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Skeletal System
Chapter 5
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Skeletal System
• Two divisions:
• Axial Skeleton (“axis”)
• Head & trunk
• Appendicular Skeleton
(“appendages”)
• limbs & their attachments
to trunk
• Also includes:
• Joints
• Cartilages
• Ligaments
1. Support
Functions of Bones
• Internal framework, cradles soft organs
2. Protection
• i.e. skull, vertebrae, rib cage
3. Movement
• Used as levers by muscles
4. Storage
• Fat is stored in yellow marrow (internal cavities)
• Minerals stored in bone (calcium & phosphorus)
5. Blood Cell Formation
• Hematopoiesis occurs
in red marrow
Classification of Bone
• 206 bones in adult skeleton
• Two types of osseous tissue:
• Compact bone – dense, looks smooth & homogenous
• Spongy bone – small needlelike pieces of bone & lots of open space
Spongy &
Compact Bone
Bone
Classification of Bones
• Classified according to shape
• Long bones – longer than they are wide; contain
mostly compact bone
• i.e. Bones of the limbs
• Short bones – cube-shaped, contain mostly
spongy bone
• i.e. Bones of wrist and ankle, patella, sesamoid bones
(within tendons)
• Flat bones – thin, flattened, usually curved;
compact bone sandwiches layer of spongy bones
• i.e. Bones of the skull, ribs, sternum
• Irregular bones – do not fit into one of the
preceding categories
• i.e. Vertebrae and hip bones
Gross Anatomy
of a Long Bone
(Hyaline)
metaphysis
(disc)
(DCT)
Microscopic Anatomy of the Bone
• Osteocytes (mature bone cells)
• Lacunae (cavities in matrix)
• Lemellae (concentric circles
around central canal)
• Central (Haversian) canal (carry
blood vessels and nerves to all
areas of the bone)
• Osteon (Haversian System)
• Canaliculi (radiate outward from
central canal to all lacunae)
• Volkmann’s canal
(communication from exterior
to interior of bone)
Bone Tissue
• Connective Tissue: calcified matrix with
abundant collagen fibers
• Four types of cells:
• Osteogenic cells: unspecialized from stem cells;
found along periosteum and endosteum
• Make osteoblasts
• Osteoblast: bone-building cells – synthesize and
secrete collagen fibers
• Osteocyte: mature cells – maintain metabolism
• Osteoclasts: huge collection of
white blood cells found in endosteum
• release acids and enzymes that digest
bone matrix (resorption)
Bone Formation
• Ossification (osteogenesis): formation
of bone
• Begins in 6th week of pregnancy
• Two patterns:
• Intramembraneous ossification: flat bones
• i.e. skull & clavicles
• Endochondral ossification: most bones
• i.e. long bones
Intramembraneous Ossification
• Flat bones form on fibrous membranes
• Mesenchymal cells cluster and form
osteoblasts which harden
• Form ossification center in membrane
• Osteoblasts secrete bone matrix which
mineralizes and traps cells in bones
(become osteocytes)
• Trabeculae then form followed by the
periosteum forming around bone
• Trabeculae then thicken to form bone
collar and deeper remain distinct as
spongy bone and eventually red marrow
Endochondral Bone Development
• Most bones develop using a hyaline cartilage model
• Fetal skeleton = hyaline cartilage (formed from mesenchyme)
• Two phases (Fig 5.5, page 140):
1. hyaline cartilage model covered with bone matrix (bone “collar”) by
osteoblasts
•Primary ossification: develops inward from outer surface & forms spongy bone
(eventually compact bone)
•Secondary ossification: develops outward (from center of epiphysis)
2. enclosed hyaline cartilage model digested away, opening up
medullary cavity within newly formed bone
•Osteoclasts break down center to form medullary cavity
•By birth or shortly after, all cartilage converted to bone except articular
cartilages (cover bone ends) and epiphyseal plates
•Adult skeleton = cartilage exists in nose, parts of ribs, and joints
Bone Growth
• Bones increase in length (interstitial growth) &
width (appositional growth)
• Epiphyseal plate: layer of hyaline cartilage in
metaphysis of growing bone; new bone forms
on diaphysis side
• Interstitial growth (length): from epiphyseal
plate
• Four zones; close around 18-21 (18-females,
21-males)
1.
2.
3.
4.
Resting cartilage: nearest epiphysis, anchor
plate to bone (not part of growing bone)
Proliferating cartilage: cells divide & replace
dying cells
Hypertrophic cartilage: matruing
chondrocytes; arranged in columns
Calcified cartilage: cells harden & die;
replaced by bone tissue
•
Osteoclasts dissolve and osteoblasts
and blood vessels enter area
Interstitial Bone Growth
Appositional Bone Growth
• Growth in thickness of the bone
• Cells in periosteum differentiate into
osteoblasts while osteoclasts increase
medullary cavity
• Osteoblasts in periosteum add bone
tissue to external face of diaphysis as
osteoclasts in endosteum remove bone
from inner surface
• Occur at about the same rate
Skull or lady in the mirror?
Bone Remodeling
• Bone resorption & deposition used to
renew and replace injured bone
• Affected by minerals, vitamins, and
hormones
• Resorption: removal of minerals and
collagen fibers by osteoclasts
• Deposition: addition of minerals and
collagen fibers by osteoblasts
Bone Remodeling
• Bones are remodeled continually in response to changes in two factors:
1. Calcium levels in the blood
•
•
•
Parathyroid gland releases PTH when blood calcium levels drop
PTH activates osteoclasts to break down bone matrix and release Ca2+ into blood
Hypercalcemia: Ca2+ is deposited into bone matrix as hard calcium salts
2. Pull of gravity and muscles on the skeleton
•
•
•
•
•
Shape of bone altered for stress
Osteoblasts lay down new matrix & become trapped within it (become osteocytes) where
bulky muscles attach (due to stress)
Bedridden/inactive: lose mass & atrophy (no stress)
Ongoing replacement of old bone tissue by new bone tissue
PTH determines when/if bone broken down or formed in response to
need for more or fewer Ca2+ ions in the blood; stress determines where
bone matrix is to be broken down or formed
•
Helps maintain skeletal strength
Bone Fracture Repair
• Repair involves four major events:
1. Hematoma forms
•
From ruptured blood vessels, cells die
that are deprived of oxygen
2. Fibrocartilage callus forms
•
•
growth of new capillaries (granulation
tissue) into clotted blood at site of
damage and disposal of dead tissue by
phagocytes.
CT cells form mass of repair tissue which
splints broken bone and closes gap
(made up of bony matrix, collagen,
collagen matrix)
Bone Fracture Repair
3. Bony callus forms
•
Fibrocartilage callus is gradually
replaced by the bony callus (made of
spongy bone) as more osteoblasts and
osteoclasts migrate to area and
multiply
4. Bone remodeling occurs
•
Bony callus is remodeled in response
to mechanical stresses placed on it
Skeletal
System
Anatomy
Bone Markings
Can you see the hidden skull?
Skeletal
System
Anatomy
206 total bones
Axial Skeleton
• Three parts:
• Skull
• Vertebral column
• Thoracic cage
Skull: 22 bones
• Cranial (8), Facial (13),
Mandible (1)
• Protects brain
• Many paired bones
• Page 148-149
• Teeth: 32 in adult (not
part of 206 total)considered part of
digestive system
Skull
• Infants have fontanels
(soft spots)
• Bones of the skull are
not fused yet
• Adults: skull bones
fused with sutures
• Immoveable joints
• Mandible: freely
moving joint
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Middle Ear: 6 bones
• Three in each ear
• Stapes (stirrup)
• Incus (anvil)
• Malleus (hammer)
Hyoid Bone
• Does not articulate
with any other bone
• Often broken during
strangulation
• CT attaches to larynx &
trachea
Vertebral Column: 26 bones
• Strong, flexible, rotates
• Protects spinal cord &
supports head
• Intervertebral discs
between made of
fibrocartilage
• Shock absorption & spine
flexibility
• High in water content
when young (spongy,
compressible); discs harden
with age
Vertebral Column: 26 bones
• Born with 33 - fuse to 26
• 7 cervical (neck)- smaller, bifed clef
• C1: atlas (articulates with
occipital condyle)
• C2: axis
• 12 thoracic (chest)- stronger, long
spine
• 5 lumbar (lower back)- strongest,
short spine
• Sacrum (5 fused by mid 20s)
• Coccyx (4 fused by 30)- tail bone
Vertebral Column:
Figure 5-1
Curvatures of the Spine
• Four normal curvatures
• Fetus has 2 primary
curvatures (thoracic and
sacral)
• Secondary curvatures
develop after birth
• Cervical (3 months) and
lumbar (6 months)
• Cervical: concave (anterior)
• Thoracic: convex
(posterior)
• Lumbar: concave (anterior)
• Sacrum: convex (posterior)
• Exaggeration of the thoracic curve
• Tuberculosis of spine• vertebral bodies partially collapse
• Degeneration of discs
• Rickets (lack of vitamin C)
• Poor posture
• Advanced osteoporosis
• Exaggeration of the lumbar curve
•Exaggerated lumbar curve
• Result of increased weight gain
• Pregnancy or extreme
obesity
• Poor posture
• Rickets
• Tuberculosis of spine
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• Lateral bending of the vertebral column
• Lateral
curvature
• Congenital malformed vertebrae
(birth defect)
• Chronic sciatica (pain down leg)
• Paralysis of muscles on one side of
backbone
• Poor posture
• One leg shorter than other
Thoracic Cage: 25 Bones
• Sternum: fusion of 3 bones,
attached to first 7 ribs
• 12 pairs of ribs (1-7 increase in
size, 8-12 decrease in size)
• Attached by costal cartilage
• True ribs (1-7)- direct
attachment
• False ribs (8-12)- no anterior
attachment
• Floating ribs (11-12)- no
anterior attachment
• All have posterior attachment
to vertebrae
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Appendicular Skeleton
126 bones: limbs,
pectoral & pelvic
girdles (attach limbs to
axial skeleton)
Pectoral Girdle: 4 bones
• Attach arms; two of each
• Clavicle: collar bones
• s-shaped- weak juncture
• Attaches to manubrium and
sternum medially and scapula
posteriorly to form shoulder
joint
• Prevents dislocation
• Scapula: shoulder bones
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Upper Limbs: 60 bones
• 8 carpals each
• 5 metacarpals
• 14 phalanges
• 2 in thumb
• Radius (thumb side)
• Ulna (pinky side)
1. Clavicle
1a. Sternal end
1b. Acromial end
2. scapula
2a. Coracoid process
2b. Acromion
2c. Subscapular fossa
2d. Infraspinous fossa
2e. Spine of scapula
2f. Supraspious fossa
2g. Glenoid cavity
3. Acromioclavicular
4. Glenohumeral joint
5. Humerus
5b. Deltoid tuberosity
5c. Head
5d. Surgical neck
5e. Anatomical neck
5f. Greater tubercle
5g. Lateral epicondyle
5h. Medial epicondyle
5i. Capitulum
5j. Trochlea
5k. Coronoid fossa
6. Ulna
6a. Olecranon
7. Radius
1. Phalanges
1a. Distal
1b. Medial
1c. Proximal
1d. Base
1e. Head
2 Interphalangeal joints
3 Sesamoid bone
4 Metacarpophalangeal joints
5 Intermetacarpal joints
6. Metacarpals
6a. Base
6b. shaft
6c. Head
7. Carpometacarpal joint
8. Trapezium
9. Trapezoid
10. Capitate
11. Hamate
12. Pisiform
13. Triquetrum
14. Lunate
15. Scaphoid
16. Midcarpal join
• median nerve which runs
through wrist to hand
becomes compressed
• involves narrow bony passage
in wrist called carpal tunnel,
through which nine tendons
and median nerve must pass
Pelvic Girdle: 2 bones
• Function: bearing weight (total
weight of upper body) & protects
reproductive and urinary systems
• United at pubic symphysis
(fibrocartilage; some flexibility)
• Attached to axial skeleton via sacral
attachment to lower lumbar
vertebrae
• Three parts fuse at birth
• Ilium
• Pubis
• Ischium
•Femur, tibia,
fibula, patella
•7 tarsals, 5
metatarsals, 14
phalanges
•Carry our total
body weight when
erect
•much
thicker/stronger
than upper limbs
Lower Limbs: 60 bones
1. Hip joint
2.Femur
2a. Head
2b. Neck
2c. Greater trochanter
2d Lesse trochanter
2e. Lateral epicondyle
2f. Medial epicondyle
2g. Adductor tubercle
2h. Medial condyle
2i. Lateral condyle
2j. Intercondylar fossa
2k. Linea aspera
3. Patella
4. Knee joint
5. Tibia
5a. Lateral condyle
5b. Medial condyle
5c. Tibial tuberosity
5d. Medial malleolus
6. Fibula
6a. Head
6b. Crest
6c. Lateral malleolus
7. Ankle joint
8 Talus
9. Calconeus
• Bones composing arch of foot held by ligaments and
tendons
• Weakened ligaments and tendons causes arch to “fall”
• Caused by excessive weight, postural abnormalities,
weakened tissue, and genetic disposition
Name those bones
Male vs. Female
• Males bones larger and
heavier
• Angle of pubic symphysis less
for men
• Page 163 lists differences
Fetal Skeleton
• Arises from
mesenchymal cells
(derived from
mesoderm)
• Intramembraneous and
endochondral
ossification
• 1st long bones = hyaline
cartilage
• 1st flat bones of skull =
fibrous membranes
• 20 million people suffer from it (250,000 hip fractures a year)
• Decreased bone mass
• Increased susceptibility to fracture
• Hormones effect osteoblast production
• Afflicts entire skeletal system
• Body build (shorter females at greater risk)
• Weight (adipose produces estrone)
• Smoking (decreases estrogen levels)
• Calcium deficiency or malabsorption
• Vitamin D deficiency
• Exercise (sedentary more at risk)
• Certain drugs (alcohol, diuretics, cortisone, tetracycline)
• Premature menopause
• Family history
• Accelerated remodeling of
bone tissue
• Osteoclast resorption
massive
• Osteoblast formation
extensive
• Irregular thickening and
softening of bones
• Greatly increased vascularity,
especially in skull, pelvis, and
extremities
Testicular tumor “face”