Introduction to the Skeletal System
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Support: Bones support the weight of the body and
structures such as the head and face
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Protection: Bones protect delicate organs such as the
brain and spinal cord, heart and lungs
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Muscle Attachment & Movement: Bones act as levers
to which muscles are attached
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Blood Production: Blood cells are produced within
red bone marrow (hematopoeisis)
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Store Minerals: Minerals such as calcium and
phosphorus, etc. are stored in bone matrix
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Many tissues are contained within bones
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Bone tissue
Nervous tissue (nerves)
Blood vessels (and blood)
Cartilage
Dense connective tissue
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There are 5 types of bones:
 Long (ex: femur)
 Short (ex: carpals)
 Flat (ex: scapula)
 Irregular (ex: vertebrae)
 Sesamoid (ex: patella)
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Those classified as Long Bones must meet
this criteria:
◦ Must have a body (diaphysis) that is longer than it is
wide; Diaphysis must have a hard covering
(periosteum) made of compact bone
◦ Must have growth plate (epiphysis) at either end;
Epiphysis must contain marrow and be made of
spongy bone with a thin covering of compact bone
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Example: femur, humerus
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Epiphysis: Expanded end of bones that form
joints with adjacent bones
◦ Covered by articular cartilage (hyaline) at the joint
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Diaphysis: The long shaft of the bone
◦ Covered in compact bone
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The epiphysis is at the end of long bones.
The epiphyseal plate is the part of the
epiphysis where lengthwise bone growth
occurs.
Usually referred to as ‘the growth plate’.
The epiphyseal plates fuse in early adulthood
and no further lengthening of the bones
occurs.
The diaphysis is the long, shaft portion of a
long bone.
 The diaphysis contains a hollow medullary
cavity that is lined with endosteum and filled
with marrow.
 A tough layer of vascular connective tissue,
called the periosteum, covers the bone and is
continuous with ligaments and tendons.
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There are two types of bone marrow:
◦ Red Bone Marrow-responsible for the production of
red blood cells, white blood cells and blood
platelets; is found in spongy bone (epiphysis)
◦ Yellow Bone Marrow-stores fat and is not active in
blood cell production; found in the medullary
cavities
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Ends of long bones:
◦ Epiphyses
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Shaft of long bones:
◦ Diaphysis
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Inner cavity of long bones:
◦ Medullary Cavity
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Outer covering of bones:
◦ Periosteum
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Inner covering of medullary cavity:
◦ Endosteum
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Osteoblasts are cells that build bone tissue
◦ They become active in connective tissue
membranes and deposit bone matrix around
themselves-this leads to the branching appearance
of spongy bone
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Osteoclasts destroy bone tissue by secreting
acid that dissolves the inorganic bone matrix
These two cells usually work well together
but malfunctions can lead to bone cancer
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Bone cells called osteocytes are located within
spaces called lacunae that lie in concentric
circles around Haversian canals that contain
blood vessels, and nerves
These systems are organized into columns
called osteons that are cemented together
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These osteonic/Haversian systems contain
blood vessels & nerve fibers and extend
longitudinally through bone
Osteonic canals are interconnected by
transverse perforating canals (Volkmann’s
canals).
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Scientists like to name things after
themselves…
Haversian canals=central canals
◦ run longitudinally through bone
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Volkmann’s canals=perforating canals
◦ these are transverse canals that run between
individual Haversian systems
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Osteocytes are scattered in no particular
order and pass nutrients and wastes back and
forth in passageways in the matrix called
canaliculi.
Important sites for spongy bone include:
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the
the
the
the
skull
ribs
vertebrae
sternum
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Compact bone
◦ Osteocytes and extracellular matrix cluster around
Haversian canals
◦ Many of these units cemented together makes up
compact bone
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Spongy bone
◦ Also composed of osteocytes and extracellular
matrix, but the units do not aggregate around
canals
◦ They form cross connections called trabeculae
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The intercellular material consists of collagen
and inorganic salts.
◦ Calcium Phosphate & Calcium Carbonate
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Most bones contain a mixture of compact and
spongy bone
◦ the epiphyses of long bones all contain spongy
bone covered by compact bone
◦ the diaphysis of a long bone is made of compact
bone
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Bones begin to form during the first few
weeks of prenatal development
Bones form in one of two ways
◦ Intramembranous bones originate between sheetlike layers of connective tissues
◦ Endochondral bones begin as masses of cartilage
that ossify
1. Membrane like layers of connective tissue
appear at the site of future bones (ex: skull)
2. Cells differentiate into osteoblasts
3. Osteoblasts deposit bony matrix around
themselves
4. Cells outside the developing bone give rise
to the outer periosteum
5. Osteoblasts on the inside of the bone form
the outer compact covering over spongy bone
6. Osteocytes form when extracellular matrix
completely surrounds osteoblasts
1. Hyaline Cartilage forms at the site of future
bones
2. Cartilage grows and begins to change
3. Blood vessels invade the tissue
4. Osetoblasts form spongy bone at the primary
ossification center of the diaphysis
5. Secondary ossification occurs in the spongy
bone of the epiphyses
6. The epiphyseal plate separates the two
ossification centers
7. Accumulation of the bone cells forces the death
of the cartilaginous cells
8. Bones continue to lengthen while
cartilaginous cells of the epiphyseal plate are
active
9. Bone growth ceases when the primary and
secondary ossification centers meet
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Epiphyseal (growth) plates are responsible for
lengthening bones while increases in
thickness are due to intramembranous
ossification underneath the periosteum.
A medullary cavity forms in the region of the
diaphysis due to the activity of the cells called
osteoclasts.
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Osteoclasts and osteoblasts remodel bone
tissue throughout life
Hormones that regulate blood calcium (PTHParathyroid Hormone and calcitonin) help
control resorption and deposition of bone
matrix
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Bones shape, support, and protect body
structures
Bones aid in body movement
Bones house tissues that produce blood cells
and store salts
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Supporting weight: bones of the lower limbs,
pelvis, and backbone
Protecting the senses: bones of the skull
protect the eyes, ears, and brain
Vital organ protection: bones of the thoracic
cavity and pectoral girdle protect the heart
and lungs
Reproductive organs: protected by the pelvic
girdle
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Blood cell formation begins in the yolk sac
As a fetus grows, blood cells are
manufactured in the liver and spleen
◦ The liver does not perform digestive processes
because the fetus does not consume meals directly,
but receives nourishment from the mother via the
placenta
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After birth, blood cells form in bone marrow
◦ Which kind?
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The thyroid gland regulates blood calcium
levels via two hormones
When blood calcium is low, parathyroid
hormone (PTH) stimulates osteoclasts to
break down bone tissue to release calcium
salts
When blood calcium is too high, calcitonin
stimulates osteoblasts to form bone tissue,
storing excess calcium salts
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Bones are good accumulators of heavy
metals such as lead, radium, or strontium
(bad news)
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The use of lead-based paints for homes,
children's toys and household furniture has
been banned in the United States since 1978
Lead-based paint is still on walls and
woodwork in many older homes and
apartments
Lead is sometimes found in toys and other
products produced abroad
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A fracture is a break in a bone
Blood vessels within the bone and the
periosteum rupture forming a 1)hematoma
Fibrocartilage fills in the gap between the
ends of the broken bone with a
2)cartilaginous callus
Osteoblasts and osteoclasts go to work and
the cartilaginous callus breaks down and a
3)bony callus fills the space
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Bone healing can be aided by doctors
◦ First cast used in Philadelphia in 1876
◦ More recently, screws and plates have been used
to internally align healing bones parts
◦ Rods, wires and nails are used by surgeons today
 They are lighter and smaller and usually built of
titanium
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Greenstick: an
incomplete
fracture
usually
occurring in
the developing
bones of
children
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Compound: a
severe fracture in
which the bone
breaks through the
skin
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Comminuted: a
fracture in which
the bone is broken
into several pieces