OH)
2
D
3
production by the kidney. In addition to hormonal
influences, changes in the concentration of ions such as
calcium, phosphate, hydrogen, and potassium exert an
effect on 1,25-(OH)
2
D
3
and 24,25-(OH)
2
D
3
production.
Under conditions of phosphate and calcium depriva-
tion, 1,25-(OH)
2
D
3
levels are increased, whereas hyper-
phosphatemia and hypercalcemia decrease the levels of
metabolite.
Joints
Joints, or articulations, are places where adjacent bones
or cartilages meet. Joints vary in the type and extent of
movements they allow. Some joints have no movement;
others allow only slight movement; and some such as
the shoulder joint are freely moveable. There are three
classes of joints based on their structure and function:
fibrous (synarthroses), cartilaginous (amphiarthoses),
and synovial (diarthroses).
SUMMARY CONCEPTS
■■
The skeletal system consists of the bones of the
skull, thorax, and vertebral column, which form
the axial skeleton, and the bones of the upper and
lower extremities, which form the appendicular
skeleton.
■■
Two types of connective tissue are found in the
skeletal system: (1) cartilage, a semirigid and
slightly flexible tissue that plays an essential
role in prenatal and childhood development
of the skeleton and serves as a surface for the
articulating ends of skeletal joints; and (2) bone,
which provides the firm structure of the skeleton
and serves as a reservoir for calcium and
phosphate storage.
■■
Bone tissue is classified as either spongy or
compact according to the relative amount of
solid matter and number and size of the spaces
they contain. Spongy bone, which forms the
interior of bones, is composed of spicules
or trabeculae that form a latticelike pattern.
Compact bone, which forms the outer shell
of a bone, has a densely calcified intercellular
matrix that makes it more rigid than spongy
bone.
■■
Bone matrix is maintained by five types of cells:
osteoprogenitor cells, which are resting cells
that differentiate into osteoblasts; osteoblasts,
which function as bone-building cells during
bone remodeling; osteocytes, or mature bone
cells; osteoclasts, which function as bone
resorption cells during bone remodeling; and
bone-lining cells, which cover the outer and
inner surfaces of bone where remodeling is not
occurring.
■■
The process of bone formation and mineral
metabolism involves the interplay among
the actions of parathyroid hormone (PTH),
calcitonin, and vitamin D. PTH acts to maintain
serum levels of ionized calcium by increasing
the release of calcium and phosphate from
bone, conserving calcium and increasing
phosphate elimination by the kidney, and
enhancing intestinal reabsorption of calcium
through vitamin D. Calcitonin inhibits the release
of calcium from bone and increases renal
elimination of calcium and phosphate, thereby
serving to lower serum calcium levels. Vitamin
D, which functions as a hormone in regulating
body calcium, increases absorption of calcium
from the intestine and promotes the actions of
PTH on bone.
