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The Skeletal System

Skull

Thoracic cage(ribs andsternum)

(a) Anterior view

Facial bonesCranium

Sacrum

Vertebralcolumn

ClavicleScapulaSternumRibHumerusVertebraRadiusUlnaCarpals

PhalangesMetacarpalsFemurPatella

TibiaFibula

TarsalsMetatarsalsPhalanges

Functions1. Support

� For the body and soft organs

2. Protection� For brain, spinal cord and vital organs

3. Movement� Levers for muscle action

4. Mineral reservoir� Calcium and phosphorus

5. Hematopoiesis� Marrow cavities

Bone

� Osseous tissue� Hydroxyapatite (85%)

� Calcium carbonate (10%)

� Inorganic minerals� Magnesium, sodium, fluoride

� Organic material� Collagen

� Chondroitin sulfate

Figure 6.3c

(c)

Yellow

bone marrow

Endosteum

Compact bone

Periosteum

Perforating

(Sharpey’s) fibers

Nutrient

arteries

Bone consists of

multiple tissues

A single bone may be

considered an organ

Figure 6.3a-b

Proximal

epiphysis

(b)

(a)

Epiphyseal

line

Articular

cartilage

Periosteum

Spongy bone

Compact bone

Medullary

cavity (lined

by endosteum)

Compact bone

Diaphysis

Distal

epiphysis

Bones are organs comprised

of more than just osseous tissue

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Figure 6.2

Bone ClassificationGeneral Bone Features

�Long bone anatomy (Humerus)� Diaphysis

� Epiphysis

� Metaphysis� Epiphyseal growth plate or line

� Articular cartilage

� Periosteum

� Medullary cavity

� Endosteum

Figure 6.3a-b

Proximal

epiphysis

(b)

(a)

Epiphyseal

line

Articular

cartilage

Periosteum

Spongy bone

Compact bone

Medullary

cavity (lined

by endosteum)

Compact bone

Diaphysis

Distal

epiphysis

Anatomy of a long bone

Figure 6.3c

(c)

Yellow

bone marrow

Endosteum

Compact bone

Periosteum

Perforating

(Sharpey’s) fibers

Nutrient

arteries

Outer layer of periosteum =

dense irregular fibrous sheath

A Closer Look

� Osteoblasts (OB)

� Osteoclasts (OC)

� Periosteum (PO)

� Sharpey’s Fibers (SF)

Proximal

epiphysis

(b)

(a)

Epiphyseal

line

Articular

cartilage

Periosteum

Spongy bone

Compact bone

Medullary

cavity (lined

by endosteum)

Compact bone

Diaphysis

Distal

epiphysis

Epiphyseal line is a remnant

of the epiphyseal plate

Metaphysis

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Epiphyseal (Growth) PlateBone Histology

� Components� Cells

� Fibers

� Ground substance

Bone Histology

� Cells� Osteogenic (osteoprogenitor) cells

� Stem cells in periosteum and endosteum → osteoblasts

� Osteoblasts� Bone forming cells

Figure 6.4a-b

(a) Osteogenic cell (b) Osteoblast

Stem cell Matrix-synthesizing

cell responsible

for bone growth

Bone Histology

� Cells� Osteocytes

� Mature bone cells

� Maintain bone matrix

Bone Histology

� Cells� Osteoclasts

� Break down (resorb) bone matrix

� Related to macrophages

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Figure 6.4c-d

(c) Osteocyte

Mature bone cell

that maintains the

bone matrix

(d) Osteoclast

Bone-resorbing cell

Bone Histology� Primary bone types

� Compact

� Spongy

Figure 6.5

Compact

bone

Trabeculae

Spongy bone

(diploë)

Compact bone

covers all spongy bone and the shafts

of long bones

Figure 6.3a-b

Proximal

epiphysis

(b)

(a)

Epiphyseal

line

Articular

cartilage

Periosteum

Spongy bone

Compact bone

Medullary

cavity (lined

by endosteum)

Compact bone

Diaphysis

Distal

epiphysis

Figure 6.7a-c

Endosteum lining bony canals

and covering trabeculae

Perforating

(Volkmann’s) canal

Perforating (Sharpey’s) fibers

Periosteal blood vesselPeriosteum

Lacuna (with

osteocyte)

(a)

(b) (c)

Lacunae

Lamellae

Nerve

Vein

Artery

Canaliculi

Osteocyte

in a lacuna

Circumferential

lamellae

Osteon

(Haversian system)

Central

(Haversian) canal

Central

canal

Interstitial lamellae

Lamellae

Compact

bone

Spongy bone

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Figure 6.6

Structures

in thecentral

canal

Artery with

capillaries

Vein

Nerve fiber

Lamellae

Collagen

fibersrun in

different

directions

Twisting

force

Figure 6.3b

(b)

Lacunae

Lamellae

Nerve

Vein

Artery

Canaliculus

Osteocyte

in a lacuna

Central

canal

Figure 6.5

Compact

bone

Trabeculae

Spongy bone

(diploë)

Bone Histology

� Bone marrow� Location

� Medullary cavity and spaces between trabeculae

Bone Histology

� Bone marrow� Types� Red

� Red blood cells

� Hematopoiesis

� Yellow� Replaces red with age

� Fat storage

� Gelatinous� Mostly water, fat, and protein

Skeletal System 17

1 - Proximal epiphysis

2 - Distal epiphysis

3 - Epiphyseal line

4 - Articular cartilage

5 - Spongy bone

6 - Compact bone

7 - Medullary canal

8 - Periosteum

9 - Endosteum

10 - Diaphysis

11 - Nutrient artery

12 - Marrow

13 - Perforating (Sharpey’s)

fibers

Label the following

Turn in

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Bone Formation & Maintenance

� Ossification (osteogenesis)� Stages

� Bone formation� Begins in the 2nd month of development

� Postnatal bone growth� Until early adulthood

� Bone remodeling and repair� Lifelong

Types of Ossification

1. Endochondral ossification� Bone forms by replacing hyaline cartilage� Forms most of the skeleton

2. Intramembranous ossification� Bone develops from fibrous membrane� Forms flat bones

Endochondral Ossification

� Uses hyaline cartilage models � Requires breakdown of hyaline cartilage prior to ossification

Figure 6.9, step 1

Bone collar forms aroundhyaline cartilage model.

1

Hyaline cartilage

Week 9

Bone collar

Primaryossificationcenter

Figure 6.9, step 2

Cartilage in the center

of the diaphysis calcifiesand then develops cavities.

2

Area of deteriorating

cartilage matrix

Figure 6.9, step 3

The periosteal bud invadesthe internal cavities andspongy bone begins to form.

3

Spongyboneformation

Bloodvessel ofperiostealbud

Month 3

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Figure 6.9, step 4

The diaphysis elongates and a medullary cavity formsas ossification continues. Secondary ossification centersappear in the epiphyses in preparation for stage 5.

4

Epiphysealblood vessel

Secondaryossificationcenter

Medullarycavity

Birth

Figure 6.9, step 5

The epiphyses ossify. When completed, hyaline cartilage

remains only in the epiphyseal plates and articular cartilages.

5

Epiphyseal plate

cartilage

Articular cartilage

Childhood to adolescence

Spongy bone

Figure 6.9

Bone collar

forms around

hyaline cartilage

model.

Cartilage in the

center of the

diaphysis calcifies

and then develops

cavities.

The periosteal

bud inavades 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.

Hyaline

cartilage

Area of

deteriorating

cartilage matrix

Epiphyseal

blood vessel

Spongy

bone

formation

Epiphyseal

plate

cartilage

Secondary

ossification

center

Blood

vessel of

periosteal

bud

Medullary

cavity

Articular

cartilage

Childhood to

adolescence

BirthWeek 9 Month 3

Spongy

bone

Bone

collarPrimary

ossification

center

1 2 3 4 5

Figure 6.10

Calcified cartilage

spicule

Osseous tissue

(bone) coveringcartilage spicules

Resting zone

Osteoblast depositing

bone matrix

Proliferation zone

Cartilage cells undergo mitosis.

Hypertrophic zone

Older cartilage cells enlarge.

Ossification zone

New bone formation is occurring.

Calcification zone

Matrix becomes calcified; cartilage cells die; matrix

begins deteriorating.

1

2

3

4

Epiphysealgrowth plates

continue to produce

cartilaginouselongation as long

as bones are

increasing in length

Intramembranous Ossification

� Forms flat bones� Skull roof, lower jaw, clavicles

� Uses a fibrous membrane model formed from mesenchymal cells

Figure 6.8, (1 of 4)

Mesenchymal

cell

Collagen

fiberOssification

center

Osteoid

Osteoblast

Ossification centers appear in the fibrous

connective tissue membrane.

• Selected centrally located mesenchymal cells clusterand differentiate into osteoblasts, forming anossification center.

1

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Figure 6.8, (2 of 4)

Osteoid

Osteocyte

Newly calcified

bone matrix

Osteoblast

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.

2

Figure 6.8, (3 of 4)

Mesenchyme

condensingto form theperiosteum

Blood vessel

Trabeculae of

woven bone

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 called woven bone.

• Vascularized mesenchyme condenses on the external faceof the woven bone and becomes the periosteum.

3

Figure 6.8, (4 of 4)

Fibrous

periosteum

Osteoblast

Plate of

compact bone

Diploë (spongy

bone) cavitiescontain red

marrow

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 boneplates.

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

4

Bone Maintenance

� Bone is dynamic throughout human lifespan

� Remodeling has several functions� Replacement and repair

� Release of calcium

� Response to stress (modification of density)

Bone Remodeling� Deposit

� Where bone is injured or added strength is required

� Osteoblasts

� Resorption� Releases minerals from

bone

� Osteoclasts

Control of Remodeling

� Hormones� Growth Hormone

� Calcitonin

� Parathyroid Hormone (PTH)

� Sex hormones

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Hormonal Control of Blood Ca2+

↓ Blood Ca2+ levels

↓

Parathyroid glands release PTH

↓

PTH stimulates osteoclasts to degrade bone matrix and release Ca2+

↓

↑ Blood Ca2+ levels

Figure 6.12

Osteoclasts

degrade bonematrix and

release Ca2+

into blood.

Parathyroid

glands

Thyroid

gland

Parathyroid

glands releaseparathyroid

hormone (PTH).

StimulusFalling blood

Ca2+ levels

PTH

Calcium homeostasis of blood: 9–11 mg/100 mlBALANCEBALANCE

Hormonal Control of Blood Ca2+

↑ Blood Ca2+ levels↓

Parafollicular cells of thyroid gland release calcitonin↓

Osteoblasts deposit calcium salts ↓

↓ Blood Ca2+ levels

Bone Maintenance

� Bone deposition� Occurs where bone is injured or added strength is needed

� Development requires proper nutrition � Vitamins C, D, and A

� Calcium and phosphorus

Bone Disorders

� Poor nutrition

� Hormonal changes/imbalances

� Trauma

� Developmental errors

� Infection

� Tumors

Nutritional Disorders

� Rickets

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Nutritional Disorders

� Osteomalacia

Hormonal Disorders

� Osteoporosis� Loss of bone mass

� Bone resorption ˃ deposit� Vertebral bodies and neck of femur

Figure 6.16

Hormonal Disorders

� Osteoporosis� Risk factors

� Lack of estrogen� Low calcium or vitamin D� Petite body form� Immobility� Low levels of TSH� Diabetes mellitus

Hormonal Disorders

� Osteoporosis� Treatment and prevention

� Calcium, vitamin D, and fluoride supplements

� ↑Weight bearing exercise throughout life

� Hormone (estrogen) replacement therapy

� Controversial because of increased cardiovascular disease, cancer

� Drugs to increase bone mineral density

� Fosamax, selective estrogen receptor modulators (SERMs), statins (don’t work)

Infections� Osteomyelitis

� Inflammation of bone due to infection

(Not normal)

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Bone Fractures� Incomplete

� Does not cross entire bone

� Complete� Bone is broken into two pieces

� Comminuted� Three or more pieces

� Displaced� Bone ends don’t line up

� Open versus closed

Bone Fractures� Transverse

� Spiral

� Impacted

� Depressed

� Greenstick

Fractures

� Pathological� Secondary to coexisting disease

� Osteoporosis

� Cancer

� Malnutrition

� Cushing’s syndrome

� Osteogenesis imperfecta

Fracture Repair� Local periosteum and surrounding blood vessels are torn

� Inflammation� Open vs. closed

� Swelling� Hemorrhage (bleeding)

� Blood clots → fracture hematoma

Fracture Repair1. Fracture hematoma forms within 6-8 hrs

� Torn blood vessels hemorrhage� Clot (hematoma) forms

� Swollen, painful and inflamed

Figure 6.15, step 1

A hematoma forms.1

Hematoma

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Fracture Repair2. Fibrocartilaginous callus forms

� Capillaries grow into area

� Osteoblasts begin forming spongy bone within 1 week

� Fibroblasts secrete collagen fibers to connect bone ends

Figure 6.15, step 2

Fibrocartilaginouscallus forms.

2

Externalcallus

Newbloodvessels

Spongybonetrabecula

Internalcallus(soft granulationtissue)

Fracture Repair3. Bony callus formation

� New trabeculae form a bony (hard) callus

� Continues until firm union is formed in ~2 months

Figure 6.15, step 3

Bony callus forms.3

Bonycallus ofspongybone

Fracture Repair4. Bone remodeling

� In response to mechanical stressors over several months

� Final structure resembles original bone

Figure 6.15, step 4

Bone remodelingoccurs.

4

Healedfracture

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Figure 6.15

Hematoma External

callusBony

callus of

spongy

boneHealed

fractureNew

blood

vessels

Spongy

bone

trabecula

Internal

callus

(fibrous

tissue and

cartilage)

A hematoma forms. Fibrocartilaginous

callus forms.

Bony callus forms. Bone

remodeling

occurs.

1 2 3 4

Treatment of Fractures

� Reduction� Realignment

� Immobilization

� Traction

� Pathological fractures� Address underlying problem

Traction

• Uses gravity and/or tension

• Immobilizes• Reduces pain

• Maintains bone length• Does not realign facture

Developmental Errors

� Skull� Microcephalus

� Anterior fontanel closes early

Developmental Errors

� Skull� Hydrocephalus

� Excess CSF causes pressure

Developmental Errors

� Skull� Cleft palate

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Figure 7.16

Cervical curvature

(concave)

7 vertebrae, C1–C7

Thoracic

curvature

(convex)

12 vertebrae,T1–T12

Lumbar curvature

(concave)

5 vertebrae, L1–L5

Sacral curvature

(convex)

5 fused vertebrae sacrum

Coccyx

4 fused vertebrae

Anterior view Right lateral view

Spinous

process

Transverse

processes

Intervertebral

discs

Intervertebral

foramen

C1

Disorders

of the

Spinal

Column

Scoliosis

Lordosis Kyphosis

Spina Bifida

Vertebral

defect

Disorders of the Spinal Column� Herniated disc

� Causes and risk factors

� Location

� Treatment

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Figure 7.17c

Vertebral spinous process(posterior aspect of vertebra)

Spinal nerve root

Anulus fibrosusof disc

Herniated portionof disc

Nucleuspulposus

of disc

Spinal cord

(c) Superior view of a herniated intervertebral disc

Transverseprocess