The Skeletal System: Bone Tissue Chapter 6

Copyright 2009, John Wiley & Sons, Inc.

The Skeletal System: Bone Tissue Chapter 6


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


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)


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


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


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


Histology of Bone Tissue n  Osteoclasts

q  Release enzymes that digest the mineral components of bone matrix (resorption)

q  Regulate blood calcium level


Histology of Bone Tissue n  Bone may be categorized as:

q  Compact q  Spongy


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


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


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




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


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


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


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


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


Osteocyte in lacuna

Canaliculus

Osteoblast

Newly calcified bone matrix


Calcification

Mandible

Flat bone of skull

2

Collagen fiber

1

Blood capillary

Ossification center



Calcification

Mandible

Flat bone of skull

2

Collagen fiber

Osteocyte in lacuna

Canaliculus

Osteoblast


Mesenchyme condenses Blood vessel

Spongy bone trabeculae Osteoblast

Formation of trabeculae 3

1

Blood capillary

Ossification center


Mesenchyme condenses Blood vessel

Spongy bone trabeculae Osteoblast

Periosteum

Spongy bone tissue

Compact bone tissue


Calcification Formation of trabeculae

Development of the periosteum

Mandible

Flat bone of skull

3

4

2

Collagen fiber

Osteocyte in lacuna

Canaliculus

Osteoblast



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


Growth of cartilage model

2

Hyaline cartilage

Uncalcified matrix

Calcified matrix

Perichondrium

Proximal epiphysis

Distal epiphysis

Diaphysis


Development of primary ossification center


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 the medullary cavity


Periosteum


2 3 4

Spongy bone

Uncalcified matrix





2 3 4

Hyaline cartilage


Uncalcified matrix

Calcified matrix

Medullary cavity


Nutrient artery

Perichondrium

Proximal epiphysis

Distal epiphysis

Diaphysis

Periosteum


Secondary ossification center


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


Uncalcified matrix

Epiphyseal artery and vein

Formation of articular cartilage and epiphyseal plate

Development of secondary ossification center

Development of cartilage model




2 3 4

5 6

Hyaline cartilage

Uncalcified matrix


Uncalcified matrix

Calcified matrix

Medullary cavity


Nutrient artery

Perichondrium

Proximal epiphysis

Distal epiphysis

Diaphysis

Periosteum


Spongy bone


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


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



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


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


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


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


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


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


Fracture and Repair of Bone


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


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


Periosteum

Fracture hematoma

Fracture hematoma


Red blood cell

Blood vessel


Phagocyte

Osteon

1

Bony callus

Spongy bone Osteoblast

Bony callus formation

Osteocyte

3


Periosteum

Fracture hematoma

Fracture hematoma


Red blood cell

Blood vessel


Phagocyte

Osteon

1

Phagocyte

Osteoblast

Fibroblast




Spongy bone Osteoblast

Osteoclast

New compact bone

Bony callus formation Bone remodeling

Osteocyte

3 4


Periosteum

Fracture hematoma

Fracture hematoma


Red blood cell

Blood vessel


Phagocyte

Osteon

1

Phagocyte

Osteoblast

Fibroblast




Bony callus


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


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


Bone’s Role in Calcium Homeostasis


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


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


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


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

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The Skeletal System: Bone Tissue Chapter 6