1. Copyright 2009, John Wiley & Sons, Inc. The Skeletal System: Bone Tissue Chapter 6

2. Copyright 2009, John Wiley & Sons, Inc. Functions of Bone and Skeletal System Support Protection Assistance in Movement Mineral Homeostasis Blood Cell Production Triglyceride Storage

3. Copyright 2009, John Wiley & Sons, Inc. Functions of Bone and Skeletal System Support  Structural framework of the body Supports soft tissues Provides attachment points for tendons of skeletal muscle Protection  Protects important internal organs Cranium protects brain Vertebrae protects spinal cord Ribs protect lungs and heart

4. Copyright 2009, John Wiley & Sons, Inc. Functions of Bone and Skeletal System Assistance in Movement  Skeletal muscle attaches to bone Skeletal muscle contraction pulls on bone producing movement Mineral Homeostasis  Bone tissue stores several minerals Acts to serve as a reservoir of critical minerals  Calcium (99% of body’s content)  Phosphorus

5. Copyright 2009, John Wiley & Sons, Inc. Functions of Bone and Skeletal System Blood Cell Production  Red bone marrow produces (Hemopoiesis) Red blood cells White blood cells Platelets Triglyceride Storage  Yellow bone marrow Triglycerides stored in adipose cells  Serves as a potential chemical energy reserve

6. Copyright 2009, John Wiley & Sons, Inc. Structure of Bone  Diaphysis  Epiphysis  Metaphysis Epiphyseal growth plate  Articular cartilage Perforating fibers  Periosteum  Medullary cavity  Endosteum Long Bone Anatomy (Humerus)

7. Copyright 2009, John Wiley & Sons, Inc. Histology of Bone Tissue Extracellular matrix surrounding widely separated cells  Matrix 25% water 25% collagen fibers 50% crystallized mineral salts The most abundant mineral salt is calcium phosphate

8. Copyright 2009, John Wiley & Sons, Inc. Histology of Bone Tissue A process called calcification is initiated by bone-building cells called osteoblasts Mineral salts are deposited and crystalize in the framework formed by the collagen fibers of the extracellular matrix Bone’s flexibility depends on collagen fibers

9. Copyright 2009, John Wiley & Sons, Inc. Histology of Bone Tissue Four types of cells are present in bone tissue Osteogenic cells  Undergo cell division; the resulting cells develop into osteoblasts Osteoblasts  Bone-building cells  Synthesize extracellular matrix of bone tissue Osteocytes  Mature bone cells  Exchange nutrients and wastes with the blood

10. Copyright 2009, John Wiley & Sons, Inc. Histology of Bone Tissue Osteoclasts  Release enzymes that digest the mineral components of bone matrix (resorption)  Regulate blood calcium level

11. Copyright 2009, John Wiley & Sons, Inc. Histology of Bone Tissue Bone may be categorized as:  Compact  Spongy

12. Copyright 2009, John Wiley & Sons, Inc. Histology of Bone Tissue Compact Bone  Resists the stresses produced by weight and movement  Components of compact bone are arranged into repeating structural units called osteons or Haversian systems  Osteons consist of a central (Haversian) canal with concentrically arranged lamellae, lacunae, osteocytes, and canaliculi

13. Copyright 2009, John Wiley & Sons, Inc. Histology of Bone Tissue Osteon  Central canals run longitudinally through bone  Around the central canals are concentric lamellae Rings of calcified matrix (like the rings of a tree trunk)  Between the lamellae are small spaces called lacunae which contain osteocytes  Radiating in all directions from the lacunae are tiny canaliculi filled with extracellular fluid

14. Copyright 2009, John Wiley & Sons, Inc. Histology of Bone Tissue Osteon  Canaliculi connect lacunae, forming a system of interconnected canals Providing routes for nutrients and oxygen to reach the osteocytes  The organization of osteons changes in response to the physical demands placed on the skeleton

15. Copyright 2009, John Wiley & Sons, Inc. Histology of Bone Tissue Spongy Bone  Lacks osteons  Lamellae are arranged in a lattice of thin columns called trabeculae Spaces between the trabeculae make bones lighter Trabeculae of spongy bone support and protect the red bone marrow Hemopoiesis (blood cell production) occurs in spongy bone

16. Copyright 2009, John Wiley & Sons, Inc. Histology of Bone Tissue Spongy Bone  Within each trabecula are lacunae that contain osteocytes  Osteocytes are nourished from the blood circulating through the trabeculae  Interior bone tissue is made up primarily of spongy bone  The trabeculae of spongy bone are oriented along lines of stress helps bones resist stresses without breaking

17. Copyright 2009, John Wiley & Sons, Inc. Blood and Nerve Supply of Bone Bone is richly supplied with blood  Periosteal arteries accompanied by nerves supply the periosteum and compact bone  Epiphyseal veins carry blood away from long bones Nerves accompany the blood vessels that supply bones  The periosteum is rich in sensory nerves sensitive to tearing or tension

18. Copyright 2009, John Wiley & Sons, Inc. Bone Formation The process by which bone forms is called ossification Bone formation occurs in four situations:  1) Formation of bone in an embryo  2) Growth of bones until adulthood  3) Remodeling of bone  4) Repair of fractures

19. Copyright 2009, John Wiley & Sons, Inc. Bone Formation Formation of Bone in an Embryo  Cartilage formation and ossification occurs during the sixth week of embryonic development

20. Copyright 2009, John Wiley & Sons, Inc. Bone Formation Formation of Bone in an Embryo  Bone formation follows one of two patterns Intramembranous ossification  Flat bones of the skull and mandible are formed in this way  “Soft spots” that help the fetal skull pass through the birth canal later become ossified forming the skull Endochondral ossification  The replacement of cartilage by bone  Most bones of the body are formed in this way including long bones

21. 1 Development of cartilage model Hyaline cartilage Perichondrium Proximal epiphysis Distal epiphysis Diaphysis 1 Development of cartilage model Growth of cartilage model 2 Hyaline cartilage Uncalcified matrix Calcified matrix Perichondrium Proximal epiphysis Distal epiphysis Diaphysis 1 Development of cartilage model Development of primary ossification center Growth of cartilage model 23 Hyaline cartilage Uncalcified matrix Calcified matrix Nutrient artery Perichondrium Proximal epiphysis Distal epiphysis Diaphysis Periosteum Primary ossification center Spongy bone 1 Hyaline cartilage Calcified matrix Periosteum (covering compact bone) Uncalcified matrix Calcified matrix Medullary cavity Nutrient artery and vein Nutrient artery Perichondrium Proximal epiphysis Distal epiphysis Diaphysis Development of cartilage model Development of primary ossification center Development of the medullary cavity Growth of cartilage model Periosteum Primary ossification center 234 Spongy bone Uncalcified matrix 1 Development of cartilage model Development of primary ossification center Development of the medullary cavity Growth of cartilage model 234 Hyaline cartilage Calcified matrix Periosteum (covering compact bone) Uncalcified matrix Calcified matrix Medullary cavity Nutrient artery and vein Nutrient artery Perichondrium Proximal epiphysis Distal epiphysis Diaphysis Periosteum Primary ossification center Secondary ossification center Nutrient artery and vein Uncalcified matrix Epiphyseal artery and vein Development of secondary ossification center 5 Spongy bone Uncalcified matrix 1 Articular cartilage Spongy bone Epiphyseal plate Secondary ossification center Nutrient artery and vein Uncalcified matrix Epiphyseal artery and vein Formation of articular cartilage and epiphyseal plate Development of secondary ossification center Development of cartilage model Development of primary ossification center Development of the medullary cavity Growth of cartilage model 234 56 Hyaline cartilage Uncalcified matrix Calcified matrix Periosteum (covering compact bone) Uncalcified matrix Calcified matrix Medullary cavity Nutrient artery and vein Nutrient artery Perichondrium Proximal epiphysis Distal epiphysis Diaphysis Periosteum Primary ossification center Spongy bone

22. Copyright 2009, John Wiley & Sons, Inc. Bone Growth During Infancy, Childhood and Adolescence Growth in Length The growth in length of long bones involves two major events:  1) Growth of cartilage on the epiphyseal plate  2) Replacement of cartilage by bone tissue in the epiphyseal plate

23. Copyright 2009, John Wiley & Sons, Inc. Bone Growth During Infancy, Childhood and Adolescence Osteoclasts dissolve the calcified cartilage, and osteoblasts invade the area laying down bone matrix The activity of the epiphyseal plate is the way bone can increase in length At adulthood, the epiphyseal plates close and bone replaces all the cartilage leaving a bony structure called the epiphyseal line

24. Copyright 2009, John Wiley & Sons, Inc. Bone Growth During Infancy, Childhood and Adolescence Growth in Thickness  Bones grow in thickness at the outer surface Remodeling of Bone  Bone forms before birth and continually renews itself  The ongoing replacement of old bone tissue by new bone tissue  Old bone is continually destroyed and new bone is formed in its place throughout an individual’s life

25. Copyright 2009, John Wiley & Sons, Inc. Bone Growth During Infancy, Childhood and Adolescence A balance must exist between the actions of osteoclasts and osteoblasts  If too much new tissue is formed, the bones become abnormally thick and heavy  Excessive loss of calcium weakens the bones, as occurs in osteoporosis  Or they may become too flexible, as in rickets and osteomalacia

26. Copyright 2009, John Wiley & Sons, Inc. Factors Affecting Bone Growth and Bone Remodeling Normal bone metabolism depends on several factors Minerals  Large amounts of calcium and phosphorus and smaller amounts of magnesium, fluoride, and manganese are required for bone growth and remodeling Vitamins  Vitamin A stimulates activity of osteoblasts  Vitamin C is needed for synthesis of collagen  Vitamin D helps build bone by increasing the absorption of calcium from foods in the gastrointestinal tract into the blood  Vitamins K and B12 are also needed for synthesis of bone proteins

27. Copyright 2009, John Wiley & Sons, Inc. Factors Affecting Bone Growth and Bone Remodeling Hormones  During childhood, the hormones most important to bone growth are growth factors (IGFs), produced by the liver IGFs stimulate osteoblasts, promote cell division at the epiphyseal plate, and enhance protein synthesis  Thyroid hormones also promote bone growth by stimulating osteoblasts  Insulin promotes bone growth by increasing the synthesis of bone proteins

28. Copyright 2009, John Wiley & Sons, Inc. Factors Affecting Bone Growth and Bone Remodeling Hormones  Estrogen and testosterone cause a dramatic effect on bone growth Cause of the sudden “growth spurt” that occurs during the teenage year Promote changes in females, such as widening of the pelvis Shut down growth at epiphyseal plates  Parathyroid hormone, calcitriol, and calcitonin are other hormones that can affect bone remodeling

29. Copyright 2009, John Wiley & Sons, Inc. Fracture and Repair of Bone Fracture Types  Open (compound) fracture The broken ends of the bone protrude through the skin  Closed (simple) fracture Does not break the skin  Comminuted fracture The bone is splintered, crushed, or broken into pieces  Greenstick fracture A partial fracture in which one side of the bone is broken and the other side bends  Impacted fracture One end of the fractured bone is forcefully driven into another  Pott’s fracture Fracture of the fibula, with injury of the tibial articulation  Colles’ fracture A fracture of the radius in which the distal fragment is displaced  Stress fracture A series of microscopic fissures in bone

30. Copyright 2009, John Wiley & Sons, Inc. Fracture and Repair of Bone

31. Copyright 2009, John Wiley & Sons, Inc. Fracture and Repair of Bone Calcium and phosphorus needed to strengthen and harden new bone after a fracture are deposited only gradually and may take several months The repair of a bone fracture involves the following steps  1) Formation of fracture hematoma Blood leaks from the torn ends of blood vessels, a clotted mass of blood forms around the site of the fracture  2) Fibrocartilaginous callus formation Fibroblasts invade the fracture site and produce collagen fibers bridging the broken ends of the bone  3) Bony callus formation Osteoblasts begin to produce spongy bone trabeculae joining portions of the original bone fragments  4) Bone remodeling Compact bone replaces spongy bone

32. Compact bone Spongy bone Periosteum Fracture hematoma Fracture hematoma Bone fragment Osteocyte Red blood cell Blood vessel Formation of fracture hematoma Phagocyte Osteon 1 Phagocyte Osteoblast Fibroblast Fibrocartilaginous callus Collagen fiber Chondroblast Cartilage Fibrocartilaginous callus formation 2 Compact bone Spongy bone Periosteum Fracture hematoma Fracture hematoma Bone fragment Osteocyte Red blood cell Blood vessel Formation of fracture hematoma Phagocyte Osteon 1 Bony callus Spongy bone Osteoblast Bony callus formation Osteocyte 3 Compact bone Spongy bone Periosteum Fracture hematoma Fracture hematoma Bone fragment Osteocyte Red blood cell Blood vessel Formation of fracture hematoma Phagocyte Osteon 1 Phagocyte Osteoblast Fibroblast Fibrocartilaginous callus Collagen fiber Chondroblast Cartilage Fibrocartilaginous callus formation 2 Spongy bone Osteoblast Osteoclast New compact bone Bony callus formationBone remodeling Osteocyte 34 Compact bone Spongy bone Periosteum Fracture hematoma Fracture hematoma Bone fragment Osteocyte Red blood cell Blood vessel Formation of fracture hematoma Phagocyte Osteon 1 Phagocyte Osteoblast Fibroblast Fibrocartilaginous callus Collagen fiber Chondroblast Cartilage Fibrocartilaginous callus formation 2 Bony callus

33. Copyright 2009, John Wiley & Sons, Inc. Bone’s Role in Calcium Homeostasis Bone is the body’s major calcium reservoir Levels of calcium in the blood are maintained by controlling the rates of calcium resorption from bone into blood and of calcium deposition from blood into bone  Both nerve and muscle cells depend on calcium ions (Ca 2+ ) to function properly  Blood clotting also requires Ca 2+  Many enzymes require Ca 2+ as a cofactor

34. Copyright 2009, John Wiley & Sons, Inc. Bone’s Role in Calcium Homeostasis Actions that help elevate blood Ca 2+ level  Parathyroid hormone (PTH) regulates Ca 2+ exchange between blood and bone tissue PTH increases the number and activity of osteoclasts PTH acts on the kidneys to decrease loss of Ca 2+ in the urine PTH stimulates formation of calcitriol a hormone that promotes absorption of calcium from foods in the gastrointestinal tract

35. Copyright 2009, John Wiley & Sons, Inc. Bone’s Role in Calcium Homeostasis

36. Copyright 2009, John Wiley & Sons, Inc. Bone’s Role in Calcium Homeostasis Actions that work to decrease blood Ca 2+ level  The thyroid gland secretes calcitonin (CT) which inhibits activity of osteoclasts  The result is that CT promotes bone formation and decreases blood Ca 2+ level

37. Copyright 2009, John Wiley & Sons, Inc. Exercise and Bone Tissue Bone tissue alters its strength in response to changes in mechanical stress  Under stress, bone tissue becomes stronger through deposition of mineral salts and production of collagen fibers by osteoblasts  Unstressed bones diminishes because of the loss of bone minerals and decreased numbers of collagen fibers The main mechanical stresses on bone are those that result from the pull of skeletal muscles and the pull of gravity Weight-bearing activities help build and retain bone mass

38. Copyright 2009, John Wiley & Sons, Inc. Aging and Bone Tissue The level of sex hormones diminishes during middle age, especially in women after menopause  A decrease in bone mass occurs  Bone resorption by osteoclasts outpaces bone deposition by osteoblasts Female bones generally are smaller and less massive than males  Loss of bone mass in old age has a greater adverse effect in females

39. Copyright 2009, John Wiley & Sons, Inc. Aging and Bone Tissue There are two principal effects of aging on bone tissue:  1) Loss of bone mass Results from the loss of calcium from bone matrix The loss of calcium from bones is one of the symptoms in osteoporosis  2) Brittleness Results from a decreased rate of protein synthesis Collagen fibers gives bone its tensile strength The loss of tensile strength causes the bones to become very brittle and susceptible to fracture