The Skeletal
System
Functions of the Skeletal System
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•
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•
•
•
Support
Permission of movement
Blood cell formation (hematopoiesis)
Protection
Detoxification (removal of poisons)
Provision for muscle attachment
Mineral storage
Bone Classification
• Bones are classified according to shape
• Long bones are long with expanded ends,
Ex: forearm and thigh bone
• Short bones are cube like, Ex: wrist, ankle
• Flat bones are broad and plate like, Ex: ribs,
scapulae, and some skull bones
• Irregular bones vary in shape, Ex: vertebrae
• Sesamoid or round bones are small bones
embedded in tendons, Ex: kneecap (patella)
Parts of Bone
• Epiphysis: end of
the bone which
articulates (forms
a joint) with
another bone.
• The epiphyses
are composed of
spongy bone and
covered with
hyaline cartilage
called articular
cartilage.
Figure 7.2
Parts of Bone
• Diaphysis:
shaft of the
bone between
the epiphyses,
composed of
compact bone
with a
medullary
cavity in the
center.
Parts of Bone
• Periosteum:
fibrous tissue
covering of the
bone.
Compact Bone
• Osteocytes and layers of intercellular material lie
in concentric rings around an osteonic canal.
• This unit is called an osteon or Haversian
system.
• Osteonic canals contain blood vessels and nerve
fibers and are interconnected by transverse
perforating (Volkmann’s) canals.
Microscopic Structure of Bone:
Compact Bone
Figure 6.6a, b
Spongy Bone
• Osteocytes lie within trabeculae, branching
bony plates.
• Nutrients diffuse into canaliculi that lead to
the trabeculae.
Bone Growth and Development
• The skeletal system begins to form during
the first weeks of prenatal development.
• Some bones originate within sheets of
connective tissue (intramembranous bones).
• Some bones begin as models of hyaline
cartilage that are replaced by bone
(endochrondral bones).
Intramembranous Bones
• Broad, flat skull bones are intramembranous
bones.
• During osteogenesis layers of primitive,
connective tissue supplied with blood
vessels appear at the site of future bone.
Intramembranous Bones
• Cells differentiate into osteoblasts (bonebuilding cells) which deposit spongy bone.
• Osteoblasts become osteocytes when
surrounded by bony matrix in lacunae.
Intramembranous Bones
• Connective tissue on the surface of the bone
forms the periosteum.
• Osteoblasts on the inside of the periosteum
deposit compact bone over spongy bone.
• This process is called intramembranous
ossification.
Endochondral Bones
• Hyaline cartilage forms a model of the bone
during embryonic development.
• Cartilage degenerates, periosteum forms.
• Periosteal blood vessels and osteoblasts
invade the bone forming a primary
ossification center in the diaphysis.
• Secondary ossification centers develop in
the epiphyses.
Endochondral Bones
• Osteoblasts form spongy bone in the space
occupied by cartilage.
• Osteoblasts become osteocytes when bony
matrix surrounds them.
Endochondral Bones
• Osteoblasts beneath the periosteum deposit
compact bone around spongy bone.
• A band of cartilage remains between the
diaphysis and epiphyses as the epiphyseal
disk.
Bone Growth
• Growth of long bones occurs along four
layers of cartilage in the epiphyseal disk.
• First Layer: resting cells that do not grow.
• Second Layer: young mitotic cells.
• Third Layer: older cells that enlarge.
• Fourth Layer: dead cells and calcified
intercellular substances.
Formation of the Bony Skeleton
• Begins at week 8 of embryo development
• Intramembranous ossification – bone
develops from a fibrous membrane
• Endochondral ossification – bone forms by
replacing hyaline cartilage
Intramembranous Ossification
• Formation of most of the flat bones of the
skull and the clavicles
• Fibrous connective tissue membranes are
formed by mesenchymal cells
Stages of Intramembranous
Ossification
• An ossification center appears in the fibrous
connective tissue membrane
• Bone matrix is secreted within the fibrous
membrane
• Woven bone and periosteum form
• Bone collar of compact bone forms, and red
marrow appears
Stages of Intramembranous
Ossification
Figure 6.7.1
Stages of Intramembranous
Ossification
Figure 6.7.2
Stages of Intramembranous
Ossification
Figure 6.7.3
Stages of Intramembranous
Ossification
Figure 6.7.4
Endochondral Ossification
• Begins in the second month of development
• Uses hyaline cartilage “bones” as models
for bone construction
• Requires breakdown of hyaline cartilage
prior to ossification
Stages of Endochondral
Ossification
• Formation of bone collar
• Cavitation of the hyaline cartilage
• Invasion of internal cavities by the periosteal bud,
and spongy bone formation
• Formation of the medullary cavity; appearance of
secondary ossification centers in the epiphyses
• Ossification of the epiphyses, with hyaline
cartilage remaining only in the epiphyseal plates
Stages of Endochondral Ossification
Secondary
ossification
center
Epiphyseal
blood vessel
Deteriorating
cartilage matrix
Hyaline
cartilage
Spongy
bone
formation
Primary
ossification
center
Bone
collar
Articular
cartilage
Spongy
bone
Medullary
cavity
Epiphyseal
plate
cartilage
Blood
vessel of
periostea
l bud
1 Formation
of bone
collar
around
hyaline
cartilage
model.
2 Cavitation
of the
hyaline
cartilage
within the
cartilage
model.
3 Invasion of
internal cavities
by the
periosteal bud
and spongy
bone formation.
4 Formation of the
medullary cavity as
ossification continues;
appearance of
secondary ossification
centers in the
epiphyses in
preparation for stage 5.
5 Ossification of the
epiphyses; when
completed, hyaline
cartilage remains
only in the
epiphyseal plates
and articular
cartilages
Figure 6.8
Postnatal Bone Growth
• Growth in length of long bones
– Cartilage on the side of the epiphyseal plate
closest to the epiphysis is relatively inactive
– Cartilage abutting the shaft of the bone
organizes into a pattern that allows fast,
efficient growth
– Cells of the epiphyseal plate proximal to the
resting cartilage form three functionally
different zones: growth, transformation, and
osteogenic
Functional Zones in Long Bone
Growth
• Growth zone – cartilage cells undergo
mitosis, pushing the epiphysis away from
the diaphysis
• Transformation zone – older cells enlarge,
the matrix becomes calcified, cartilage cells
die, and the matrix begins to deteriorate
• Osteogenic zone – new bone formation
occurs
Long Bone Growth and
Remodeling
• Growth in length – cartilage continually
grows and is replaced by bone as shown
• Remodeling – bone is resorbed and added
by appositional growth as shown
Long Bone Growth and
Remodeling
Figure 6.10
Appositional Growth of Bone
Central canal of osteon
Periosteal ridge
Artery
Periosteum
1 Osteoblasts beneath
the periosteum
secrete bone matrix,
forming ridges that
follow the course of
periosteal blood
vessels.
Penetrating canal
2 As the bony ridges
enlarge and meet,
the groove
containing the
blood vessel
becomes a tunnel.
3 The periosteum
lining the tunnel is
transformed into an
endosteum and the
osteoblasts just
deep to the tunnel
endosteum secrete
bone matrix,
narrowing the canal.
4 As the osteoblasts
beneath the endosteum
form new lamellae, a new
osteon is created.
Meanwhile new
circumferential lamellae
are elaborated beneath
the periosteum and the
process is repeated,
continuing to enlarge
bone diameter.
Figure 6.11
Bone Remodeling
• Remodeling units – adjacent osteoblasts and
osteoclasts deposit and resorb bone at
periosteal and endosteal surfaces
Bone Homeostasis
• After bone formation, osteoclasts and
osteoblasts continue to remodel the bone.
• Resorption and deposition are regulated to
keep bone mass constant.
Nutrition and Bone Development
• Vitamin D is necessary to absorb calcium in
the small intestine.
• Vitamin D deficiency leads in rickets in
children and osteomalacia in adults.
Nutrition and Bone Development
• Vitamin A is necessary for osteoblast and
osteoclast activity.
• Vitamin C is necessary for collagen
synthesis.
Hormones and Bone
• Growth Hormone (GH) stimulates
epiphyseal cartilage cell division.
• Deficiency of G H: pituitary dwarfism.
Excess GH: pituitary gigantism in children
and acromegaly in adults.
Hormones and Bone
• Thyroid hormone stimulates cartilage
replacement in the epiphyseal disks.
• Sex steroids promote formation of bone
tissue close the epiphyseal disk.
Physical Factors Affecting Bone
• Physical stress stimulates bone growth.
• Weight bearing exercise stimulates bone
tissue to thicken and strengthen
(hypertrophy).
• Lack of exercise leads to bone wasting
(atrophy).
Bone Function
• Bones shape, support, and protect body
structures.
• Bones act as levers to create body
movement with muscles.
• Bones house blood cell producing tissue.
• Bones store inorganic salts.
Support and Protection
• Bones give shape to the head, face, chest,
and limbs.
• Bones of the skull protect structures like the
eyes, ears, and brain.
• Bones of the rib cage and shoulder protect
the heart and lungs.
• Bones of the pelvic girdle protect the
abdominal and reproductive organs.
Body Movement
• When body parts move, bones and muscle
act as levers.
Body Movement
• A lever has
four parts: a
bar, a fulcrum,
an object
moved, a force
to supply
energy.
• There are
three classes
of levers.
Figure 7.13
Body Movement
Body Movement
Body Movement
Figure 7.14
Body Movement
Fracture Repair
• Blood escapes from damaged blood vessels
and forms a hematoma.
• Spongy bone forms in regions near blood
vessels and fibrocartilage forms farther away.
• A bony callus replaces the fibrocartilage.
• Osteoclasts remove excess bony tissue,
restoring new bone much like the original.
Blood Cell Formation
• Blood cell formation (hematologists) occurs
in yolk sac in early development.
• Later it occurs in the liver and spleen.
• In the adult red and white blood cells are
formed in the red bone marrow.
Blood Cell Formation
• Red marrow fills the cavity in the diaphesis
of the long bones in infants. In adults it is
replaced with yellow marrow (fat).
• Adult red marrow is found in spongy bone
of the skull, ribs, sternum, vertebrae, pelvis.
Inorganic Salt Storage
• Salts account for 70% of the bone matrix.
• These salts are mostly calcium phosphate
crystals called hydroxyapatite.
Inorganic Salt Storage
• Parathyroid hormone stimulates osteoclasts
to break down bone when Ca levels are low.
• Calcitonin stimulates osteoblasts to build
bone when Ca levels are high.
• Bone contains Mg, Na, K, and carbonate
ions.
Axial Skeleton
• The skeleton has two divisions: the axial
and the appendicular skeleton.
• The axial skeleton consists bones that
support organs of the head, neck, and trunk.
• Skull :cranium and facial bones.
• Hyoid bone.
• Vertebral column.
• Thoracic cage: ribs and sternum.
Appendicular Skeleton
• The appendicular skeleton consists of the
bones of the limbs and bones that anchor
the limbs to the axial skeleton.
• Pectoral girdle: scapula, clavicle.
• Upper limbs: humerus, radium, ulna,
carpals, metacarpals, phalanges.
• Pelvic girdle: coxal bones.
• Lower limbs: femur, tibia, fibula, patella,
tarsals, metatarsals, phalanges.
Figure 7.17
Figure 7.17
Cranium
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•
Frontal bone: forehead
Parietal bones: top of the skull
Occipital bone: back of the skull
Temporal bones: side of skull, near ears
Sphenoid bone:base of the cranium
Ethmoid bone: roof of the nasal cavity
Figure 7.19
Figure 7.19
Figure 7.21
Figure 7.21
Figure 7.22
Figure 7.22
Facial Skeleton
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Maxillary bones: upper jaw, hard palate
Palatine bones: hard palate, nasal cavity
Zygomatic bones: cheek bones
Lacrimal bones: orbit of the eye
Nasal bones: bridge of the nose
Vomer bone: nasal septum
Nasal conchae:walls of the nasal cavity
Mandible: lower jaw
Figure 7.29
Figure 7.29
Figure 7.29
Infantile Skull
• The skull at birth is not fully developed.
• Fibrous membranes, fontanels, connect the
cranial bones.
• The fontanels allow movement of the bones
to enable the skull to pass through the birth
canal.
• The fontanels close as cranial bones grow.
Figure 7.33
Figure 7.33
Vertebral Column
• Cervical vertebrae: seven vertebrae of the
neck, includes atlas and axis
• Thoracic vertebrae: twelve vertebrae that
articulate with the ribs
• Lumbar vertebrae: five vertebrae that make
up the small of the back
Vertebral Column
• Sacrum: five
vertebrae that
fuse in early
adulthood, part
of the pelvis
• Coccyx: four
small fused
vertebrae
Figure 7.34
Thoracic Cage
• Ribs: twelve pair of ribs attached to each
thoracic vertebrae.
• Seven pairs: true ribs and attach to the
sternum by costal cartilage.
• Two pairs: false ribs that attach to cartilage.
Thoracic Cage
• Two pairs: floating ribs that do not attach to
the sternum or its cartilage.
• Sternum: the manubrium, the body, and the
xyphoid process.
Pectoral Girdle
• Clavicles: collar bones that attach the
sternum to the shoulder anteriorly.
• Scapulae: shoulder blades with two
processes.
• Acromion process: tip of the shoulder.
• Coracoid process: attaches to the clavicle
and provides attachments for muscles.
• Glenoid fossa articulates with the humerus.
Figure 7.42
Upper limb
• Humerus: upper arm bone, articulates with
the glenoid fossa of the scapula
Upper limb
• Radius: thumb side of the forearm,
articulates with the capitulum of the
humerus and the radial notch of the ulna
• Ulna: longer bone of the forearm, olecranon
and coronoid processes articulate with the
humerus
Hand
• Carpal bones: eight small bones of the
wrist.
• Metacarpal bones: five bones, the
framework of the palm.
• Phalanges: finger bones, three in each finger
(proximal, middle, distal phalanx), two in
the thumb.
Figure 7.47
Pelvic Girdle
• Coxal bones: two hips bones composed of
three fused bones.
• Ilium: superior part of the coxal bone.
• Ischium: lowest portion of the coxal bone.
• Pubis: anterior part of the coxal bone. The
two pubic bones joint at the symphysis
pubis.
Figure 7.49
Figure 7.49
Male and Female Pelvis
• Female iliac bones are more flared.
• The female pubic arch angle is greater.
• There is a greater distance between the
ischial spines and tuberosities in the female.
• The sacral curvature is shorter and flatter.
• The differences create a wider pelvic cavity.
Figure 7.51
Lower Limb
• Femur: thigh bone, longest bone
• Patella: kneecap, located in a tendon, femur,
tibia, and patella form the knee joint
• Tibia: shinbone, lateral malleolus forms the
ankle
• Fibula: slender bone lateral to the tibia, not
part of the knee joint
Figure 7.52
Foot
• Tarsal bones: seven small bones in the
ankle. The calcaneus (heel bone) is the
largest, located below the talus.
• Metatarsal bones: elongated bones that form
the arch of the foot.
• Phalanges: each toe has three except the
great tow which has two.
Figure 7.55
Life-Span Changes
• Calcium levels
fall through life
and the skeleton
loses strength.
• Osteoclasts
outnumber
osteoblasts.
Life-Span Changes
• By age 35,
everyone loses
bone mass.
Women lose bone
mass faster
between
menopause and
age seventy.
• Trabecular bone
is lost before
compact bone.