Chapter 42
Structure and Function of the
Skeletal System
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Connective Tissue
• Cells
• Extracellular matrix
– Ground substance
– Protein fibers
• Characteristics of the tissue depend on the
extracellular matrix
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Cartilage
• Ground substance is a gel containing 65%–80% water
• Oxygen and food can diffuse through this gel
• Therefore, the tissue does not need blood vessels
• Different types of cartilage are distinguished by how
many protein fibers are in the extracellular matrix
– Hyaline cartilage: very few fibers
– Fibrocartilage: some elastin fibers
– Elastic cartilage: many elastin fibers
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Bone
• Ground substance contains crystals of calcium salts
• These make the bone rigid
• They block oxygen and food diffusion, so bone must
contain blood vessels
• Types of bone are distinguished by the pattern the
crystals are laid down in:
– Cancellous bone: a lattice of tiny struts of bone,
like a sponge
– Compact bone: laid down in layers like an onion
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Bone Cells
• Osteogenic cells: divide to form the other bone
cells
• Osteoblasts: lay down the extracellular matrix
• Osteocytes: live in the mature bone and
maintain the extracellular matrix
• Osteoclasts: break down the extracellular matrix
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Question
Which type of bone cells remain in the extracellular matrix
and function as strain sensors?
a. Osteogenic
b. Osteoblasts
c. Osteoclasts
d. Osteocytes
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Answer
d. Osteocytes
Osteocytes are osteoblasts that become trapped in the
matrix as tissue is ossified. They live in lacunae, and
function as strain sensors (stimulating or inhibiting other
bone cells in response to stress).
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Bone Remodeling
• Osteoblasts and osteoclasts are formed when they are
needed to remodel bone
• Osteoclasts remove damaged areas of bone
• Osteoblasts come after them and replace the extracellular
matrix
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Parathyroid Hormone
• PTH is released
when blood calcium
levels decrease
• Sustained PTH raises
blood calcium levels
three ways:
º From bone
º From kidneys
º From intestines
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Scenario
Two men have bone problems…
• One man developed a lung tumor that secreted
constant high levels of parathyroid hormone; he
became very weak and developed weak bones
• A second man suffered complete kidney failure;
he also developed weak bones
Question
• Why did these men both develop weak bones?
• Which of them is more likely to have increased
blood calcium levels?
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Vitamin D
• Vitamin D is needed to absorb dietary calcium
• Created in skin cells under the influence of
sunlight
• Absorbed from the diet as a fat-soluble vitamin
• Activated in a two-step process:
– Liver
– Kidneys
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Calcitonin
• Released by thyroids when blood calcium is too high
• Inhibits the release of calcium from bone
• Reduces osteoclast activity
• Inhibits vitamin D activation in the kidney
• Inhibits calcium reabsorption by the kidney
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Question
Tell whether the following statement is true or false.
PTH is released as a result of hypocalcemia.
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Answer
True
PTH is released by the parathyroid gland when serum
calcium levels are low. PTH increases calcium levels by
acting on bone (bone cells release more calcium),
kidneys (more calcium is reabsorbed), and intestines
(more calcium is reabsorbed).
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Skeletal Structures
• Epiphysis = end
• Diaphysis = shaft
– Contains bone marrow
º Red marrow: makes
blood cells
º Yellow marrow:
contains fat
• Metaphysis = widening
before the end of the bone
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Tendons and Ligaments
• Extracellular matrix is filled with collagen fibers
• Strong and not elastic
• Tendons connect muscles to bone
• Ligaments connect bones to one another
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Joints
• Synarthroses: little mobility
• Diarthroses: highly mobile
– Joint capsule connects the two bones
– Synovial membrane lines joint capsule
– Synovial fluid in the joint capsule lubricates joint
– Menisci are protective smooth cartilage plates
between the two bones
– Bursae are small synovial sacs cushioning tendons
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Question
Which of the following is an example of a synarthrodial
joint?
a. Elbow
b. Wrist
c. Sutures of the skull
d. Hip/coxal
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Answer
c. Sutures of the skull
Synarthrodial joints have limited movement. The sutures
of the skull do not move. All of the other joints are
freely movable (diarthrodial) joints.
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins