Download Chapter 5—Skeletal System. I. Bone functions. a. The skeleton is

Chapter 5—Skeletal System.
I.
II.
III.
IV.
Bone functions.
a. The skeleton is composed of 206 bones (in adults) that perform the following functions:
i. Support.
ii. Movement in association with the skeletal muscles.
iii. Protection.
iv. Storage of minerals.
v. Storage of fat.
vi. Blood cell production.
Bone structure. Fig. 5.1.
a. Compact bone: Very dense bone that makes up the shafts (diaphyses) of the long bones and the
surfaces of all bones.
i. All compact bone (with the exception of joint surfaces and attachment points for tendons
and ligaments) is covered with a CT layer called the periosteum.
1. Periosteum is a dense irregular CT that contains blood vessels, nerves, and CT
cells called fibroblasts.
ii. Compact bone is organized into microscopic structures called osteons (Haversian canal
systems). [Discuss terminology: osteon, osteocyte, lacuna, lamella, canaliculi, etc.].
b. Spongy bone looks like a lattice with bridges and plates of bone interspersed with open spaces—
just like a sponge.
i. Spongy bone makes up the interior of the irregularly shaped bones in the body and the
heads (epiphyses) of the long bones.
c. Red marrow—found in the hollow spaces of spongy bone and (in younger people) in the hollow
shafts of long bones. Red blood cell (RBC) production occurs in the red marrow.
d. Yellow marrow—found in the hollow shafts of long bones in older people. Yellow marrow is
mostly fat (adipose). It may be converted to red marrow in times of need.
Cartilage model. Fig. 5.2.
a. Bone is laid down on a solid hyaline cartilage model during embryonic development. Fig. 5.3.
b. The periosteum produces osteoblasts (bone producing cells).
c. Osteoblasts form a shell of bone around the cartilage; the cartilage is slowly replaced with bone.
d. At the same time, blood vessels penetrate the bone, and the marrow cavity forms due to the
action of osteoclasts (bone destroying cells).
e. Eventually, the osteoblasts surround themselves with matrix (collagen fibers, elastic fibers,
calcium salts, phosphorus salts) and mature into osteocytes.
i. The osteocytes occupy tiny hollow spaces called lacunae within the solid matrix.
ii. The osteocytes communicate with each other and the Haversian system via canaliculi.
f. The ends of long bones are called epiphyses, the shafts of long bones are called diaphyses.
g. In children and adolescents, the epiphyses are separated from the diaphysis by an epiphyseal
plate (growth plate) of hyaline cartilage.
h. As long as the epiphyseal plate is present and maintained by growth hormone, the bone can
lengthen.
i. In late adolescence, the epiphyseal plate is replaced by bone tissue, and the bone cannot grow
any longer.
Hormones and bone growth.
a. Growth hormone (secreted by the anterior pituitary gland) stimulates bone growth during
childhood.
i. Growth hormone stimulates the liver to release growth factors to cause a surge of growth
at the growth plates.
b. Thyroid hormones (secreted by the thyroid gland) ensure the bones grow in the proper
proportions.
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VIII.
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c. At puberty, sex hormones (testosterone [in males] and estrogen [in females]) initiate the “growth
spurt” and further stimulates the growth plates.
i. The growth spurt ceases at the end of puberty, the growth plate is replaced by bone, and
the diaphysis fuses with the epiphyses.
1. Bones cannot lengthen after this occurs, but they can widen.
Bone remodeling—the deposition and withdrawal of calcium and phosphate in bone tissue to allow
for the growth and repair of bones.
a. Osteoblasts deposit bone.
b. Osteoclasts break down bone.
c. When blood calcium levels fall below a set point, parathyroid hormone (PTH, secreted by the
parathyroid glands) stimulates the following:
i. Osteoclasts to break down bone.
ii. The kidneys to reabsorb additional calcium during urine formation.
iii. An increase in the rate of calcium absorption from the digestive tract.
1. Calcium from these sources is picked up by the blood stream, until homeostasis is
achieved. (Negative feedback loop).
d. If the level of blood calcium is too high, calcitonin (secreted by the thyroid gland) stimulates the
following:
i. Osteoblasts to deposit calcium in new bone tissue.
ii. The inhibition of osteoclast activity.
iii. An increase of calcium excretion in urine.
1. This decreases blood calcium levels until homeostasis is restored.
e. Mechanical stress through exercise causes mature bones to become more dense and stronger as
additional bone tissue is laid down.
i. In women, estrogen plays a role in bone remodeling, because it promotes calcium
absorption from the digestive tract, stimulates bone formation, and inhibits osteoclasts.
ii. If osteoclast activity outpaces osteoblast activity, osteoporosis results.
1. Some medications used to treat osteoporosis inhibit osteoclast activity, while
others mimic the effects of estrogen. Calcitonin can also be used, as it stimulates
osteoblasts.
Repairing fractures and bone remodeling. Fig. 5.4.
a. When a bone breaks, blood from broken vessels within the periosteum forms a clot.
b. Within days, fibroblasts from the periosteum invade the clot and secrete collagen fibers to form a
callus, which bridges the break.
i. Some fibroblasts differentiate into cells that produce hyaline cartilage within the callus.
ii. The periosteum produces osteoblasts that then replace the hyaline cartilage with bone.
The skeleton. Fig. 5.5.
a. The adult skeleton has 206 bones.
b. The skeleton is subdivided into two parts:
i. Axial skeleton—skull, ribcage, and vertebrae.
ii. Appendicular skeleton—shoulder girdle, arm bones, pelvic girdle, and leg bones.
c. Ligaments—made of dense regular CT, they link bones together at the joints.
d. Tendons—attach muscles to bones, and sometimes muscles to muscles. Also made of dense
regular CT.
Axial skeleton. [Reference for lab work].
Appendicular skeleton. [Reference for lab work].
Joints—points of contact or near contact between bones.
a. Synovial joints. Fig. 5.13.
i. Freely movable.
ii. Bones forming the joint are separated by a cavity.
iii. Articulating ends of the bones are covered with hyaline cartilage, and are stabilized by
ligaments.
iv. A CT capsule surrounds the joint, which secretes synovial fluid into the joint cavity.
v. Types of synovial joints:
1. Ball and socket—provide a wide range of movement, including rotation. Hip
joint.
2. Hinge joint—move in a single plane. Knee and elbow.
vi. Synovial joint disorders:
1. Osteoarthritis—degeneration of cartilage covering the bone ends inside the joint.
2. Rheumatoid arthritis—autoimmune disorder that attacks synovial joint tissues,
and destroys them.
b. Cartilaginous joints—cartilage fills the space between the bones of this joint type. Permits only
slight movement. Found between the vertebrae, and between the sternum and ribs.
c. Fibrous joints—fibrous CT joins the bones, without the presence of a cavity.
i. Fontanels—joints between cranial bones of babies. They fuse and harden to form:
ii. Sutures—immovable joints tied together firmly with dense fibrous CT. Fig. 5.12.
d. Movements about synovial joints. Fig. 5.14.
i. Flexion, extension, hyperextension.
ii. Circumduction, rotation.
iii. Abduction, adduction
iv. Supination, pronation.
Study suggestions for this chapter: In the textbook at the end of the chapter, the sections entitled 1)
Highlighting the Concepts, 2) Recognizing Key Terms, and 3) Reviewing the Concepts are all good for you to
gauge your comprehension and focus your study efforts.