5
The 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
Spongy
bone
Compact
bone
Figure 5.1
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
Classification of Bones
• 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
Classification of Bones
• Flat bones
– Thin, flattened, and usually curved
– Two thin layers of compact bone surround a layer
of spongy bone
– Example:
• Skull
• Ribs
• Sternum
Spongy
bone
Compact
bone
Figure 5.1
Figure 5.2c
Classification of Bones
• 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
Articular
cartilage
Proximal
epiphysis
Diaphysis
Spongy bone
Epiphyseal
line
Periosteum
Compact bone
Medullary
cavity (lined
by endosteum)
Distal
epiphysis
(a)
Figure 5.3a
Anatomy of a Long Bone
• 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
Endosteum
Yellow
bone marrow
Compact bone
Periosteum
Perforating
(Sharpey’s)
fibers
Nutrient
arteries
(c)
Figure 5.3c
Anatomy of a Long Bone
• Articular cartilage
– Covers the external surface of the epiphyses
– Made of hyaline cartilage
– Decreases friction at joint surfaces
Articular
cartilage
Compact bone
Spongy bone
(b)
Figure 5.3b
Anatomy of a Long Bone
• Epiphyseal plate
– Flat plate of hyaline cartilage seen in young,
growing bone
• Epiphyseal line
– Remnant of the epiphyseal plate
– Seen in adult bones
Articular
cartilage
Proximal
epiphysis
Diaphysis
Spongy bone
Epiphyseal
line
Periosteum
Compact bone
Medullary
cavity (lined
by endosteum)
Distal
epiphysis
(a)
Figure 5.3a
Anatomy of a Long Bone
• 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
Articular
cartilage
Proximal
epiphysis
Diaphysis
Spongy bone
Epiphyseal
line
Periosteum
Compact bone
Medullary
cavity (lined
by endosteum)
Distal
epiphysis
(a)
Figure 5.3a
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”
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
Osteon
(Haversian system)
Lamellae
Blood vessel continues into
medullary cavity containing marrow
Spongy bone
Perforating
fibers
Compact
bone
Periosteal
blood vessel
Periosteum
(a)
Central (Haversian) canal
Perforating
(Volkmann’s) canal
Blood vessel
Figure 5.4a
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
Lamella
Osteocyte
(b)
Canaliculus
Lacuna
Central (Haversian) canal
Figure 5.4b
Osteon
Lacuna
(c) Central
canal
Interstitial
lamellae Figure 5.4c
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
Lamella
Osteocyte
(b)
Canaliculus
Lacuna
Central (Haversian) canal
Figure 5.4b
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)
Articular
cartilage
Hyaline
cartilage
Spongy
bone
New center of
bone growth
New bone
forming
Epiphyseal
plate
cartilage
Growth
in bone
width
Medullary
cavity
Bone starting
to replace
cartilage
Growth
in bone
length
New bone
forming
Bone collar
Hyaline
cartilage
model
In an embryo
Invading
blood
vessels
Epiphyseal
plate cartilage
In a fetus
In a child
Figure 5.5
Bone starting
to replace
cartilage
Bone collar
Hyaline
cartilage
model
In an embryo
Figure 5.5, step 1
Hyaline
cartilage
New center of
bone growth
Medullary
cavity
Invading
Growth blood
in bone vessels
length
In a fetus
Figure 5.5, step 2
Articular
cartilage
Spongy
bone
New bone
forming
Epiphyseal
plate
cartilage
Growth
in bone
width
Invading
blood
vessels
New bone
forming
Epiphyseal
plate cartilage
In a child
Figure 5.5, step 3
Bone growth
Bone grows in
length because:
1 Cartilage
grows here.
2 Cartilage
is replaced
by bone here.
3 Cartilage
grows here.
4 Cartilage
is replaced by
bone here.
Bone remodeling
Growing shaft is
remodeled as:
Articular cartilage
Epiphyseal plate
1 Bone is
resorbed here.
2 Bone is added
by appositional
growth here.
3 Bone is
resorbed here.
Figure 5.6
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
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
Hematoma
External
callus
New
blood
vessels
Internal
callus
(fibrous
tissue and
cartilage)
1 Hematoma
forms.
Bony
callus of
spongy
bone
Healed
fracture
Spongy
bone
trabecula
2 Fibrocartilage
callus forms.
3 Bony callus
forms.
4 Bone remodeling
occurs.
Figure 5.7
Hematoma
1 Hematoma
forms.
Figure 5.7, step 1
Hematoma
External
callus
New
blood
vessels
Internal
callus
(fibrous
tissue and
cartilage)
1 Hematoma
forms.
Spongy
bone
trabecula
2 Fibrocartilage
callus forms.
Figure 5.7, step 2
Hematoma
External
callus
New
blood
vessels
Internal
callus
(fibrous
tissue and
cartilage)
1 Hematoma
forms.
Bony
callus of
spongy
bone
Spongy
bone
trabecula
2 Fibrocartilage
callus forms.
3 Bony callus
forms.
Figure 5.7, step 3
Hematoma
External
callus
New
blood
vessels
Internal
callus
(fibrous
tissue and
cartilage)
1 Hematoma
forms.
Bony
callus of
spongy
bone
Healed
fracture
Spongy
bone
trabecula
2 Fibrocartilage
callus forms.
3 Bony callus
forms.
4 Bone remodeling
occurs.
Figure 5.7, step 4