BONES
AND
SKELETAL TISSUES
SKELETAL CARTILAGES
• Skeletal cartilages:
– Made of some variety of cartilage
– Consists primarily of water
• High water content accounts for its resilience
– Ability to spring back to its original shape after being
compressed
– Contains no nerves or blood vessels
– Surrounded by a layer of dense irregular connective
tissue called the perichondrium
• Acts like a girdle to resist outward expansion when the
cartilage is compressed
• Contains the blood vessels from which nutrients diffuse
through the matrix to reach the cartilage cells
– This mode of nutrient delivery limits cartilage thickness
SKELETAL CARTILAGES
• Three types:
– Hyaline
– Elastic
– Fibrocartilage
• All three types have the same basic
components:
– Cells called chondrocytes
• Encased in small cavities (lacunae) within an
extracellular matrix containing a jellylike ground
substance and fibers
HYALINE CARTILAGE
• Looks like a frosted glass
when freshly exposed
• Provides support with
flexibility and resilience
• Most abundant skeletal
cartilage
– Chondrocytes appear
spherical
– Only fiber type in the
matrix is fine collagen
fibers
HYALINE CARTILAGE
HYALINE CARTILAGE
• Includes:
– Articular cartilages:
• Cover the ends of most
bones at movable joints
– Costal cartilages:
• Connect the ribs to the
sternum (breastbone)
– Respiratory cartilages:
• Form the skeleton of the
larynx (voicebox)
– Reinforces other
respiratory passageways
– Nasal cartilages:
• Support the external nose
BONE CARTILAGE
ELASTIC CARTILAGE
• Looks very much like
hyaline cartilages, but
they contain more
stretchy elastic
fibers and so are
better able to stand
up to repeated
bending
ELASTIC CARTILAGE
ELASTIC CARTILAGE
• More flexible than
hyaline
• Located only in the:
– External ear
– Epiglottis of the
larynx:
• Flap that bends to
cover the opening of
the larynx each time we
swallow
BONE CARTILAGE
FIBROCARTILAGE
• Highly compressible
and have great
tensile strength
• Perfect intermediate
between hyaline and
elastic cartilages
• Consist of roughly
parallel rows of
chondrocytes
alternating with
thick collagen fibers
FIBROCARTILAGE
FIBROCARTILAGE
• Occur in sites that
are subjected to
both heavy pressure
and stretch:
– Padlike cartilages
(menisci) of the knee
– Discs between
vertebrae
BONE CARTILAGE
Growth of Cartilage
• Growth occurs in two ways:
– Appositional growth (growth from the outside)
results in outward expansion due to the production of
cartilage matrix on the outside of the tissue
• Cartilage-forming cells in the surrounding
perichondrium secrete new matrix against the external
face of the existing cartilage tissue
– Interstitial growth (growth from inside) results in
expansion from within the cartilage matrix due to
division of lacunae-bound chondrocytes and secretion
of matrix
• Lacunae-bound chondrocytes divide and secrete new
matrix, expanding the cartilage from within
Growth of Cartilage
• Typically, cartilage growth ends during
adolescence when the skeleton stops
growing
• Under certain conditions—during normal
bone growth in youth and during old age—
calcium salts may be deposited in the
matrix and cause it to harden, a process
called calcification
– NOTE: calcified cartilage is NOT bone
CLASSIFICATION OF BONES
•
206 named bones of the human
skeleton are divided into two
groups:
– Axial skeleton:
• Forms the long axis of the body
• Protect, support, or carry other
body parts
• Includes:
– Skull
– Vertebral column
– Rib cage
– Appendicular skeleton:
• Bones of the upper and lower
limbs, and the girdles
(pectoral/shoulder and pelvic/hip)
that attach them to the axial
skeleton
• Help us to get from place to place
(location)
BONE CARTILAGE
CLASSIFICATION OF BONES
• Classified by shape:
– Long
– Short
– Flat
– Irregular
BONE SHAPE
Long Bones
• Longer than they are
wide:
– Have a definite shaft and
two ends
– Consist of all limb bones
except patellas, the
carpels (wrist), and
tarsals (ankles)
• Named for their elongated
shape, NOT their overall
size
– Example: the three
bones in your fingers
(digits) are long bones,
even though they are
very small
BONE SHAPE
Short Bones
• Somewhat cube-shaped
• Include:
– Carpals: wrist
– Tarsals: ankle
– Sesamoid bone:
• Shaped like a sesame
seed
• Special type of bone that
forms in a tendon
– Example: patella
• Vary in size
• Some clearly act to alter
direction of pull of a
tendon
• Functions of others is not
known
BONE SHAPE
Flat Bone
• Thin, flattened and
often curved
• Include:
– Most skull bones
– Sternum
(breastbone)
– Scapulae (shoulder
blades)
– Ribs
BONE SHAPE
Irregular Bones
• Complicated
shapes that do not
fit in any of the
previous classes
– Example:
• Vertebrae
• Coxae (hip
bone)
BONE SHAPE
FUNCTIONS OF BONES
• Besides contributing to body shape and
form, our bones perform several important
functions:
– 1. Support
– 2. Protection
– 3. Movement
– 4. Mineral storage
– 5. Blood cell formation
Support
• Bones provide a framework that
supports the body and cradles its soft
organs
• Examples:
– Bones of the lower limbs act as pillars to
support the body trunk
– Rib cage supports the thoracic wall
Protection
• Fused bones at the skull protect the brain
• Vertebrae surround the spinal cord
• Rib cage helps protect the vital organs of
the thorax
Movement
• Skeletal muscles, which attach to bones
by tendons, use bones as levers to move
the body and its parts
– As a result, we can walk, grasp objects, and
breathe
Mineral Storage
• Bone is a reservoir for minerals
– Most important are calcium and
phosphates
– Stored minerals are released into the
bloodstream as needed for distribution to
all parts of the body:
• Deposits and withdrawals of minerals to and
from the bones go on almost continuously
Blood Cell Formation
• Most blood cell formation
(hematopoiesis) occurs in the marrow
cavities of certain bones
Bone Structure
• Because bones contain various types
of tissue, bones are organs:
– Bone (osseous) tissue
– Nervous tissue in their nerves
– Cartilage tissue in their articular cartilages
– Fibrous connective tissue lining their
cavities
– Muscle and epithelial tissues in their blood
vessels
Gross Anatomy
• Bone markings are projections,
depressions, and openings found on the
surface of bones that function as sites of
muscle, ligament, and tendon
attachment, as joint surfaces, and as
openings for the passage of blood
vessels and nerves
Names of Bone Markings
•
Projections (bulges): grow
outward from the bone surface
– Projections that sites of muscle
and ligament attachment:
• Tuberosity: elevated round
swelling
– Large and rounded
– May be roughened
• Crest: elongated prominence
– Narrow ridge
– Usually prominent
• Trochanter: to run
–
–
–
–
Very large
Blunt
Irregularly shaped
ONLY in the femur
• Line:
– Narrow ridge
– Less prominent than crest
• Tubercle: little swelling
– Small and rounded
• Epicondyle: above a knuckle
(condyle)
– Raised area on or above a
condyle
• Spine:
– Sharp, slender, often pointed
projection
• Process:
– Any bone prominence
Names of Bone Markings
• Projections That Help to Form Joints:
articulation
– Head:
• Bony expansion carried on a narrow neck
– Facet: small face
• Smooth, nearly flat articular surface
– Condyle: knuckle
• Rounded articular projection
– Ramus: branch
• Armlike bar of bone
Names of Bone Markings
• Depressions and openings:
– Allow blood vessels and nerves to pass
• Meatus: passage/opening
– Canal-like passageway
• Sinus: curve, hollow
– Cavity within a bone, filled with air and lined with mucous membrane
• Fossa: furrow or shallow depression
– Shallow, basinlike depression in a bone
– Often serves as an articular surface
• Groove: ditch
– Furrow
• Fissure: slender deep furrow
– Narrow, slitlike opening
• Foramen: passage/opening
– Round or oval opening through a bone
Bone Textures
•
Compact:
– All bones have a dense
outer layer consisting of
compact bone that appears
smooth and solid
• Spongy Bone:
– Internal to compact bone is
spongy bone, which
consists of honeycomb,
needle-like, or flat pieces,
called trabeculae (little
beam)
• In living bones the open
spaces between
trabeculae are filled with
red or yellow bone
marrow
Compact/Spongy Bone
Typical Long Bone Structure
•
All long bones have the same
general structure:
–
Diaphysis: dia (through) / physis
(growth)
•
•
•
Tubular shaft
Forms the long axis of the bone
Constructed of a relatively thick collar
of compact bone that surrounds a
central medullary cavity or marrow
cavity
–
–
In adults, the medullary cavity
contains fat (yellow marrow) and is
called the yellow bone marrow
Epiphyses: epi (upon) / epiphyses
(singular)
•
•
•
The ends of the bone
Consist of internal spongy bone
covered by an outer layer of compact
bone
Joint surfaces of each epiphysis is
covered with a thin layer of articular
(hyaline) cartilage, which cushions
the opposing bone ends during joint
movement and absorbs stress
LONGBONE
Typical Long Bone Structure
• Epiphyseal line:
sometimes called the
metaphysis
– Located between the
epiphyses and
diaphysis in an adult
• Is the remnant of the
epiphyseal plate, a
disc of hyaline cartilage
that grows during
childhood to lengthen
the bone
LONGBONE
Typical Long Bone Structure
• Membranes: Periosteum
– The external surface of the
entire bone except the joint
surfaces is covered by a
glistening white, doublelayered membrane called the
periosteum (peri=around /
osteo=bone)
• Outer fibrous layer is dense
irregular connective tissue
• Inner osteogenic layer
abutting the bone surface
consists primarily of:
– Bone-forming cells:
osteoblasts
– Bone-destroying cells:
osteoclasts
LONGBONE
Typical Long Bone Structure
• Membranes: Periosteum
– Richly supplied with nerve
fibers, lymphatic vessels, and
blood vessels, which enter the
diaphysis via a nutrient
foramen
– Secured to the underlying
bone by perforating
(Sharpey’s) fibers
• Tufts of collagen fibers that
extend from its fibrous layer
into the bone matrix
– Provides anchoring points
for tendons and ligaments
• At these points the
perforating fibers are
exceptionally dense
LONGBONE
Typical Long Bone Structure
• Membranes:
Endosteum (within
bone)
– The internal surface of the
bone is lined by a
connective tissue
membrane called the
endosteum
• Covers the trabeculae of
spongy bone and lines the
canals that pass through
the compact bone
• Like the periosteum, the
endosteum contains both
osteoblasts and
osteoclasts
LONGBONE
Structure of Short, Flat, and
Irregular Bones
• Short, flat, and irregular bones
consist of thin plates of
periosteum-covered
compact bone on the
outside, and endosteumcovered spongy bone inside,
which houses bone marrow
between the trabeculae (no
marrow cavity is present)
• Not cylindrical
• No shaft or epiphyses
• Called the diploe (folded)
– Arrangement resembles a
sandwich
FLATBONE
Location of Hematopoietic Tissue in
Bones
• Hematopoietic tissue of bones, red bone marrow, is located
within:
• The trabecular cavities of the spongy bone in flat bones
• The trabecular cavities of the spongy bone of the epiphyses in the long
bones
– Red bone marrow is found in:
• All flat bones
• Epiphyses, and medullary cavities of infants
• In adults, distribution is restricted to flat bones and the proximal
epiphyses of the humerus and femur
– Hence, blood cell production in adult long bones routinely occurs only in the
head of the femur and humerus
– Red marrow found in the diploe of flat bones (such as the sternum) and in
some irregular bones (such as the hip bones) is much more active in
hematopoiesis
» These are the sites used for obtaining red marrow samples
– Yellow marrow in the medullary cavity can revert to red marrow if a
person becomes very anemic and needs enhanced red blood cell
production
Microscope Anatomy of Bone
• Although compact
bone looks dense
and solid, a
microscope reveals
that it is riddled with
passageways that
serve as conduits for
nerves, blood
vessels, and
lymphatic vessels
COMPACT BONE
Microscope Anatomy of Bone
Compact Bone
•
The structural unit of compact bone
is the osteon, or Haversian system
–
Each osteon is an elongated
cylinder oriented parallel to the long
axis of the bone
•
•
Tiny weight –bearing pillars
Group of hollow tubes of bone
matrix, one placed outside the next
like the growth rings of a tree trunk
–
•
–
In diagram: osteon are drawn as if
pulled out like a telescope to illustrate
the individual lamellae
Each matrix tube is a lamella (little
plate), and for this reason compact
bone is often called lamellar bone
Although all of the collagen fibers in
a particular lamella run in a single
direction, the collagen fibers in
adjacent lamella always run in
opposite directions
•
This alternating pattern is beautifully
designed to withstand torsion (twisting)
stresses—the adjacent lamella
reinforce one another to resist
twisting
OSTEON
Microscope Anatomy of Bone
Compact Bone
•
Collagen fibers are not the only part
of bone lamellae that are beautifully
ordered
–
•
Running through the core of each
osteon is:
–
•
•
Tiny crystals of bone salts align with
the collagen fibers and thus also
alternate their direction in adjacent
lamellae
The Central (Haversian) Canal that
containing small blood vessels and
nerve fibers that serve the needs of
the osteon’s cells
Perforating (Volkmann’s) Canals lie
at right angles to the long axis of the
bone, and connect the blood and
nerve supply of the periosteum to
that of the central canals and
medullary cavity
BOTH Haversian and Volkmann Canal
are lined with endosteum
COMPACT BONE
Microscope Anatomy of Bone
Compact Bone
•
(b):Osteocytes (spider-shaped
mature bone cells) occupy
lacunae (small space, cavity, or
depression occupied by cells) at
the junctions of the lamellae
– Hair-like canals called canaliculi
connect the lacunae to each
other and to the central canal
• Tie all the osteocytes in an
osteon together, permitting
nutrients and wastes to be relayed
from one osteocyte to the next
throughout the osteon
• Although bone matrix is hard and
impermeable to nutrients, its
canaliculi and cell-to-cell relays
(via gap junctions) allow bone
cells to be well nourished
– Function is to maintain the
bone matrix:
• If they die, the surrounding matrix
is resorbed (remove-assimilated)
COMPACT BONE
Microscope Anatomy of Bone
Compact Bone
• Not all the lamellae in compact
bone are part of osteons
– (c): Lying between intact
osteons are incomplete
lamella called interstitial
lamella
• These either fill the gaps
between forming osteons or
are remnants of osteons
that have been cut through by
bone remodeling
• (a): Circumferential lamellae
are located just beneath the
periosteum, extending
around the entire
circumference of the bone
– Effectively resist twisting of the
long bone
COMPACT BONE
Spongy Bone
•
•
Lacks osteons
Trabeculae (honeycomb
network) align along lines of
stress and help the bone resist
stress as much as possible
– These tiny bone struts are as
carefully positioned as the
flying buttresses of a Gothic
cathedral
•
•
Irregularly arranged lamella and
osteocytes interconnected by
canaliculi
Nutrients reach the osteocytes
by diffusing through the
canaliculi from capillaries in the
endosteum surrounding the
trabeculae
LONGBONE
FLATBONE
COMPACT BONE
Chemical Composition of Bone
• Organic components:
– Cells (osteoblasts, osteocytes, and osteoclasts)
– Osteoid: nonliving
• Composed of secretions from the osteoblasts which
contribute to the flexibility and tensile strength of bone that
allows the bone to resist stretch and twisting
– Ground substance: proteoglycans and glycoproteins
– Collagen fibers:
» Bonds in or between collagen molecules break easily
on impact dissipating energy to prevent the force from
rising to a fracture value
» In the absence of continued or additional trauma, most
of the bonds reform
Chemical Composition of Bone
• Inorganic components:
– Make up 65% of bone by mass
• Consist of hydroxyapatite (mineral salts) that is
largely calcium phosphate, which accounts for the
hardness and compression resistance of bone
– Present in the form of tightly packed tiny crystals
surrounding the collagen fibers in the extracellular matrix
• Because of the salts they contain, bones last long after
death and provide an enduring “monument”
• Healthy bone is half as strong as steel in resisting
compression and fully as strong as steel in resisting
tension (stretching)
BONE DEVELOPMENT
• Ossification and osteogenesis are
synonyms meaning the process of bone
formation (os=bone / genesis=beginning)
– In embryos: leads to the formation of the
skeleton
– Early adulthood: bones increase in length
– Throughout life: bones are capable of
growing in thickness
– Adults: ossification serves mainly for bone
remodeling and repair
Formation of the Bony Skeleton
• Before week 8, the skeleton of a human embryo
is constructed entirely from fibrous
membranes and hyaline cartilage
– Bone tissue begins to develop at about this time
and eventually replaces most of the existing fibrous or
cartilage structures
– When a bone develops from a fibrous membrane,
the process is intramembranous ossification, and the
bone is called a membrane bone
– Bone development by replacing hyaline cartilage
is called endochondral ossification (endo=within /
chondo=cartilage), and the resulting bone is called a
cartilage (endochondral) bone
Intramembranous Ossification
• Results in the formation
of cranial bones of the
skull (frontal, parietal,
occipital, and temporal
bones) and the clavicles
• All bones formed by
this process are flat
bones
• Four Major Steps: 1, 2, 3,
4
Intramembranous Ossification
Intramembranous Ossification
Endochondral Ossification
• Replaces hyaline cartilage, forming all
bones below the skull except for the
clavicles
• Begins in the second month of
development
• Five Steps: 1,2,3,4,5
Endochondral Ossification
• 1. Initially, osteoblasts secrete osteoid, creating
a bone collar around the diaphysis of the
hyaline cartilage model
Endochondral Ossification
Endochondral Ossification
• 2. Cartilage in the center of the diaphysis
calcifies:
– Because calcified cartilage matrix is impermeable
to diffusing nutrients, the chondrocytes die and
deteriorate forming cavities
Endochondral Ossification
Endochondral Ossification
• 3. The periosteal bud (nutrient artery and vein,
lymphatics, nerve fibers, red marrow elements,
osteoblast, and osteoclasts) invades the internal
cavities and spongy bone forms around the
remaining fragments of hyaline cartilage
Endochondral Ossification
Endochondral Ossification
• 4. The diaphysis elongates as the cartilage in
the epiphyses continue to lengthen and a
medullary cavity forms through the action of
osteoclasts within the center of the diaphysis
Endochondral Ossification
Endochondral Ossification
• 5. The epiphyses ossify shortly after birth through the
development of secondary ossification centers
– When complete, hyaline cartilage remains only at two places:
• On the epiphyseal surfaces (articular cartilages)
• Junction of the diaphysis and epiphysis, where it forms the
epiphyseal plates
Endochondral Ossification
Postnatal Bone Growth
• During infancy and youth:
– Long bones lengthen entirely by interstitial
growth of the epiphyseal plates
– All bones grow in thickness by appositional
growth
Growth in Length of Long Bones
• Side of the epiphyseal plate
cartilage facing the
epiphysis, the cartilage is
relatively quiescent and
inactive
• Side of the epiphyseal plate
cartilage abutting the
diaphysis organizes into a
pattern that allows fast,
efficient growth (osteogenic
zone)
– As the cells divide the
epiphysis is pushed away
from the diaphysis
• Long bone lengthens
LENGTH GROWTH
Bone Growth
Growth in Length of Long Bones
• During growth, the
epiphyseal plate
maintains a constant
thickness because the
rate of cartilage growth
on its epiphyseal-facing
side is balanced by its
replacement with bony
tissue on its diaphysisfacing side
Bone Growth
Growth in Length of Long Bones
• As adolescence draws to an end, the
chondroblasts of the epiphyseal plates
divide less often and the plates become
thinner and thinner until they are entirely
replaced by bone tissue
• Longitudinal bone growth ends when the
bone of the epiphysis and diaphysis fuses
– This process, called epiphyseal plate closure,
happens at about 18 years of age in females and 21
years of age in males
– However, an adult bone can still increase in
diameter or thickness by appositional growth if
stressed by excessive activity or body weight
Growth in Width (Thickness)
• Growing bones widen as they lengthen
• Increases in thickness by appositional
growth
APPOSITIONAL GROWTH
Growth in Width (Thickness)
• Osteoblast beneath the
periosteum secrete
bone matrix on the
external bone surface
– Osteoclasts on the
endosteal surface of the
diaphysis remove bone
• There is normally
slightly less breaking
down than building up
– This unequal process
produces a thicker,
stronger bone but
prevents it from
becoming too heavy
Appositional Growth
Hormonal Regulation of Bone
Growth
• During infancy and childhood, the most important stimulus
of epiphyseal plate activity is growth hormone from the
anterior pituitary, whose effects are modulated by thyroid
hormone, ensuring that the skeleton has proper proportions
as it grows
• At puberty, male and female sex hormones (testosterone
and estrogen) are released in increasing amounts
– Initially these sex hormones promote the growth spurt
typical of adolescence, as well as the masculinization or
feminization of specific parts of the skeleton
– Ultimately these hormones induct the closure of the
epiphyseal plate ending longitudinal bone growth
BONE HOMEOSTASIS
• Every week we recycle 5 to 7% of our bone
mass, and as much as half a gram of calcium
may enter or leave the adult skeleton each day
• Spongy bone is replaced every 3-4 years
• Compact bone, is replaced approximately every 10 years
– This is fortunate because when bone remains in place
for long periods the calcium crystallizes and
becomes very brittle—ripe conditions for fracture
• When we break bones (most common disorder of
bones), they undergo a remarkable process of self-repair
Bone Remodeling
• In the adult skeleton, bone deposit and bone
resorption (removal) occur BOTH at the surface of
the periosteum and the surface of the endosteum
– These two processes constitute bone remodeling:
• They are coupled and coordinated by remodeling units
(osteoblasts and osteoclasts)
– Osteoblast: bone forming cells
– Osteoclast: large cells that resorb or break down bone matrix
– In adult skeletons, bone remodeling is balanced bone
deposit and removal, bone deposit occurs at a greater rate
when bone is injured, and bone resorption allows minerals
of degraded bone matrix to move into the blood
Bone Remodeling
• Bone deposit: involves osteoblasts
– Occurs wherever bone is injured or added
bone strength is required
– Optimal bone deposit requires:
•
•
•
•
•
Healthy diet rich in proteins
Vitamin C
Vitamin D
Vitamin A
Minerals: calcium, phosphorus, magnesium, and
manganese
Bone Remodeling
• Bone Resorption: accomplished by
osteoclasts
– Move along a bone surface, digging grooves called
resorption bays as they break down the bone matrix
– Secretes:
• Lysosomal enzymes that digest the organic matrix
• Hydrochloric acid that converts the calcium salts into
soluble forms that pass easily into solution
– May also phagocytize the demineralized matrix
and dead osteocytes
Control of Remolding
• Regulated by two control loops:
– A negative feedback hormonal mechanism that maintains
Ca2+ homeostasis in the blood
• Calcium is important in many physiological processes:
–
–
–
–
–
Nerve impulses
Muscle contraction
Blood coagulation
Secretion by glands, nerve cells
Cell division
– Responses to mechanical and gravitational forces acting on
the skeleton
• Daily calcium requirement is:
– 400-800 mg from birth until the age of 10
– 1200-1500 mg from ages 11 to 24
Hormonal Mechanism
• Mostly used to maintain blood calcium
homeostasis, and balances activity of
parathyroid hormone (PTH) and calcitonin
(thyroid)
Hormonal Mechanism
• Increased parathyroid
hormone (PTH) level
stimulates osteoclasts to
resorb bone, releasing
calcium to the blood
– Osteoclasts are no respectors
of matrix age
• They break down both old
and new matrix
• ONLY osteoid
(unmineralized matrix),
which lacks calcium salts,
escapes digestion
• As blood concentrations of
calcium rise, the stimulus for
PTH release ends
HORMONE CONTROL
Hormonal Mechanism
• Calcitonin (Thyroid):
– Secreted when blood
calcium levels rise
– Inhibits bone resorption
– Encourages calcium salt
deposit in bone matrix,
effectively reducing blood
calcium levels
– As blood calcium levels
fall, calcitonin release
wanes
HORMONE CONTROL
REMODELING
Hormonal Mechanism
• These hormonal controls act not to
preserve the skeleton’s strength or
well-being but rather to maintain blood
calcium homeostasis
– In fact, if blood calcium levels are low for an
extended time, the bones become so
demineralized that they develop large,
punched-out-looking holes
• Thus, the bones serve as a storehouse from
which ionic calcium is drawn as needed
Response to Mechanical Stress
and Gravity
•
•
Wolff’s Law: Response to
mechanical stress (muscle pull)
and gravity serves the needs of
the skeleton by keeping the
bones strong where stressors
are acting
A bone’s anatomy reflects the
common stresses it
encounters:
– Example: a bone is loaded
(stressed) whenever weight bears
down on it or muscles pull on it
• Tends to bend the bone
• Compresses the bone on one
side and subjects it to tension
(stretching) on the other side
– Both forces are minimal toward
the center of the bone (cancel
each other out)
BONE STRESS
Wolff’s Law
• 1. Long bones are thickest midway along the
diaphysis, exactly where bending stresses are greatest
(bend a stick and it will split near the middle
• 2. Curved bones are thickest where they are most
likely to buckle
• 3. Trabeculae of spongy bone form trusses, or struts,
along lines of compression
• 4. Large, bony projections occur where heavy, active
muscles attach
– Bones of weight lifters have enormous thickenings at the
attachment sites of the most used muscles
• Also explains the featureless bones of the fetus and the
atrophied bones of bedridden people—situations in
which bones are not stressed
Control of Remolding
• Skeleton is continuously subjected to both hormonal
influences and mechanical forces
• The hormonal loop determines whether and when
remodeling occurs in response to changing blood
calcium levels
• Mechanical stress determines where it occurs
• Example:
– When bone must be broken down to increase blood calcium
levels, PTH is released and targets the osteoclasts
– Mechanical forces determine which osteoclasts are most
sensitive to PTH stimulation, so that bone in the least stressed
areas (temporarily dispensable) is broken down
Bone Repair
• Fractures are breaks in bones:
– Due to trauma to bones or thin, weaken
bones
Classification of Fracture
• Position of the bone ends after fracture:
– Nondisplaced: bone ends retain their normal position
– Displaced: bone ends are out of normal alignment
• Completeness of break:
– Complete: bone is broken through
– Incomplete: bone is not broken through
• Greenstick: bone breaks incompletely (like green twig
breaks)
– Only one side of the shaft breaks; the other side bends
• Orientation of the break relative to the long axis of the bone:
– Linear: parallel fracture
– Transverse: break is perpendicular to the bone’s long axis
Classification of Fracture
• Whether the bone ends penetrate the skin:
– Open (compound): penetrates the skin
– Closed (simple): does not penetrate the skin
• Location:
– Arm, leg, etc.
– Epiphyseal: epiphysis separates from the diaphysis
along the epiphyseal plate
• Depressed: skull bones pushed in
• External appearance
• Nature of break:
– Comminuted: bone fractures into 3 or more pieces
– Spiral: angular
Bone Repair
• Repair of fractures involves four major stages:
– 1. Hematoma formation: mass of clotted blood
• Because blood vessels are damaged
• Bone cells deprived of nutrients die at the site
• Tissue at the site become swollen, painful, and inflamed
BONE HEALING
Bone Repair
•
2. Fibrocartilaginous callus formation:
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Formation of soft granulation tissue (soft callus)
Capillaries grow into the hematoma
Phagocytes invade the area
Fibroblasts:
• Produce collagen fibers that span the break and connect the bone ends
– Osteoblasts:
• Begin forming spongy bone
BONE HEALING
Bone Repair
• 3. Bony callus formation:
– New bone trabeculae begins to form and is gradually
converted to bony (hard) callus
BONE HEALING
Bone Repair
• 4. Remodeling of the bony callus:
– Excess material on the diaphysis exterior and within
the medullary cavityis removed
– Compact bone is laid down to reconstruct the shaft
walls
BONE HEALING
Bone Repair
New Methods
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1. Electrical stimulation of fracture
2. Ultrasound treatments
3. Free Vascular fibular graft
4. VEGF: vascular endothelial growth
factor
• 5. Nanobiotechnology
• 6. Bone Substitutes
HOMEOSTATIC IMBALANCE
• Imbalances between bone deposit and
bone resorption underline nearly every
disease that affects the adult skeleton
Osteomalacia
• Soft bones
• Includes a number of disorders in adults in which
the bone is inadequately mineralized
• Osteoid is produced, but calcium salts are not
deposited, so bones are soft and weak
• Main symptom is pain when weight is put on the affected
bones
• Cause: insufficient calcium or by a vitamin D deficiency
(helps to absorb calcium)
• Treatment: drink vitamin D-fortified milk and exposing
the skin to sunlight which stimulates production of
vitamin D
Rickets
• Inadequate mineralization of bones in
children caused by insufficient calcium or
vitamin D deficiency
• Treatment: drink vitamin D-fortified milk and
exposing the skin to sunlight which stimulates
production of vitamin D
• Because young bones are still growing rapidly,
rickets is much more severe than adult
Osteomalacia
– Bowed legs, deformities of the pelvis, skull, and rib
cage are common
Osteoporosis
• Refers to a group of disorders in which the rate of
bone resorption exceeds the rate of formation
• Bones become so fragile that something as simple as a
hearty sneeze or stepping off a curb can cause them to
break
• Bones have normal bone matrix (intercellular material of
a tissue), but bone mass is reduced and the bones
become more porous and lighter increasing the
likelihood of fractures
– Spongy bone of the spine is most vulnerable, and compression
fractures of the vertebrae are common
– Femur, particular the neck, is also very susceptible to fracture
(broken hip)
Osteoporosis
• Older women are especially vulnerable to
osteoporosis, due to the decline in estrogen after
menopause
• Other factors that contribute to osteoporosis
include:
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A petite body form
Insufficient exercise or immobility to stress the bones
A diet poor in calcium and vitamin D
Abnormal vitamin D receptors
Smoking:
• Reduces estrogen levels
– Hormone-related conditions:
• Hyperthyroidism
• Diabetes mellitus
OSTEOPOROSIS
(a): Normal Bone
(b): Osteoporotic Bones
Paget’s disease
• Is characterized by excessive bone
deposition and resorption, with the
resulting bone abnormally high in
spongy bone
– High ratio of spongy bone to compact bones
– It is a localized condition that results in
deformation of the affected bone
• Weaken of a region of a bone
• Cause: unknown
DEVELOPMENTAL ASPECTS OF BONES:
TIMING OF EVENTS
• The skeleton derives from embryonic
mesenchymal cells, with ossification occurring at
precise times
– Most long bones have obvious primary ossification
centers by 12 weeks
• At birth, most bones are well ossified, except for
the epiphyses, which form secondary
ossification centers
• Throughout childhood, bone growth exceeds
bone resorption; in young adults, these
processes are in balance; in old age, resorption
exceeds formation
FETAL OSSIFICATION
BONE REPAIR