1. The Skeletal System: Bone Tissue Chapter 6
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2. The Skeletal System: Bone Tissue
Functions of Bone and Skeletal System
Structure of Bone
Histology of Bone Tissue
Blood and Nerve Supply of Bone
Bone Formation
Bone’s Role in Calcium Homeostasis
Exercise and Bone Tissue
Aging and Bone Tissue
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3. Functions of Bone and Skeletal System
Support: structural framework; supporting soft tissues; attachment for muscles and tendons (fascia to periosteum
Protection: internal organs from injury; cranium/brain; vertebrae/spinal cord, ribs/heart and lungs
Assistance in Movement: chapter 11pp. 338 thru 342 “levers”; skeletal muscles pull bones
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4. Functions of Bone and Skeletal System
Mineral Homeostasis: calcium and phosphorus contribute to bone strength; stores 99% of body’s calcium; releases it on demand (see PTH actions and calcium metabolism)
Blood Cell Production: red bone marrow produces RBC, WBC and platelets thru hemopoiesis; r.b.m. c/o of developing blood cells, adipocytes, fibroblasts and macrophages w/in a network of reticular fibers; found in hips, ribs, sternum, vertebrae, skull and ends of arm and thigh bones.
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5. Functions of Bone and Skeletal System
Triglyceride Storage: yellow bone marrow c/o adipose tissue; chemical energy reserve and is involved in hemopoiesis; with increasing age red marrow changes to yellow
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6. Copyright 2009, John Wiley & Sons, Inc.
Structure of Bone
Long Bone Anatomy (Humerus)
Diaphysis: shaft
Epiphysis: proximal and distal ends
Metaphysis: regions between diaphysis and epiphysis
Epiphyseal growth plate: layer of hyaline cartilage that allow diaphysis to grow in length; ceases to grow between ages 18-21; replaced by bone then becomes epiphyseal line
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7. Copyright 2009, John Wiley & Sons, Inc.
Structure of Bone
Long Bone Anatomy (Humerus)
Articular cartilage: thin layer of hyaline cartilage where bone articulates (joint) with another; reduces friction and absorbs shock
Periosteum: composed of outer fibrous layer of dense irregular connective tissue and inner osteogenic layer; allows bone growth in thickness; protects bone, assists in fracture repair, bone nourishment and attachment point for ligaments and tendons
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8. Copyright 2009, John Wiley & Sons, Inc.
Structure of Bone
Long Bone Anatomy (Humerus)
Periosteum (cont’d)
Perforating fibers: Sharpey’s fibers, attachment to underlying bone, c/o thick bundles of collagen extend into extracellular matrix of bone
Medullary cavity: hollow cylindrical space with diaphysis that contains fatty yellow marrow in adults
Endosteum: thin membrane that lines the internal surface facing the medullary cavity; made of single layers of cells and connective tissue
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10. Histology of Bone Tissue
Abundant extracellular matrix surrounding widely separated cells
Matrix
25% water
25% collagen fibers
50% crystallized mineral salts: calcium phosphate combines with salt, calcium hydroxide to form crystals of hydroxyapatite then crystals combine with calcium carbonate and ions of magnesium, fluoride, potassium and sulfate.
The most abundant mineral salt is calcium phosphate
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11. Histology of Bone Tissue
A process called calcification is initiated by bone-building cells called osteoblasts
Mineral salts are deposited and crystallize in the framework formed by the collagen fibers of the extracellular matrix
Bone’s flexibility depends on collagen fibers
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12. 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
Osteoclasts
Release enzymes that digest the mineral components of bone matrix (resorption)
Regulate blood calcium level
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13. 06_02
06_02

14. Histology of Bone Tissue
Bone may be categorized as:
Compact
Spongy
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15. 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
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16. 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
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17. 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
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18. 06_03
06_03

19. 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
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20. 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
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21. 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
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22. Copyright 2009, John Wiley & Sons, Inc.
Bone Formation
The process by which bone forms is called ossification or osteogenesis
Bone formation occurs in four situations:
Formation of bone in an embryo
Growth of bones until adulthood
Remodeling of bone
Repair of fractures
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23. Copyright 2009, John Wiley & Sons, Inc.
Bone Formation
Formation of Bone in an Embryo
Embryonic skeleton initially composed of mesenchyme (an embryonic connective tissue from which all other connective tissues arise) shaped like bones they become
Replacement of preexisting connective tissue with bone; basic structure and function of mature bone same even though there are 2 different means of formation
Cartilage formation and ossification occurs during the sixth week of embryonic development
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24. Copyright 2009, John Wiley & Sons, Inc.
Bone Formation
Formation of Bone in an Embryo
Bone formation follows one of two patterns
Intramembranous ossification
Simpler method of two
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
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25. 1. Intramembranous ossification
Development of the ossification center.
At site where bone develops specific chemical messages cause mesenchymal cells to aggregate and differentiate, first into osteogenic cells then osteoblasts.
Organic extracellular matrix secreted until bone surrounds cells.
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26. 1. Intramembranous ossification
Calcification:
Within a few days calcium and other mineral salts are deposited and extracellular matrix becomes calcified.
Formation of trabeculae:
Extracellular matrix develops into trabeculae that fuse with each other to form spongy bone
Blood vessels grow into the spaces
Connective tissue associated with blood vessels differentiates into red marrow
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27. 1. Intramembranous ossification
Development of the periosteum:
In conjunction with formation of trabeculae, the mesenchyme condenses at the periphery
Eventually a thin layer of compact bone replaces the spongy bone, but spongy bone remains in the center
Much of the new bone is remodeled as the bone is transformed into adult shape and size
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28. 1
Blood capillary
Ossification center
Mesenchymal cell
Osteoblast
Mesenchyme
condenses
Blood vessel
Spongy bone
trabeculae
Periosteum
Spongy bone tissue
Compact bone tissue
Development of ossification center
Calcification
Formation of trabeculae
Development of the periosteum
Mandible
Flat bone
of skull 3 4 2
Collagen fiber
Osteocyte in lacuna
Canaliculus
Newly calcified bone
matrix 1
Blood capillary
Ossification center
Mesenchymal cell
Osteoblast
Development of ossification center
Calcification
Mandible
Flat bone
of skull 2
Collagen fiber
Osteocyte in lacuna
Canaliculus
Newly calcified bone
matrix
Mesenchyme
condenses
Blood vessel
Spongy bone
trabeculae
Formation of trabeculae 3 1
Blood capillary
Ossification center
Mesenchymal cell
Osteoblast
Osteocyte in lacuna
Canaliculus
Newly calcified bone
matrix
Development of ossification center
Calcification
Mandible
Flat bone
of skull 2
Collagen fiber 1
Blood capillary
Ossification center
Mesenchymal cell
Osteoblast
Collagen fiber
Development of ossification center
Mandible
Flat bone
of skull

29. 2. Endochondral ossification
Replacement of hyaline cartilage by bone
Development of the cartilage model:
Specific chemical messages cause the mesenchymal cells to crow together in the shape of the future bone and develop into chondroblasts
Chondroblasts secrete cartilage extracellular matrix producing the model
A covering called perichondrium develops around the model
Copyright 2009, John Wiley & Sons, Inc.

30. 2. Endochondral ossification
Growth of the cartilage model:
Once the chondroblasts are buried deep in cartilage they become chondrocytes
Interstitial growth: chondrocytes divide accompanied by more cartilage matrix
Appositional growth: More matrix is also laid down at the periphery
Matrix begins to calcify; chondrocytes hypertrophy then die leaving lacunae which merge into small cavities
Copyright 2009, John Wiley & Sons, Inc.

31. 2. Endochondral ossification
Development of the primary ossification center:
Nutrient artery penetrates perichondrium and calcifying cartilage model through a nutrient foramen
Stimulates cells in perichondrium to differentiate into osteoblasts
Perichondrium starts forming bone then becomes periosteum
Near the middle periosteal capillaries grow into the disintegrating calcified cartilage inducing the p.c.o.
Spongy bone is formed spreading from center to the ends
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32. 2. Endochondral ossification
Development of the medullary cavity (marrow):
Osteoclasts resorb some spongy bone leaving a cavity in the diaphysis
the wall of the diaphysis is eventually replaced by compact bone
Development of the secondary ossification centers:
Branches of epiphyseal artery enter around the time of birth
Develops similar to primary center
Spongy bone remains w/o formation of medullary cavity
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33. 2. Endochondral ossification
Formation of articular cartilage and the epiphyseal growth plate:
Hyaline cartilage covers the epiphyses
Prior to adulthood, hyaline cartilage remains in metaphysis
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34. 1
Development of
cartilage model
primary ossification
center
the medullary
cavity
Growth of 2 3 4
Hyaline
cartilage
Calcified
matrix
Periosteum
(covering
compact bone)
Uncalcified
Medullary
Nutrient
artery and vein
artery
Perichondrium
Proximal
epiphysis
Distal
Diaphysis
Primary
ossification
Secondary
Epiphyseal
artery and
vein
Development of secondary
ossification center 5
Spongy
bone 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
primary ossification
the medullary
cavity
Growth of 2 3 4 5 6
Hyaline
cartilage
Calcified
Periosteum
(covering
compact bone)
Medullary
artery
Perichondrium
Proximal
epiphysis
Distal
Diaphysis
Primary
Spongy
bone 1
Hyaline
cartilage
Calcified
matrix
Periosteum
(covering
compact bone)
Uncalcified
Medullary
cavity
Nutrient
artery and vein
artery
Perichondrium
Proximal
epiphysis
Distal
Diaphysis
Development of
cartilage model
primary ossification
center
the medullary
Growth of
Primary
ossification 2 3 4
Spongy
bone 1
Development of
cartilage model
primary ossification
center
Growth of 2 3
Hyaline
cartilage
Uncalcified
matrix
Calcified
Nutrient
artery
Perichondrium
Proximal
epiphysis
Distal
Diaphysis
Periosteum
Primary
ossification
Spongy
bone 1
Development of
cartilage model
Growth of 2
Hyaline
cartilage
Uncalcified
matrix
Calcified
Perichondrium
Proximal
epiphysis
Distal
Diaphysis 1
Development of
cartilage model
Hyaline
cartilage
Perichondrium
Proximal
epiphysis
Distal
Diaphysis

35. Bone Growth During Infancy, Childhood and Adolescence
Growth in Length
The growth in length of long bones involves two major events:
Growth of cartilage on the epiphyseal plate
Replacement of cartilage by bone tissue in the epiphyseal plate
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36. 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
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37. 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
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38. 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
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39. 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
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40. 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
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41. 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
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42. 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
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43. Spongy bone
Osteoblast
Osteoclast
New compact
bone
Bony callus formation
Bone remodeling
Osteocyte 3 4
Compact bone
Periosteum
Fracture hematoma
Fracture
hematoma
Bone
fragment
Red blood
cell
Blood vessel
Formation of fracture hematoma
Phagocyte
Osteon 1
Fibroblast
Fibrocartilaginous
callus
Collagen fiber
Chondroblast
Cartilage
Fibrocartilaginous callus formation 2
Bony callus
Bony callus
Spongy bone
Osteoblast
Bony callus formation
Osteocyte 3
Compact bone
Periosteum
Fracture hematoma
Fracture
hematoma
Bone
fragment
Red blood
cell
Blood vessel
Formation of fracture hematoma
Phagocyte
Osteon 1
Fibroblast
Fibrocartilaginous
callus
Collagen fiber
Chondroblast
Cartilage
Fibrocartilaginous callus formation 2
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
Osteon 1
Compact bone
Spongy bone
Periosteum
Fracture hematoma
Fracture
hematoma
Bone
fragment
Osteocyte
Red blood
cell
Blood vessel
Formation of fracture hematoma
Phagocyte
Osteon 1

44. 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 (Ca2+) to function properly
Blood clotting also requires Ca2+
Many enzymes require Ca2+ as a cofactor
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45. Bone’s Role in Calcium Homeostasis
Actions that help elevate blood Ca2+ level
Parathyroid hormone (PTH) regulates Ca2+ exchange between blood and bone tissue
PTH increases the number and activity of osteoclasts
PTH acts on the kidneys to decrease loss of Ca2+ in the urine
PTH stimulates formation of calcitriol a hormone that promotes absorption of calcium from foods in the gastrointestinal tract
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46. Bone’s Role in Calcium Homeostasis
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47. Bone’s Role in Calcium Homeostasis
Actions that work to decrease blood Ca2+ level
The thyroid gland secretes calcitonin (CT) which inhibits activity of osteoclasts
The result is that CT promotes bone formation and decreases blood Ca2+ level
Accelerates uptake of calcium and phosphate into bone matrix
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48. 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
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49. 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
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50. 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
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51. Copyright 2009, John Wiley & Sons, Inc.
End of Chapter 6
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