The Skeletal System

5The 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 body• Protect soft organs– Skull and vertebrae for brain and spinal cord– Rib cage for thoracic cavity organs

• Allow movement due to attached skeletal muscles• Store minerals and fats– Calcium and phosphorus– Fat in the internal marrow cavity

• Blood cell formation (hematopoiesis)

Bones of the Human Body

• The adult skeleton has 206 bones• Two basic types of bone tissue– Compact bone• Homogeneous

– Spongy bone• Small needle-like pieces of bone• Many open spaces

Figure 5.1

Spongybone

Compactbone

Classification of Bones on the Basis of Shape

• Bones are classified as:– Long– Short– Flat– Irregular

Figure 5.2

Classification of Bones

• Long bones– Typically longer than they are wide– Shaft with heads situated at both ends– Contain mostly compact bone– All of the bones of the limbs (except wrist, ankle,

and kneecap bones)– Example:• Femur• Humerus

Figure 5.2a


• Short bones– Generally cube-shaped– Contain mostly spongy bone– Includes bones of the wrist and ankle– Sesamoid bones are a type of short bone which

form within tendons (patella)– Example:• Carpals• Tarsals

Figure 5.2d


• Flat bones– Thin, flattened, and usually curved– Two thin layers of compact bone surround a layer

of spongy bone– Example: • Skull• Ribs• Sternum

Figure 5.1

Spongybone

Compactbone

Figure 5.2c


• Irregular bones– Irregular shape– Do not fit into other bone classification categories– Example:• Vertebrae • Hip bones

Figure 5.2b

Anatomy of a Long Bone

• Diaphysis– Shaft– Composed of compact bone

• Epiphysis – Ends of the bone– Composed mostly of spongy bone

Figure 5.3a

Distalepiphysis

Diaphysis

Proximalepiphysis

Articularcartilage

Spongy boneEpiphyseallinePeriosteumCompact boneMedullarycavity (linedby endosteum)

(a)


• Periosteum– Outside covering of the diaphysis– Fibrous connective tissue membrane

• Perforating (Sharpey’s) fibers– Secure periosteum to underlying bone

• Arteries– Supply bone cells with nutrients

Figure 5.3c

Yellowbone marrowCompact bone

Perforating(Sharpey’s)fibersNutrientarteries

Periosteum

Endosteum

(c)


• Articular cartilage– Covers the external surface of the epiphyses– Made of hyaline cartilage– Decreases friction at joint surfaces

Figure 5.3b

Compact bone

Spongy bone

Articularcartilage

(b)


• Epiphyseal plate– Flat plate of hyaline cartilage seen in young,

growing bone• Epiphyseal line– Remnant of the epiphyseal plate– Seen in adult bones

Figure 5.3a

Distalepiphysis

Diaphysis

Proximalepiphysis

Articularcartilage


(a)


• Marrow (medullary) cavity – Cavity inside of the shaft– Contains yellow marrow (mostly fat) in adults– Contains red marrow for blood cell formation in

infants• In adults, red marrow is situated in cavities of

spongy bone and epiphyses of some long bones

Figure 5.3a

Distalepiphysis

Diaphysis

Proximalepiphysis

Articularcartilage


(a)

Bone Markings

• Surface features of bones– Sites of attachments for muscles, tendons, and

ligaments– Passages for nerves and blood vessels

• Categories of bone markings– Projections or processes—grow out from the bone

surface• Terms often begin with “T”

– Depressions or cavities—indentations• Terms often begin with “F”

Microscopic Anatomy of Compact Bone

• Osteon (Haversian system)– A unit of bone containing central canal and matrix

rings• Central (Haversian) canal– Opening in the center of an osteon– Carries blood vessels and nerves

• Perforating (Volkmann’s) canal– Canal perpendicular to the central 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– Rings around the central canal– Sites of lacunae

Figure 5.4b

Lamella

CanaliculusLacunaCentral (Haversian) canal

(b)

Osteocyte

Figure 5.4c

Osteon

Lacuna

Centralcanal

Interstitiallamellae

(c)

Microscopic Anatomy of Bone

• Canaliculi – Tiny canals– Radiate from the central canal to lacunae– Form a transport system connecting all bone cells

to a nutrient supply

Figure 5.4b

Lamella

CanaliculusLacunaCentral (Haversian) canal

(b)

Osteocyte

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 5.5, step 1

In an embryo

Bone collarHyalinecartilagemodel

Bone startingto replacecartilage

Figure 5.5, step 2

In a fetus

Medullarycavity

New center ofbone growth

Hyalinecartilage

Growthin bonelength


Figure 5.5, step 3

In a child

Epiphysealplate cartilage

New boneforming


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.

Types of Bone Cells

• Osteocytes—mature bone cells• Osteoblasts—bone-forming cells• Osteoclasts—giant bone-destroying cells– Break down bone matrix for remodeling and

release of calcium in response to parathyroid hormone

• Bone remodeling is performed by both osteoblasts and osteoclasts

Bone Fractures

• Fracture—break in a bone• Types of bone fractures– Closed (simple) fracture—break that does not

penetrate the skin– Open (compound) fracture—broken bone

penetrates through the skin• Bone fractures are treated by reduction and

immobilization

Common Types of Fractures

• Comminuted—bone breaks into many fragments

• Compression—bone is crushed• Depressed—broken bone portion is pressed

inward• Impacted—broken bone ends are forced into

each other• Spiral—ragged break occurs when excessive

twisting forces are applied to a bone• Greenstick—bone breaks incompletely

Repair of Bone Fractures

• Hematoma (blood-filled swelling) is formed• Break is splinted by fibrocartilage to form a

callus• Fibrocartilage callus is replaced by a bony

callus• Bony callus is remodeled to form a permanent

patch

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

Figure 5.7, step 1

Hematomaforms.

Hematoma

1

Figure 5.7, step 2


Hematomaforms.


Hematoma

Spongybonetrabecula

Newbloodvessels

Externalcallus

1 2

Figure 5.7, step 3


Hematomaforms.


Bony callus forms.


Spongybonetrabecula

Newbloodvessels

Externalcallus

1 2 3

Figure 5.7, step 4


Hematomaforms.


Bony callus forms.

Bone remodeling occurs.


Spongybonetrabecula

Newbloodvessels

Externalcallus

Healedfracture

1 2 3 4

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The Skeletal System