Lecture 13
Bones
Function of Bones
 Support – form the framework that supports the body and
cradles soft organs
 Protection – provide a protective case for the brain, spinal
cord, and vital organs
 Movement – provide levers for muscles
 Mineral storage – reservoir for minerals, especially
calcium and phosphorus
 Blood cell formation – hematopoiesis occurs within the
marrow cavities of bones
Bones and Cartilages of the Human Body
Composition of Bone
Organic
 Osteoblasts – boneforming cells
 Osteocytes – mature
bone cells
 Osteoclasts – large cells
that resorb or break down
bone matrix
Inorganic
 Mineral salts
 Sixty-five percent of
bone by mass
 Mainly calcium
phosphates
 Responsible for bone
hardness and its
resistance to
compression
Gross Anatomy of Bones: Bone Textures
 Compact bone
dense outer layer
 Spongy bone
honeycomb of trabeculae
filled with yellow bone
marrow
Microscopic Structure of Compact Bone
 Haversian system, or
osteon:
 The structural unit of
compact bone
 Lamella: weight-bearing,
column-like matrix tubes
composed mainly of
collagen
 Haversian, or central
canal: central channel
containing blood vessels
and nerves
Structure of Long Bone
 Long bones consist of a
diaphysis and an epiphysis
 Diaphysis
 Tubular shaft that forms the
axis of long bones
 Composed of compact bone
that surrounds the medullary
cavity
 Yellow bone marrow (fat) is
contained in the medullary
cavity
 Epiphyses
 Expanded ends of long bones
 Exterior is compact bone, and
the interior is spongy bone
 Joint surface is covered with
articular (hyaline) cartilage
 Epiphyseal line separates the
diaphysis from the epiphyses
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
Two control loops regulate bone remodeling
Hormonal mechanism maintains calcium homeostasis in the blood
Mechanical and gravitational forces acting on the skeleton
Structure of Short, Irregular, and Flat Bones
 Thin plates of compact
bone on the outside with
spongy bone (diploë) on
the inside
 Have no diaphysis or
epiphyses
 Contain Red bone marrow
between the trabeculae
Location of Hematopoietic Tissue (Red Marrow)
 In infants
 Found in the medullary cavity and all areas of spongy bone
 In adults
 Found in the diploë of flat bones, and the head of the femur and humerus
Importance of Ionic Calcium in the Body
 Calcium is necessary for:
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Transmission of nerve impulses
Muscle contraction
Blood coagulation
Secretion by glands and nerve cells
Cell division
Hormonal Mechanism
 Rising blood Ca2+ levels
trigger the thyroid to
release calcitonin
 Calcitonin stimulates
calcium salt deposit in
bone
 Falling blood Ca2+ levels
signal the parathyroid
glands to release PTH
 PTH signals osteoclasts
to degrade bone matrix
and release Ca2+ into the
blood
Response to Mechanical Stress
 Wolff’s law: a bone grows or
remodels in response to the forces
or demands placed upon it
 Observations supporting Wolff’s law
include
 Long bones are thickest midway
along the shaft (where bending
stress is greatest)
 Curved bones are thickest where
they are most likely to buckle
 Trabeculae form along lines of
stress
 Large, bony projections occur where
heavy, active muscles attach
Common Types of Fractures
Homeostatic Imbalances
 Osteomalacia
 Bones are inadequately mineralized causing softened, weakened
bones
 Main symptom is pain when weight is put on the affected bone
 Caused by insufficient calcium in the diet, or by vitamin D deficiency
 Rickets
 Bones of children are inadequately mineralized causing softened,
weakened bones
 Bowed legs and deformities of the pelvis, skull, and rib cage are
common
 Caused by insufficient calcium in the diet, or by vitamin D deficiency
Osteoporosis
 Group of diseases in which bone
reabsorption outpaces bone deposit
 Spongy bone of the spine is most
vulnerable
 Occurs most often in postmenopausal
women
 Bones become so fragile that
sneezing or stepping off a curb can
cause fractures
 A single gene that codes for vitamin D
docking determines
 The tendency to accumulate bone
mass early in life
 The risk for osteoporosis later in life
 Treatments
 Calcium and vitamin D supplements
 Increased weight-bearing exercise
 Hormone (estrogen) replacement
therapy (HRT) slows bone loss
 Natural progesterone cream prompts
new bone growth
 Statins increase bone mineral density
Equal osteoblast
and osteoclast
activities
Excess osteoclast
activity
Paget’s Disease
 Characterized by excessive bone formation and breakdown
 Pagetic bone with an excessively high ratio of woven to
compact bone is formed
 Pagetic bone, along with reduced mineralization, causes
spotty weakening of bone
 Osteoclast activity wanes, but osteoblast activity continues
to work
 Usually localized in the spine, pelvis, femur, and skull
 Unknown cause (possibly viral)
 Treatment includes the drugs Didronate and Fosamax
Developmental Aspects of Bones
 Mesoderm gives rise to embryonic mesenchymal cells, which produce
membranes and cartilages that form the embryonic skeleton
 The embryonic skeleton ossifies in a predictable timetable that allows
fetal age to be easily determined from sonograms
 At birth, most long bones are well ossified (except for their epiphyses)
 During infancy and childhood, epiphyseal plate activity is stimulated by
growth hormone
 During puberty, testosterone and estrogens:
 Initially promote adolescent growth spurts
 Cause masculinization and feminization of specific parts of the skeleton
 Later induce epiphyseal plate closure, ending longitudinal bone growth
 By age 25, nearly all bones are completely ossified
 In old age, bone resorption predominates
Axial and Appendicular Skeletons
 Axial skeleton – bones of
the skull, vertebral
column, and rib cage
 Appendicular skeleton –
bones of the upper and
lower limbs, shoulder, and
hip
The Axial Skeleton
 Forms the longitudinal axis of
the body
 80 bones
 Skull:
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8 cranial bones
14 facial bones
6 auditory ossicles
Hyoid bone
 Vertebral column:
 24 vertebrae
 Sacrum
 Coccyx
 Thoracic cage:
 24 ribs
 Sternum
 Supports and protects organs in body cavities
 Attaches to muscles of:
 Head, neck, and trunk
 Respiration
 Appendicular skeleton
The Skull
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The skull protects:
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22 bones:
 The brain
 Entrances to
respiratory system
 Entrance to
digestive system
 8 cranial bones:
 Form the
braincase or
cranium
 14 facial bones:
 Separate the
oral and nasal
cavities
 Form the nasal
septum
 Sinuses
 Cavities which decrease the weight of the skull:
 lined with mucus membranes
 protect the entrances of the respiratory system
The Vertebral Column
 Formed from 26 irregular bones
(vertebrae) connected in such a way
that a flexible curved structure results
 Cervical vertebrae – 7 bones of the
neck
 Thoracic vertebrae – 12 bones of the
torso
 Lumbar vertebrae – 5 bones of the
lower back
 Sacrum – bone inferior to the lumbar
vertebrae that articulates with the hip
bones
 Posteriorly concave curvatures –
cervical and lumbar
 Posteriorly convex curvatures –
thoracic and sacral
Abnormal spine curvatures
 Scoliosis (abnormal
lateral curve),
 Kyphosis
(hunchback), and
 Lordosis
(swayback)
Damage to Intervertebral Discs
 Slipped disc:
 bulge in anulus fibrosus
 invades vertebral canal
 Herniated disc:
 nucleus pulposus breaks through anulus fibrosus
 presses on spinal cord or nerves
The Appendicular Skeleton
Synovial Joints: General Structure
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Synovial joints all
have the following
1. Articular cartilage
2. Joint (synovial)
cavity
3. Articular capsule
4. Synovial fluid
5. Reinforcing
ligaments
 Joints in which the articulating bones are separated by a fluidcontaining joint cavity
 All are freely movable diarthroses
 Examples – all limb joints, and most joints of the body
Injuries
Sprains
 The ligaments reinforcing a joint are stretched or torn
 Partially torn ligaments slowly repair themselves
 Completely torn ligaments require prompt surgical repair
Cartilage Injuries
 The snap and pop of overstressed cartilage
 Common aerobics injury
 Repaired with arthroscopic surgery
Dislocations
 Occur when bones are forced out of alignment
 Usually accompanied by sprains, inflammation, and joint immobilization
 Caused by serious falls and are common sports injuries
 Subluxation – partial dislocation of a joint
Inflammatory and Degenerative Conditions
 Bursitis
 An inflammation of a bursa, usually caused by a blow or friction
 Symptoms are pain and swelling
 Treated with anti-inflammatory drugs; excessive fluid may be
aspirated
 Tendonitis
 Inflammation of tendon sheaths typically caused by overuse
 Symptoms and treatment are similar to bursitis
 Arthritis
 More than 100 different types of inflammatory or degenerative
diseases that damage the joints
 Most widespread crippling disease in the U.S.
 Symptoms – pain, stiffness, and swelling of a joint
 Acute forms are caused by bacteria and are treated with antibiotics
 Chronic forms include osteoarthritis, rheumatoid arthritis, and gouty
arthritis
Osteoarthritis (OA)
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Most common chronic arthritis; often called “wear-and-tear” arthritis
Affects women more than men
85% of all Americans develop OA
More prevalent in the aged, and is probably related to the normal aging process
Course
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OA reflects the years of abrasion and compression causing increased
production of metalloproteinase enzymes that break down cartilage
As one ages, cartilage is destroyed more quickly than it is replaced
The exposed bone ends thicken, enlarge, form bone spurs, and restrict
movement
Joints most affected are the cervical and lumbar spine, fingers, knuckles,
knees, and hips
Treatment
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OA is slow and irreversible
Treatments include:
 Mild pain relievers, along with moderate activity
 Magnetic therapy
 Glucosamine sulfate decreases pain and inflammation
Rheumatoid Arthritis (RA)
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Chronic, inflammatory, autoimmune disease of unknown cause, with an
insidious onset
Usually arises between the ages of 40 to 50, but may occur at any age
Signs and symptoms include joint tenderness, anemia, osteoporosis, muscle
atrophy, and cardiovascular problems
 The course of RA is marked with exacerbations and remissions
 Course
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RA begins with synovitis of the affected joint
Inflammatory chemicals are inappropriately released
Inflammatory blood cells migrate to the joint, causing swelling
Inflamed synovial membrane thickens into a pannus
Pannus erodes cartilage, scar tissue forms, articulating bone ends connect
The end result, ankylosis, produces bent, deformed fingers
 Treatment
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Conservative therapy – aspirin, long-term use of antibiotics, and physical
therapy
Progressive treatment – anti-inflammatory drugs or immunosuppressants
The drug Enbrel, a biological response modifier, neutralizes the harmful
properties of inflammatory chemicals
Gouty Arthritis
 Deposition of uric acid crystals in joints and soft tissues,
followed by an inflammation response
 Typically, gouty arthritis affects the joint at the base of the
great toe
 In untreated gouty arthritis, the bone ends fuse and
immobilize the joint
 Treatment – colchicine, nonsteroidal anti-inflammatory
drugs, and glucocorticoids