Chapter 6
The Skeletal System: Bone Tissue
Lecture Outline
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INTRODUCTION
 Bone is made up of several different tissues
working together: bone, cartilage, dense
connective tissue, epithelium, various blood
forming tissues, adipose tissue, and nervous
tissue.
 Each individual bone is an organ; the bones,
along with their cartilages, make up the
skeletal system.
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The Skeletal System:Bone Tissue
 Dynamic and ever-changing throughout life
 Skeleton composed of many different tissues
 cartilage, bone tissue, epithelium, nerve, blood forming
tissue, adipose, and
dense
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Functions of Bone
 Supporting & protecting soft tissues
 Attachment site for muscles making
movement possible
 Storage of the minerals, calcium &
phosphate -- mineral homeostasis
 Blood cell production occurs in red bone
marrow (hemopoiesis)
 Energy storage in yellow bone marrow
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Anatomy of a
Long Bone
 diaphysis = shaft
 epiphysis = one end of a
long bone
 metaphyses are the
areas between the
epiphysis and diaphysis
and include the
epiphyseal plate in
growing bones.
 Articular cartilage over
joint surfaces acts as
friction reducer & shock
absorber
 Medullary cavity =
marrow cavity
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Anatomy of a
Long Bone
 Endosteum = lining
of marrow cavity
 Periosteum = tough
membrane covering
bone but not the
cartilage


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fibrous layer =
dense irregular CT
osteogenic layer =
bone cells & blood
vessels that nourish
or help with repairs
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Histology of Bone
 A type of connective tissue
as seen by widely spaced
cells separated by matrix
 Matrix of 25% water, 25%
collagen fibers & 50%
crystalized mineral salts
 4 types of cells in bone
tissue
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HISTOLOGY OF BONE TISSUE
 Bone (osseous) tissue consists of widely separated
cells surrounded by large amounts of matrix.
 The matrix of bone contains inorganic salts, primarily
hydroxyapatite and some calcium carbonate, and
collagen fibers.
 These and a few other salts are deposited in a
framework of collagen fibers, a process called
calcification or mineralization.
 The process of calcification occurs only in the
presence of collagen fibers.
 Mineral salts confer hardness on bone while
collagen fibers give bone its great tensile strength.
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bone cells.(Figure 6.2)
1. Osteogenic cells undergo cell division and
develop into osteoblasts.
2. Osteoblasts are bone-building cells.
3. Osteocytes are mature bone cells and the
principal cells of bone tissue.
4. Osteoclasts are derived from monocytes and
serve to break down bone tissue.
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Cells of Bone
 Osteoprogenitor cells ---- undifferentiated cells
 can divide to replace themselves & can become osteoblasts
 found in inner layer of periosteum and endosteum
 Osteoblasts--form matrix & collagen fibers but can’t divide
 Osteocytes ---mature cells that no longer secrete matrix
 Osteoclasts---- huge cells from fused monocytes (WBC)
 function in bone resorption at surfaces such as endosteum
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Cells
of
Bone
Osteoblasts Osteocytes
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Osteoclasts
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Matrix of Bone
 Inorganic mineral salts provide bone’s hardness

hydroxyapatite (calcium phosphate) & calcium
carbonate
 Organic collagen fibers provide bone’s flexibility


their tensile strength resists being stretched or torn
remove minerals with acid & rubbery structure results
 Bone is not completely solid since it has small
spaces for vessels and red bone marrow


spongy bone has many such spaces
compact bone has very few such spaces
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Compact Bone
 Compact bone is arranged in units called
osteons or Haversian systems (Figure 6.3a).
 Osteons contain blood vessels, lymphatic
vessels, nerves, and osteocytes along with
the calcified matrix.
 Osteons are aligned in the same direction
along lines of stress. These lines can slowly
change as the stresses on the bone changes.
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Compact
or Dense
Bone
 Looks like solid hard layer of bone
 Makes up the shaft of long bones and the
external layer of all bones
 Resists stresses produced by weight and
movement
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Histology of Compact Bone
 Osteon is concentric rings (lamellae) of calcified
matrix surrounding a vertically oriented blood vessel
 Osteocytes are found in spaces called lacunae
 Osteocytes communicate through canaliculi filled
with extracellular fluid that connect one cell to the
next cell
 Interstitial lamellae represent older osteons that have
been partially removed during tissue remodeling
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Spongy Bone
 Spongy (cancellous) bone does not contain
osteons. It consists of trabeculae surrounding
many red marrow filled spaces (Figure 6.3b).
 It forms most of the structure of short, flat,
and irregular bones, and the epiphyses of
long bones.
 Spongy bone tissue is light and supports and
protects the red bone marrow.
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The Trabeculae of Spongy Bone
 Latticework of thin plates of bone called trabeculae oriented
along lines of stress
 Spaces in between these struts are filled with red marrow
where blood cells develop
 Found in ends of long bones and inside flat bones such as
the hipbones, sternum, sides of skull, and ribs.
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No true Osteons.
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Blood and Nerve Supply of Bone
 Periosteal arteries
 supply periosteum
 Nutrient arteries
 enter through nutrient
foramen
 supplies compact bone
of diaphysis & red
marrow
 Metaphyseal &
epiphyseal aa.

supply red marrow &
bone tissue of
epiphyses
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BONE FORMATION
 All embryonic connective tissue begins as
mesenchyme.
 Bone formation is termed osteogenesis or
ossification and begins when mesenchymal
cells provide the template for subsequent
ossification.
 Two types of ossification occur.
 Intramembranous ossification is the formation
of bone directly from or within fibrous
connective tissue membranes.
 Endochondrial ossification is the formation of
bone from hyaline cartilage models.
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Intramembranous
 Intramembranous ossification forms the flat bones of
the skull and the mandible (Figure 6.5).
 An ossification center forms from mesenchymal
cells as they convert to osteoblasts and lay down
osteoid matrix.
 The matrix surrounds the cell and then calcifies as
the osteoblast becomes an osteocyte.
 The calcifying matrix centers join to form bridges
of trabeculae that constitute spongy bone with red
marrow between.
 On the periphery the mesenchyme condenses and
develops into the periosteum.
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Intramembranous
Bone Formation
 Mesenchymal cells become osteoprogenitor
cells then osteoblasts.
 Osteoblasts surround themselves with matrix
to become osteocytes.
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 Matrix calcifies into trabeculae with
spaces holding red bone marrow.
 Mesenchyme condenses as
periosteum at the bone surface.
 Superficial layers of spongy bone
are replaced with compact bone.
Intramembranous Bone
Formation (cont.)
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Intramembranous
Bone Formation
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Endochondrial
 Endochondrial ossification involves
replacement of cartilage by bone and forms
most of the bones of the body (Figure 6.6).
 The first step in endochondrial ossification is
the development of the cartilage model.
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Endochondral Bone Formation
 Development of Cartilage model
Mesenchymal cells form a
cartilage model of the bone during
development
 Growth of Cartilage model
 in length by chondrocyte cell
division and matrix formation (
interstitial growth)
 in width by formation of new matrix
on the periphery by new
chondroblasts from the
perichondrium (appositional
growth)
 cells in midregion burst and
change pH triggering calcification
and chondrocyte death

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Endochondral Bone Formation
 Development of Primary
Ossification Center
 perichondrium lays down
periosteal bone collar
 nutrient artery penetrates
center of cartilage model
 periosteal bud brings
osteoblasts and osteoclasts
to center of cartilage model
 osteoblasts deposit bone
matrix over calcified cartilage
forming spongy bone
trabeculae
 osteoclasts form medullary
cavity
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Endochondral Bone Formation
 Development of Secondary Ossification Center
 blood vessels enter the epiphyses around time of birth
 spongy bone is formed but no medullary cavity
 Formation of Articular Cartilage
Anatomy and Physiology,
 cartilage on ends of Principles
boneof Human
remains
as articular cartilage.
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Bone Scan
 Radioactive tracer is given intravenously
 Amount of uptake is related to amount of
blood flow to the bone
 “Hot spots” are areas of increased
metabolic activity that may indicate cancer,
abnormal healing or growth
 “Cold spots” indicate decreased
metabolism of decalcified bone, fracture or
bone infection
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BONE GROWTH
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Growth in Length
 To understand how a bone grows in length, one needs to
know details of the epiphyseal or growth plate (Figure
6.7).
 The epiphyseal plate consists of four zones: (Figure 6.7b)
 zone of resting cartilage,
 zone of proliferation cartilage,
 zone of hypertrophic cartilage, and
 zone of calcified cartilage The activity of the epiphyseal
plate is the only means by which the diaphysis can
increase in length.
 When the epiphyseal plate closes, is replaced by bone,
the epiphyseal line appears and indicates the bone has
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completed its growth in length.
Bone Growth in Length
 Epiphyseal plate or cartilage growth
plate
 cartilage cells are produced by
mitosis on epiphyseal side of plate
 cartilage cells are destroyed and
replaced by bone on diaphyseal
side of plate
 Between ages 18 to 25, epiphyseal
plates close.
 cartilage cells stop dividing and
bone replaces the cartilage
(epiphyseal line)
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 Growth in length stops
at age
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 Zone of resting cartilage
Zones of Growth in
Epiphyseal Plate
anchors growth plate to bone
 Zone of proliferating cartilage
 rapid cell division (stacked
coins)
 Zone of hypertrophic cartilage
 cells enlarged & remain in
columns
 Zone of calcified cartilage
 thin zone, cells mostly dead
since matrix calcified
 osteoclasts removing matrix
 osteoblasts & capillaries
move in to create bone over
and Physiology,
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Growth in Thickness
 Bone can grow in thickness or diameter only
by appositional growth (Figure 6.8).
 The steps in thes process are:




Periosteal cells differentiate into osteoblasts
which secrete collagen fibers and organic
molecules to form the matrix.
Ridges fuse and the periosteum becomes the
endosteum.
New concentric lamellae are formed.
Osetoblasts under the peritsteum form new
circumferential lamellae.
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Bone Growth in Width
 Only by appositional growth at the bone’s surface
 Periosteal cells differentiate into osteoblasts and form bony
ridges and then a tunnel around periosteal blood vessel.
 Concentric lamellae fill in the tunnel to form an osteon.
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Factors Affecting Bone Growth
 Nutrition
adequate levels of minerals and vitamins
 calcium and phosphorus for bone growth
 vitamin C for collagen formation
 vitamins K and B12 for protein synthesis
 Sufficient levels of specific hormones
 during childhood need insulinlike growth factor
 promotes cell division at epiphyseal plate
 need hGH (growth), thyroid (T3 &T4) and insulin
 sex steroids at puberty
 At puberty the sex hormones, estrogen and testosterone,
stimulate sudden growth and modifications of the skeleton
to create the male and female forms.

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Hormonal Abnormalities
 Oversecretion of hGH during childhood
produces giantism
 Undersecretion of hGH or thyroid hormone
during childhood produces short stature
 Both men or women that lack estrogen
receptors on cells grow taller than normal

estrogen is responsible for closure of growth
plate
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BONES AND HOMEOSTASIS
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Bone Remodeling
 Remodeling is the ongoing replacement of
old bone tissue by new bone tissue.



Old bone is constantly destroyed by
osteoclasts, whereas new bone is constructed
by osteoblasts.
In orthodontics teeth are moved by brraces.
This places stress on bone in the sockets
causing osteoclasts and osteablasts to
remodel the sockets so that the teeth can be
properly aligned (Figure 6.2)
Several hormones and calcitrol control bone
growth and bone remodeling (Figure 6.11)
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Bone Remodeling
 Ongoing since osteoclasts carve out small tunnels
and osteoblasts rebuild osteons.




osteoclasts form leak-proof seal around cell edges
secrete enzymes and acids beneath themselves
release calcium and phosphorus into interstitial
fluid
osteoblasts take over bone rebuilding
 Continual redistribution of bone matrix along lines
of mechanical stress

distal femur is fully remodeled every 4 months
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Fracture and Repair of Bone
A fracture is any break in a bone.
 Fracture repair (Figure 6.10)involves
formation of a clot called a fracture
hematoma, organization of the fracture
hematoma into granulation tissue called a
procallus (subsequently transformed into a
fibrocartilaginous [soft] callus), conversion of
the fibrocartilaginous callus into the spongy
bone of a bony (hard) callus, and, finally,
remodeling of the callus to nearly original
form.
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Fracture & Repair of Bone
 Healing is faster in bone than in
cartilage due to lack of blood vessels
in cartilage
 Healing of bone is still slow process
due to vessel damage
 Clinical treatment


closed reduction = restore pieces to
normal position by manipulation
open reduction = realignment during
surgery
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Fractures
 Named for shape or position of fracture
line
 Common types of fracture


greenstick -- partial fracture
impacted -- one side of fracture driven into
the interior of other side
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Fractures
 Named for shape or position of fracture line
 Common types of fracture



closed -- no break in skin
open fracture --skin broken
comminuted -- broken ends of bones are
fragmented
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Fractures
 Named for shape or position of fracture line
 Common types of fracture



Pott’s -- distal fibular fracture
Colles’s -- distal radial fracture
stress fracture -- microscopic fissures from
repeated strenuous activities
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Repair
of a
Fracture
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Repair of a Fracture
 Formation of fracture hematoma
 damaged blood vessels produce clot in 6-8 hours, bone cells
die
 inflammation brings in phagocytic cells for clean-up duty
 new capillaries grow into damaged area
 Formation of fibrocartilagenous callus formation
 fibroblasts invade the procallus & lay down collagen fibers
 chondroblasts produce fibrocartilage to span the broken ends
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of the bone
Repair of a Fracture
 Formation of bony callus
 osteoblasts secrete spongy bone that joins 2
broken ends of bone
 lasts 3-4 months
 Bone remodeling
 compact bone replaces the spongy in the bony
callus
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 surface is remodeled
back to normal shape
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Calcium Homeostasis & Bone Tissue
 Skeleton is a reservoir of Calcium & Phosphate
 Calcium ions involved with many body systems
 nerve & muscle cell function
 blood clotting
 enzyme function in many biochemical reactions
 Small changes in blood levels of Ca+2 can be
deadly (plasma level maintained 9-11mg/100mL)


cardiac arrest if too high
respiratory arrest if too low
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Hormonal Influences
 Parathyroid hormone (PTH) is
secreted if Ca+2 levels falls


PTH gene is turned on & more
PTH is secreted from gland
osteoclast activity increased,
kidney retains Ca+2 and produces
calcitriol
 Calcitonin hormone is secreted
from parafollicular cells in thyroid if
Ca+2 blood levels get too high


inhibits osteoclast activity
increases bone formation by
osteoblasts
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EXERCISE AND BONE TISSUE
 Within limits, bone has the ability to alter its strength
in response to mechanical stress by increasing
deposition of mineral salts and production of collagen
fibers.
 Removal of mechanical stress leads to weakening of
bone through demineralization (loss of bone minerals)
and collagen reduction.
 reduced activity while in a cast
 astronauts in weightless environment
 bedridden person
 Weight-bearing activities, such as walking or moderate
weightlifting, help build and retain bone mass.
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Development of Bone Tissue
 Both types of bone formation
Mesenchymal Cells
begin with mesenchymal
cells
 Mesenchymal cells
transform into chondroblasts
which form cartilage
OR
 Mesenchymal cells become
osteoblasts which form bone
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Developmental Anatomy
5th Week =limb bud appears
as mesoderm covered
with ectoderm
6th Week = constriction
produces hand or foot
plate
and skeleton now totally
cartilaginous
7th Week = endochondral
ossification begins
8th Week = upper & lower
limbs appropriately
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AGING AND BONE TISSUE
 Of two principal effects of aging on bone, the first is the
loss of calcium and other minerals from bone matrix
(demineralization), which may result in osteoporosis.
 very rapid in women 40-45 as estrogens levels
decrease
 in males, begins after age 60
 The second principal effect of aging on the skeletal
system is a decreased rate of protein synthesis
 decrease in collagen production which gives bone its
tensile strength
 decrease in growth hormone
 bone becomes
brittle
&Anatomy
susceptible
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Osteoporosis
 Decreased bone mass resulting in porous bones
 Those at risk



white, thin menopausal, smoking, drinking female with
family history
athletes who are not menstruating due to decreased
body fat & decreased estrogen levels
people allergic to milk or with eating disorders whose
intake of calcium is too low
 Prevention or decrease in severity


adequate diet, weight-bearing exercise, & estrogen
replacement therapy (for menopausal women)
behavior when young may be most important factor
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Disorders of Bone Ossification
 Rickets



calcium salts are not deposited properly
bones of growing children are soft
bowed legs, skull, rib cage, and pelvic
deformities result
 Osteomalacia


new adult bone produced during remodeling
fails to ossify
hip fractures are common
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end
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