Copyright 2009, John Wiley & Sons, Inc. The Skeletal System: Bone Tissue Chapter 6
Copyright 2009, John Wiley & Sons, Inc.
The Skeletal System: Bone Tissue Chapter 6
Copyright 2009, John Wiley & Sons, Inc.
Functions of Bone and Skeletal System
n Support n Protection n Assistance in Movement n Mineral Homeostasis n Blood Cell Production(Hemopoiesis)
q Triglyceride Storage q Yellow bone marrow
n Triglycerides stored in adipose cells q Serves as a potential chemical energy reserve
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Structure of Bone
q Diaphysis q Epiphysis q Metaphysis
n Epiphyseal growth plate q Articular cartilage
n Perforating fibers q Periosteum q Medullary cavity q Endosteum
n Long Bone Anatomy (Humerus)
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Histology of Bone Tissue n Extracellular matrix surrounding widely
separated cells q Matrix
n 25% water n 25% collagen fibers n 50% crystallized mineral salts
n The most abundant mineral salt is calcium phosphate
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Histology of Bone Tissue n A process called calcification is initiated
by bone-building cells called osteoblasts
n Mineral salts are deposited and crystalize in the framework formed by the collagen fibers of the extracellular matrix
n Bone’s flexibility depends on collagen fibers
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Histology of Bone Tissue n Four types of cells are present in bone tissue n Osteogenic cells
q Undergo cell division; the resulting cells develop into osteoblasts
n Osteoblasts q Bone-building cells q Synthesize extracellular matrix of bone tissue
n Osteocytes q Mature bone cells q Exchange nutrients and wastes with the blood
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Histology of Bone Tissue n Osteoclasts
q Release enzymes that digest the mineral components of bone matrix (resorption)
q Regulate blood calcium level
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Histology of Bone Tissue n Bone may be categorized as:
q Compact q Spongy
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Histology of Bone Tissue n Compact Bone
q Resists the stresses produced by weight and movement
q Components of compact bone are arranged into repeating structural units called osteons or Haversian systems
q Osteons consist of a central (Haversian) canal with concentrically arranged lamellae, lacunae, osteocytes, and canaliculi
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Histology of Bone Tissue n Osteon
q Central canals run longitudinally through bone q Around the central canals are concentric
lamellae n Rings of calcified matrix (like the rings of a tree
trunk) q Between the lamellae are small spaces called
lacunae which contain osteocytes q Radiating in all directions from the lacunae are
tiny canaliculi filled with extracellular fluid
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Histology of Bone Tissue n Osteon
q Canaliculi connect lacunae, forming a system of interconnected canals n Providing routes for nutrients and oxygen to reach
the osteocytes
q The organization of osteons changes in response to the physical demands placed on the skeleton
Copyright 2009, John Wiley & Sons, Inc.
Copyright 2009, John Wiley & Sons, Inc.
Copyright 2009, John Wiley & Sons, Inc.
Histology of Bone Tissue n Spongy Bone
q Lacks osteons q Lamellae are arranged in a lattice of thin
columns called trabeculae n Spaces between the trabeculae make bones
lighter n Trabeculae of spongy bone support and protect
the red bone marrow n Hemopoiesis (blood cell production) occurs in
spongy bone
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Histology of Bone Tissue n Spongy Bone
q Within each trabecula are lacunae that contain osteocytes
q Osteocytes are nourished from the blood circulating through the trabeculae
q Interior bone tissue is made up primarily of spongy bone
q The trabeculae of spongy bone are oriented along lines of stress n helps bones resist stresses without breaking
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Blood and Nerve Supply of Bone n Bone is richly supplied with blood
q Periosteal arteries accompanied by nerves supply the periosteum and compact bone
q Epiphyseal veins carry blood away from long bones
n Nerves accompany the blood vessels that supply bones q The periosteum is rich in sensory nerves sensitive
to tearing or tension
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Bone Formation n The process by which bone forms is
called ossification n Bone formation occurs in four situations:
q 1) Formation of bone in an embryo q 2) Growth of bones until adulthood q 3) Remodeling of bone q 4) Repair of fractures
Copyright 2009, John Wiley & Sons, Inc.
Bone Formation n Formation of Bone in an Embryo
q Cartilage formation and ossification occurs during the sixth week of embryonic development
1
Blood capillary
Ossification center
Mesenchymal cell Osteoblast
Collagen fiber
Development of ossification center
Mandible
Flat bone of skull
1
Blood capillary
Ossification center
Mesenchymal cell Osteoblast
Osteocyte in lacuna
Canaliculus
Osteoblast
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
Development of ossification center
Calcification
Mandible
Flat bone of skull
2
Collagen fiber
Osteocyte in lacuna
Canaliculus
Osteoblast
Newly calcified bone matrix
Mesenchyme condenses Blood vessel
Spongy bone trabeculae Osteoblast
Formation of trabeculae 3
1
Blood capillary
Ossification center
Mesenchymal cell Osteoblast
Mesenchyme condenses Blood vessel
Spongy bone trabeculae Osteoblast
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
Osteoblast
Newly calcified bone matrix
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Bone Formation n Formation of Bone in an Embryo
q Bone formation follows one of two patterns n Intramembranous ossification
q Flat bones of the skull and mandible are formed in this way
q “Soft spots” that help the fetal skull pass through the birth canal later become ossified forming the skull
n Endochondral ossification q The replacement of cartilage by bone q Most bones of the body are formed in this way including
long bones
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
2 3
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
2 3 4
Spongy bone
Uncalcified matrix
1 Development of cartilage model
Development of primary ossification center
Development of the medullary cavity
Growth of cartilage model
2 3 4
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
2 3 4
5 6
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
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Bone Growth During Infancy, Childhood and Adolescence
n Growth in Length n The growth in length of long bones
involves two major events: q 1) Growth of cartilage on the
epiphyseal plate q 2) Replacement of cartilage by
bone tissue in the epiphyseal plate
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Bone Growth During Infancy, Childhood and Adolescence n Osteoclasts dissolve the calcified cartilage, and osteoblasts invade the
area laying down bone matrix
n The activity of the epiphyseal plate is the way bone can increase in length
n At adulthood, the epiphyseal plates close and bone replaces all the cartilage leaving a bony structure called the epiphyseal line
Copyright 2009, John Wiley & Sons, Inc.
Copyright 2009, John Wiley & Sons, Inc.
Bone Growth During Infancy, Childhood and Adolescence n Growth in Thickness
q Bones grow in thickness at the outer surface n Remodeling of Bone
q Bone forms before birth and continually renews itself
q The ongoing replacement of old bone tissue by new bone tissue
q Old bone is continually destroyed and new bone is formed in its place throughout an individual’s life
Copyright 2009, John Wiley & Sons, Inc.
Bone Growth During Infancy, Childhood and Adolescence n A balance must exist between the actions of
osteoclasts and osteoblasts
q If too much new tissue is formed, the bones become abnormally thick and heavy
q Excessive loss of calcium weakens the bones, as occurs in osteoporosis
q Or they may become too flexible, as in rickets and osteomalacia
Copyright 2009, John Wiley & Sons, Inc.
Factors Affecting Bone Growth and Bone Remodeling n Normal bone metabolism depends on several factors n Minerals
q Large amounts of calcium and phosphorus and smaller amounts of magnesium, fluoride, and manganese are required for bone growth and remodeling
n Vitamins q Vitamin A stimulates activity of osteoblasts q Vitamin C is needed for synthesis of collagen q Vitamin D helps build bone by increasing the
absorption of calcium from foods in the gastrointestinal tract into the blood
q Vitamins K and B12 are also needed for synthesis of bone proteins
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Factors Affecting Bone Growth and Bone Remodeling n Hormones
q During childhood, the hormones most important to bone growth are growth factors (IGFs), produced by the liver n IGFs stimulate osteoblasts, promote cell division at the
epiphyseal plate, and enhance protein synthesis
q Thyroid hormones also promote bone growth by stimulating osteoblasts
q Insulin promotes bone growth by increasing the synthesis of bone proteins
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Factors Affecting Bone Growth and Bone Remodeling n Hormones
q Estrogen and testosterone cause a dramatic effect on bone growth n Cause of the sudden “growth spurt” that occurs
during the teenage year n Promote changes in females, such as widening of
the pelvis n Shut down growth at epiphyseal plates
q Parathyroid hormone, calcitriol, and calcitonin are other hormones that can affect bone remodeling
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Fracture and Repair of Bone n Fracture Types
q Open (compound) fracture n The broken ends of the bone protrude through the skin
q Closed (simple) fracture n Does not break the skin
q Comminuted fracture n The bone is splintered, crushed, or broken into pieces
q Greenstick fracture n A partial fracture in which one side of the bone is broken and the other side
bends q Impacted fracture
n One end of the fractured bone is forcefully driven into another q Pott’s fracture
n Fracture of the fibula, with injury of the tibial articulation q Colles’ fracture
n A fracture of the radius in which the distal fragment is displaced q Stress fracture
n A series of microscopic fissures in bone
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Fracture and Repair of Bone
Copyright 2009, John Wiley & Sons, Inc.
Fracture and Repair of Bone n Calcium and phosphorus needed to strengthen and
harden new bone after a fracture are deposited only gradually and may take several months
n The repair of a bone fracture involves the following steps q 1) Formation of fracture hematoma
n Blood leaks from the torn ends of blood vessels, a clotted mass of blood forms around the site of the fracture
q 2) Fibrocartilaginous callus formation n Fibroblasts invade the fracture site and produce collagen
fibers bridging the broken ends of the bone q 3) Bony callus formation
n Osteoblasts begin to produce spongy bone trabeculae joining portions of the original bone fragments
q 4) Bone remodeling n Compact bone replaces spongy bone
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Fracture and Repair of Bone
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 formation Bone remodeling
Osteocyte
3 4
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
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Bone’s Role in Calcium Homeostasis n Bone is the body’s major calcium reservoir n 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 q Both nerve and muscle cells depend on
calcium ions (Ca2+) to function properly q Blood clotting also requires Ca2+
q Many enzymes require Ca2+ as a cofactor
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Bone’s Role in Calcium Homeostasis n Actions that help elevate blood Ca2+ level
q Parathyroid hormone (PTH) regulates Ca2+ exchange between blood and bone tissue n PTH increases the number and activity of
osteoclasts n PTH acts on the kidneys to decrease loss of
Ca2+ in the urine n PTH stimulates formation of calcitriol a
hormone that promotes absorption of calcium from foods in the gastrointestinal tract
Copyright 2009, John Wiley & Sons, Inc.
Bone’s Role in Calcium Homeostasis
Copyright 2009, John Wiley & Sons, Inc.
Bone’s Role in Calcium Homeostasis n Actions that work to decrease blood Ca2+
level
q The thyroid gland secretes calcitonin (CT) which inhibits activity of osteoclasts
q The result is that CT promotes bone formation and decreases blood Ca2+ level
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Exercise and Bone Tissue n Bone tissue alters its strength in response to
changes in mechanical stress q Under stress, bone tissue becomes stronger through
deposition of mineral salts and production of collagen fibers by osteoblasts
q Unstressed bones diminishes because of the loss of bone minerals and decreased numbers of collagen fibers
n The main mechanical stresses on bone are those that result from the pull of skeletal muscles and the pull of gravity
n Weight-bearing activities help build and retain bone mass
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Aging and Bone Tissue n The level of sex hormones diminishes during
middle age, especially in women after menopause q A decrease in bone mass occurs q Bone resorption by osteoclasts outpaces bone
deposition by osteoblasts n Female bones generally are smaller and less
massive than males q Loss of bone mass in old age has a greater
adverse effect in females
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Aging and Bone Tissue n There are two principal effects of aging on bone tissue:
q 1) Loss of bone mass n Results from the loss of calcium from bone matrix n The loss of calcium from bones is one of the symptoms in
osteoporosis q 2) Brittleness
n Results from a decreased rate of protein synthesis n Collagen fibers gives bone its tensile strength n The loss of tensile strength causes the bones to become very brittle
and susceptible to fracture