ch06 bones-ap1 spring2020

Copyright 2009, John Wiley & Sons, Inc.

Chapter 6:The Skeletal System:

Bone Tissue


Functions of Bone and Skeletal System

n Supportn Protectionn Assistance in Movementn Mineral Homeostasisn Blood Cell Production(Hemopoiesis)

q Triglyceride Storageq Yellow bone marrow

n Triglycerides stored in adipose cellsq Serves as a potential chemical energy reserve


Structure of Bone

q Diaphysisq Epiphysisq Metaphysis

n Epiphyseal growth plateq Articular cartilage

n Perforating fibersq Periosteum q Medullary cavityq Endosteum

n Long Bone Anatomy (Humerus)


Histology of Bone Tissuen Extracellular matrix surrounding widely

separated cellsq Matrix

n 25% watern 25% collagen fibersn 50% crystallized mineral salts

n The most abundant mineral salt is calcium phosphate


Histology of Bone Tissuen 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 Tissuen Four types of cells are present in bone tissuen Osteogenic cells

q Undergo cell division; the resulting cells develop into osteoblasts

n Osteoblastsq Bone-building cellsq Synthesize extracellular matrix of bone tissue

n Osteocytesq Mature bone cellsq Exchange nutrients and wastes with the blood

Histology of Bone Tissuen Osteoclasts

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

q Regulate blood calcium level


Histology of Bone Tissuen Bone may be categorized as:

q Compactq Spongy


Histology of Bone Tissuen 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 Tissuen Osteon

q Central canals run longitudinally through boneq Around the central canals are concentric

lamellaen Rings of calcified matrix (like the rings of a tree

trunk)q Between the lamellae are small spaces called

lacunae which contain osteocytesq Radiating in all directions from the lacunae are

tiny canaliculi filled with extracellular fluid


Histology of Bone Tissuen Osteon

q Canaliculi connect lacunae, forming a system of interconnected canalsn 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 Tissuen Spongy Bone

q Lacks osteonsq Lamellae are arranged in a lattice of thin

columns called trabeculaen Spaces between the trabeculae make bones

lightern Trabeculae of spongy bone support and protect

the red bone marrown Hemopoiesis (blood cell production) occurs in

spongy bone


Histology of Bone Tissuen 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 stressn helps bones resist stresses without breaking


Blood and Nerve Supply of Bonen 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 bonesq The periosteum is rich in sensory nerves

sensitive to tearing or tension


Bone Formationn The process by which bone forms is

called ossificationn Bone formation occurs in four situations:

q 1) Formation of bone in an embryoq 2) Growth of bones until adulthoodq 3) Remodeling of boneq 4) Repair of fractures


Bone Formationn 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 cellOsteoblast

Collagen fiber

Development of ossification center

Mandible

Flat boneof skull

1

Blood capillary

Ossification center


Osteocyte in lacuna

Canaliculus

Osteoblast

Newly calcified bonematrix


Calcification

Mandible

Flat boneof skull

2

Collagen fiber

1

Blood capillary

Ossification center



Calcification

Mandible

Flat boneof skull

2

Collagen fiber

Osteocyte in lacuna

Canaliculus

Osteoblast


MesenchymecondensesBlood vessel

Spongy bonetrabeculaeOsteoblast

Formation of trabeculae3

1

Blood capillary

Ossification center


MesenchymecondensesBlood vessel

Spongy bonetrabeculaeOsteoblast

Periosteum

Spongy bone tissue

Compact bone tissue


Calcification Formation of trabeculae

Development of the periosteum

Mandible

Flat boneof skull

3

4

2

Collagen fiber

Osteocyte in lacuna

Canaliculus

Osteoblast



Bone Formationn Formation of Bone in an Embryo

q Bone formation follows one of two patternsn 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 ossificationq The replacement of cartilage by boneq Most bones of the body are formed in this way including

long bones

1 Development ofcartilage model

Hyalinecartilage

Perichondrium

Proximalepiphysis

Distalepiphysis

Diaphysis


Growth ofcartilage model

2

Hyalinecartilage

Uncalcifiedmatrix

Calcifiedmatrix

Perichondrium

Proximalepiphysis

Distalepiphysis

Diaphysis


Development ofprimary ossificationcenter


2 3

Hyalinecartilage

Uncalcifiedmatrix

Calcifiedmatrix

Nutrientartery

Perichondrium

Proximalepiphysis

Distalepiphysis

Diaphysis

Periosteum

Primaryossificationcenter

Spongybone

1

Hyalinecartilage

CalcifiedmatrixPeriosteum(covering compact bone)

Uncalcifiedmatrix

Calcifiedmatrix

Medullarycavity

Nutrientartery and vein

Nutrientartery

Perichondrium

Proximalepiphysis

Distalepiphysis

Diaphysis

Development ofcartilage model


Development ofthe medullarycavity


Periosteum


2 3 4

Spongybone

Uncalcifiedmatrix





2 3 4

Hyalinecartilage


Uncalcifiedmatrix

Calcifiedmatrix

Medullarycavity


Nutrientartery

Perichondrium

Proximalepiphysis

Distalepiphysis

Diaphysis

Periosteum


Secondaryossificationcenter


Uncalcifiedmatrix

Epiphysealartery andvein

Development of secondaryossification center

5

Spongybone

Uncalcifiedmatrix

1

Articular cartilage

Spongy bone

Epiphyseal plate

Secondaryossificationcenter


Uncalcifiedmatrix

Epiphysealartery andvein

Formation of articular cartilageand epiphyseal plate

Development of secondaryossification center

Development ofcartilage model




2 3 4

5 6

Hyalinecartilage

Uncalcifiedmatrix


Uncalcifiedmatrix

Calcifiedmatrix

Medullarycavity


Nutrientartery

Perichondrium

Proximalepiphysis

Distalepiphysis

Diaphysis

Periosteum


Spongybone


Bone Growth During Infancy, Childhood and Adolescence

n Growth in Lengthn The growth in length of long bones

involves two major events:q 1) Growth of cartilage on the

epiphyseal plateq 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 Adolescencen Growth in Thickness

q Bones grow in thickness at the outer surfacen 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 Adolescencen 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 Remodelingn Normal bone metabolism depends on several factorsn Minerals

q Large amounts of calcium and phosphorus and smaller amounts of magnesium, fluoride, and manganese are required for bone growth and remodeling

n Vitaminsq Vitamin A stimulates activity of osteoblastsq Vitamin C is needed for synthesis of collagenq 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 Remodelingn Hormones

q During childhood, the hormones most important to bone growth are growth factors (IGFs), produced by the livern 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 Remodelingn Hormones

q Estrogen and testosterone cause a dramatic effect on bone growthn Cause of the sudden “growth spurt” that occurs

during the teenage yearn Promote changes in females, such as widening of

the pelvisn Shut down growth at epiphyseal plates

q Parathyroid hormone, calcitriol, and calcitonin are other hormones that can affect bone remodeling


Fracture and Repair of Bonen Fracture Types

q Open (compound) fracturen The broken ends of the bone protrude through the skin

q Closed (simple) fracturen Does not break the skin

q Comminuted fracturen The bone is splintered, crushed, or broken into pieces

q Greenstick fracturen A partial fracture in which one side of the bone is broken and the other side

bendsq Impacted fracture

n One end of the fractured bone is forcefully driven into anotherq Pott’s fracture

n Fracture of the fibula, with injury of the tibial articulationq Colles’ fracture

n A fracture of the radius in which the distal fragment is displacedq Stress fracture

n A series of microscopic fissures in bone

Fracture and Repair of Bone


Fracture and Repair of Bonen 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 stepsq 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 formationn Fibroblasts invade the fracture site and produce collagen

fibers bridging the broken ends of the boneq 3) Bony callus formation

n Osteoblasts begin to produce spongy bone trabeculae joining portions of the original bone fragments

q 4) Bone remodelingn Compact bone replaces spongy bone


Fracture and Repair of Bone

Compact boneSpongy bone

Periosteum

Fracture hematoma

Fracturehematoma

BonefragmentOsteocyte

Red bloodcell

Blood vessel

Formation of fracture hematoma

Phagocyte

Osteon

1

Phagocyte

Osteoblast

Fibroblast

Fibrocartilaginouscallus

Collagen fiberChondroblastCartilage

Fibrocartilaginous callus formation2


Periosteum

Fracture hematoma

Fracturehematoma


Red bloodcell

Blood vessel


Phagocyte

Osteon

1

Bony callus

Spongy boneOsteoblast

Bony callus formation

Osteocyte

3


Periosteum

Fracture hematoma

Fracturehematoma


Red bloodcell

Blood vessel


Phagocyte

Osteon

1

Phagocyte

Osteoblast

Fibroblast




Spongy boneOsteoblast

Osteoclast

New compactbone

Bony callus formation Bone remodeling

Osteocyte

3 4


Periosteum

Fracture hematoma

Fracturehematoma


Red bloodcell

Blood vessel


Phagocyte

Osteon

1

Phagocyte

Osteoblast

Fibroblast




Bony callus


Bone’s Role in Calcium Homeostasisn Bone is the body’s major calcium reservoirn 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 boneq Both nerve and muscle cells depend on

calcium ions (Ca2+) to function properlyq Blood clotting also requires Ca2+

q Many enzymes require Ca2+ as a cofactor


Bone’s Role in Calcium Homeostasisn Actions that help elevate blood Ca2+ level

q Parathyroid hormone (PTH) regulatesCa2+ exchange between blood and bone tissuen PTH increases the number and activity of

osteoclastsn PTH acts on the kidneys to decrease loss of

Ca2+ in the urinen 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 Homeostasisn 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 Tissuen Bone tissue alters its strength in response to

changes in mechanical stressq 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 Tissuen The level of sex hormones diminishes during

middle age, especially in women after menopauseq A decrease in bone mass occursq Bone resorption by osteoclasts outpaces bone

deposition by osteoblastsn Female bones generally are smaller and less

massive than malesq Loss of bone mass in old age has a greater

adverse effect in females

Aging and Bone Tissuen There are two principal effects of aging on bone tissue:

q 1) Loss of bone massn Results from the loss of calcium from bone matrixn The loss of calcium from bones is one of the symptoms in

osteoporosisq 2) Brittleness

n Results from a decreased rate of protein synthesisn Collagen fibers gives bone its tensile strengthn The loss of tensile strength causes the bones to become very brittle

and susceptible to fracture

ch06 bones-ap1 spring2020

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