CHAPTER 5

Skeletal System

The Skeletal System

Parts of the skeletal system Bones (skeleton) Joints Cartilages Ligaments

Two subdivisions of the skeleton Axial skeleton Appendicular skeleton

Functions of Bones

Support the bodyProtect soft organsActs as a lever for attached skeletal musclesStore minerals and fatsBlood cell formation (hematopoiesis)

Bones of the Human Body

The adult skeleton has 206 bonesTwo basic types of bone tissue

Compact bone Spongy bone

Figure 5.3a

Distalepiphysis

Diaphysis

Proximalepiphysis

Articularcartilage

Spongy boneEpiphyseallinePeriosteumCompact boneMedullarycavity (linedby endosteum)

(a)

Anatomy of a Long Bone

Diaphysis Shaft Composed of compact bone

Epiphysis Ends of the bone Composed mostly of spongy bone

Figure 5.3c

Yellowbone marrow

Compact bone

Perforating(Sharpey’s)fibers

Nutrientarteries

Periosteum

Endosteum

(c)

Anatomy of a Long Bone

Periosteum Fibrous connective tissue membrane, covers

outside of the diaphysisPerforating (Sharpey’s) fibersHyaline cartilage lines joint surfacesArteries

Anatomy of a Long Bone

Epiphyseal plate (Young) Flat plate of hyaline

cartilage Epiphyseal line

(Adults) Remnant of the

epiphyseal plate

Distalepiphysis

Diaphysis

Proximalepiphysis

Articularcartilage

Spongy boneEpiphyseallinePeriosteumCompact boneMedullarycavity (linedby endosteum)

(a)

Anatomy of a Long Bone

Marrow (medullary) cavity Contains yellow marrow (mostly fat) in adults Infants: red marrow for blood cell formation

In adults: red marrow is cavities of spongy bone

Microscopic Anatomy of Compact Bone

Osteon (Haversian system) Central canal and matrix rings Carries blood vessels and nerves

Perforating (Volkmann’s) canal Carries blood vessels and nerves

Figure 5.4a

CompactbonePeriostealblood vesselPeriosteum

Perforatingfibers

Central (Haversian) canalPerforating(Volkmann’s) canalBlood vessel

Spongy bone

Blood vessel continues intomedullary cavity containing marrow

Lamellae

(a)

Osteon(Haversian system)

Microscopic Anatomy of Bone

Lacunae Cavities containing bone cells (osteocytes) Arranged in concentric rings called lamellae

Lamellae Calcified cartilage surrounding lacunae

Formation of the Human Skeleton

In embryos, the skeleton is primarily hyaline cartilage

During development, much of this cartilage is replaced by bone

Cartilage remains in isolated areas Bridge of the nose Parts of ribs Joints

Bone Growth (Ossification)

Epiphyseal plates allow for lengthwise growth of long bones during childhood New cartilage is continuously formed Older cartilage becomes ossified

Cartilage is broken down Enclosed cartilage is digested away, opening up a

medullary cavity Bone replaces cartilage through the action of osteoblasts

Bone Growth (Ossification)

Bones are remodeled and lengthened until growth stops Bones are remodeled in response to two factors

Blood calcium levels Pull of gravity and muscles on the skeleton

Bones grow in width (called appositional growth)

Figure 5.5

In a fetusIn an embryo

Bone collarHyalinecartilagemodel

Bone startingto replacecartilage

In a child

Medullarycavity

New center ofbone growth

Hyalinecartilage

Epiphysealplate cartilage

Growthin bonelength

New boneforming

Invadingbloodvessels

Epiphysealplatecartilage

Articularcartilage

Spongybone

New boneforming

Growthin bonewidth

Figure 5.6

Bone growth

Bone grows inlength because:

Bone remodeling

Growing shaft isremodeled as:

Cartilagegrows here.

Cartilageis replacedby bone here.

Cartilagegrows here.

Cartilageis replaced by bone here.

1

2

3

4

1

2

3 Bone isresorbed here.

Epiphyseal plate

Articular cartilage

Bone isresorbed here.

Bone is addedby appositionalgrowth here.

Bone Remodeling• Osteocytes are mature bone cells

• Osteoclasts digest organic matric in response to PTH

• Increases blood calcium levels

• Osteoblasts form organic matric in response to calcitonin

• Decrease blood calcium levels

• Mechanical and gravitational forces also affect remodeling

17

Bone Remodeling

• Balance between resorption and formation

18

Bone Fractures

Fracture—break in a boneTypes of bone fractures

Closed (simple) fracture—break that does not penetrate the skin

Open (compound) fracture—broken bone penetrates through the skin

Common Types of Fractures

Comminuted—bone breaks into many fragments

Compression—bone is crushedDepressed—broken bone portion is pressed

inwardImpacted—broken bone ends are forced into

each otherSpiral—ragged break occurs when excessive

twisting forces are applied to a boneGreenstick—bone breaks incompletely

Figure 5.7

Internalcallus(fibroustissue andcartilage)

Hematomaforms.

Fibrocartilage callus forms.

Bony callus forms.

Bone remodeling occurs.

1 2 3 4

HematomaBonycallus ofspongybone

Spongybonetrabecula

Newbloodvessels

Externalcallus

Healedfracture

Inflammatory Conditions Associated with Joints

Bursitis—inflammation of a bursa usually caused by a blow or friction

Tendonitis—inflammation of tendon sheathsArthritis—inflammatory or degenerative

diseases of joints Over 100 different types Autoimmune disease

Clinical Forms of Arthritis

Osteoarthritis Most common chronic arthritis, related to normal

aging processesRheumatoid arthritis

An autoimmune disease—the immune system attacks the joints

Gouty arthritis Deposition of uric acid crystals from the blood

Figure 5.33