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C H A P T E R

6Bones andSkeletal Tissues

The skeletal system

• Components :• Bones of the skeleton• Cartilages• Ligaments• Other connective tissues that stabilize and connect

• Functions :• Support – provides hard framework• Movement – skeletal muscles use bones as levers• Protection of underlying organs• Mineral storage – reservoir for important minerals• Blood-cell formation – bone contains red marrow

Cartilage • Resist compression and tension• Cartilage, firm but flexible and cells separated by abundant

extracellular matrix (holds 80% water)- no blood vessels or nerves. • Chondrocytes• Lacunae• Chondroblasts• perichondrium

• Hyaline cartilage – (glass) – most abundant cartilage• Provides support through flexibility

• Elastic cartilage – contains many elastic fibers• Able to tolerate repeated bending

• Fibrocartilage – resists strong compression and strong tension• An intermediate between hyaline and elastic cartilage

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Cartilage inexternal ear

Cartilages innose

Articular cartilageof a joint

Costal cartilageCartilagein intervertebraldisc

CartilageEpiglottis

Larynx

Trachea

Cricoidcartilage

Lung

Thyroidcartilage

Hyaline cartilages

Elastic cartilages

Fibrocartilages

Cartilages

Pubicsymphysis

Articular cartilageof a joint

Meniscus (padlikecartilage inknee joint) Respiratory

tube cartilagesin neck and thorax

Gelatinous groundsubstance

Chondrocyte in a lacunaElastic fibers

Lacuna

Matrix

Chondrocytein a lacuna

Perichondrium

(a) Hyaline cartilage (180×)

Cartilages in the Adult Body

Chondrocyte in a lacuna

Collagen fibers

(b) Elastic cartilage (470×)

(c) Fibrocartilage (285×)Figure 6.2

Classification of Bones

• Long bones – longer than wide – a shaft plus ends• Short bones – roughly cube-shaped• Flat bones – thin and flattened, usually curved• Irregular bones – various shapes, do not fit into

other categories

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Classification of Bones

Figure 6.3

Cells in bone

• Osteocytes = mature bone cells• In lacunae, Connected by canaliculi• Osteoblasts synthesize new matrix (Osteogenesis)• Osteoclasts dissolve bone matrix (Osteolysis)• Osteoprogenitor cells differentiate into

osteoblasts

Gross Anatomy of Bones

• Compact bone – dense outer layer of bone• Spongy (cancellous) bone – internal network of

bone

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Structure of a Long Bone

Proximalepiphysis

(b)

EndosteumEpiphysealline

Articular cartilage

Periosteum

Spongy bone

Compact boneMedullarycavity (lined

Compact bone

Endosteum

Figure 6.4a–c

(b)

(c)(a)

Yellowbone marrow

by endosteum)

Compact bone

Periosteum

Perforating(Sharpey’s)fibersNutrientarteries

Diaphysis

Distalepiphysis

Spongy bone(di l ë)

Structure of Short, Irregular, and Flat Bones

• Flat bones, short bones, and irregular bones• Contain bone

marrow but noCompact bone

Trabeculae

(diploë)

Figure 6.5

marrow but no marrow cavity

• Diploë• Internal spongy

bone of flat bones

Load here(body weight)

Head offemur

Gross Anatomy of Bones

• Bone design and stress• Anatomy of a bone

reflects stresses• Compression and tension

greatest at external

Compressionhere

Point of no stress

Tensionhere

(a)

greatest at external surfaces

Figure 6.6a

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Gross Anatomy of Bones

Compact bone

Endosteum lining bony canals and covering trabeculae

Perforating (Volkmann’s) canal

Circumferentiallamellae

Osteon(Haversian system)

Central(Haversian) canal

Spongy bone

Microscopic Structure of Compact Bones

Figure 6.7

(a)Perforating (Sharpey’s) fibersPeriosteal blood vesselPeriosteum

Lamellae

(c)Interstitial lamellae

Lacunae

LamellaeCentralcanal

Lacuna (with osteocyte) (b)

NerveVeinArtery

CanaliculiOsteocytein a lacuna

Lacunae

LamellaeCentral canal

Structuresin thecentralcanal

Artery withcapillaries

VeinNerve fiber

Lamellae

Collagenfibers

Microscopic Structure of Compact Bones

run indifferentdirections

Twistingforce

Figure 6.8

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Trabecula

Osteocytes

Marrow space

Microscopic Structure of Spongy Bone

Endosteum

(b)

(a)Osteoblasts

Figure 6.9

Chemical Composition of Bone

• 35% organic components• Composed of cells, fibers, and organic substances• Collagen – abundant

• 65% inorganic mineral salts• Primarily calcium phosphate• Resists compression

Bone Development

• Ossification (osteogenesis) – bone-tissue formation• Membrane bones – formed directly from

mesenchyme• Intramembranous ossification

• Other bones – develop initially from hyalineOther bones develop initially from hyaline cartilage• Endochondral ossification

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Intramembranous Ossification

MesenchymalcellCollagenfiberOssificationcenter

Osteoid

Osteocyte

Osteoblast

Figure 6.10, steps 1–2

center

Osteoid

Osteoblast

Newly calcifiedbone matrix

Ossification centers appear in the fibrous connective tissue membrane.• Selected centrally located mesenchymal cells cluster

and differentiate into osteoblasts, forming an ossification center.

Bone matrix (osteoid) is secreted within the fibrous membrane and calcifies.• Osteoblasts begin to secrete osteoid, which is

calcified within a few days.• Trapped osteoblasts become osteocytes.

1 2

Intramembranous Ossification

Mesenchymecondensingto form theperiosteum

Blood vessel

Trabeculae ofwoven bone

FibrousperiosteumOsteoblast

Plate ofcompact bone

Diploë (spongybone) cavities

Figure 6.10, steps 3–4

Blood vessel bone) cavitiescontain redmarrow

Woven bone and periosteum form.• Accumulating osteoid is laid down between embryonic

blood vessels in a random manner. The result is a network (instead of lamellae) of trabeculae calledwoven bone.

• Vascularized mesenchyme condenses on the externalface of the woven bone and becomes the periosteum.

Lamellar bone replaces woven bone, just deep to the periosteum. Red marrow appears. • Trabeculae just deep to the periosteum thicken and

are later replaced with mature lamellar bone, forming compact bone plates.

• Spongy bone (diploë), consisting of distinct trabeculae, persists internally, and its vascular tissue becomes red marrow.

3 4

Hyalinecartilage

Area ofdeterioratingcartilage matrix

Epiphysealblood vessel

Epiphysealplatecartilage

Secondaryossificationcenter

Medullary

Articularcartilage

Childhood to adolescenceBirthWeek 9 Month 3

Spongybone

Stages in Endochondral Ossification

cartilageSpongyboneformation

Bloodvessel ofperiostealbud

Medullarycavity

Bone collarPrimaryossificationcenter

Bone collar forms around hyaline cartilage model.

Cartilage in the center of the diaphysis calcifies and then develops cavities.

The periosteal bud invades the internal cavities, and spongy bone begins to form.

The diaphysis elongates and a medullary cavity forms as ossification continues. Secondary ossification centers appear in the epiphyses in preparation for stage 5.

The epiphyses ossify. When completed, hyaline cartilage remains only in the epiphyseal plates and articular cartilages.

1 2 3 4 5

Figure 6.11

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Growth of Endochondral Bones• Appositional growth – growth of a bone by addition

of bone tissue to its surface

Hormonal Regulation of Bone Growth

• Growth hormone – produced by the pituitary gland• Stimulates epiphyseal plates

• Thyroid hormone – ensures that the skeleton t i tiretains proper proportions

• Sex hormones• Promote bone growth• Later induces closure of epiphyseal plates

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Bones are Continuously changing

• Bone deposit and removal• Occurs at periosteal and endosteal surfaces• Bone deposition – accomplished by osteoblasts• Bone reabsorption – accomplished by osteoclast

• Bone remodeling• Helps maintain constant concentration of Ca2+ andHelps maintain constant concentration of Ca and

PO43-

• PTH and calcitonin affect osteoclast activity• Response to mechanical stress

Repair of Bone Fractures

• Simple and compound fractures• Treatment by reduction

• Closed reduction• Open reduction

Stages of Healing a Fracture

Hematoma Externalcallus

NewInternal

Bonycallus ofspongyboneH l d

Figure 6.15

Newbloodvessels

Spongybonetrabecula

Internalcallus(fibroustissue andcartilage)

A hematoma forms. Fibrocartilaginous callus forms.

Bony callus forms.

Healedfracture

Bone remodeling occurs.

1 2 3 4

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Common Types of Fractures

Table 6.2 (1 of 3)

Common Types of Fractures

Table 6.2 (2 of 3)

Common Types of Fractures

Table 6.2 (3 of 3)

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Disorders of Bones

• Osteoporosis – characterized by low bone mass• Bone reabsorption outpaces bone deposition• Occurs most of in women after menopause

Figure 6.16

Disorders of Bones

• Osteomalacia – occurs in adults – bones are inadequately mineralized• Rickets – occurs in children – analogous to

osteomalacia • Paget's disease characterized by excessive rate• Paget's disease – characterized by excessive rate

of bone deposition • Osteosarcoma – a form of bone cancer