1. Chapter 42Structure and Function of the Skeletal System

2. Connective Tissue • Cells • Extracellular matrix • Ground substance • Protein fibers • Characteristics of the tissue depend on the extracellular matrix

3. 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

4. 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

5. 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

6. Question Which type of bone cells remain in the extracellular matrix and function as strain sensors? • Osteogenic • Osteoblasts • Osteoclasts • Osteocytes

7. Answer • 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).

8. 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

9. Parathyroid Hormone • PTH is released when blood calcium levels decrease • Sustained PTH raises blood calcium levels three ways: • From bone • From kidneys • From intestines

10. 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?

11. 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

12. 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

13. Question Tell whether the following statement is true or false. PTH is released as a result of hypocalcemia.

14. 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).

15. Skeletal Structures • Epiphysis = end • Diaphysis = shaft • Contains bone marrow • Red marrow: makes blood cells • Yellow marrow: contains fat • Metaphysis = widening before the end of the bone

16. 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

17. 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

18. Question Which of the following is an example of a synarthrodial joint? • Elbow • Wrist • Sutures of the skull • Hip/coxal

19. Answer • 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.