Ch 6 Bones and
Skeletal Tissue
Human Skeleton
• Initial – cartilage and fibrous membrane
• The replaced by bone
• Few areas of cartilage remains in adults
Basic Structure, Types, and Locations
•
•
•
•
Skeletal Cartilage – cartilage tissue
Consists primarily of water
No nerves or blood vessels
Surrounded by a layer of dense irregular
connective tissue – pericardium
• 3 types of cartilage –
–
–
–
–
Hyaline
Elastic
Fibrocartilage
All have condrocytes within an extracellular matrix
Cartilage in
external ear
Cartilage in
Intervertebral
disc
Cartilages in
nose
Articular
Cartilage
of a joint
Epiglottis
Thyroid
cartilage
Cricoid
cartilage
Larynx
Trachea
Lung
Costal
cartilage
Respiratory tube cartilages
in neck and thorax
Pubic
symphysis
Meniscus
(padlike
cartilage in
knee
joint)
Articular
cartilage
of a joint
Bones of skeleton
Axial skeleton
Appendicular skeleton
Cartilages
Hyaline cartilages
Elastic cartilages
Fibrocartilages
Figure 6.1
Skeletal Cartilage
1. Hyaline cartilage • Frosted glass
• Provide support with flexibility and resilience
• Condrocytes appear spherical
• Fiber in matrix – fine collage fibers
• Found –
– Articular cartilages – at the ends of most bones in
moveable joints
– Costal cartilage – connects ribs to sternum
– Respiratory cartilage – larynx, reinforce respiratory
passageways
– Nasal cartilage – support external nose
Skeletal Cartilage
2. Elastic Cartilage –
• More elastic fibers
• Repeated bending
• External ear and epiglottis
Skeletal Cartilage
3. Fibrocartilage • Highly compressible
• Great tensile strength
• roughly parallel rows of chondrocytes
alternating with thick collagen fibers
• Sites subjected to heavy pressure and stretch
• Pad like cartilage of knee
• Discs between vertebrea
Growth of Cartilage
• Flexible matrix which can accommodate mitosis
• Grows in 2 ways –
1. Oppositional growth – “growth from outside”
- Cartilage forming cells in pericardium secretes new
matrix against external face of existing cartilage
2. Interstitial Growth – lacunae – bound chondrocytes
- Divide and secrete new matrix
- Expanding from within
- Typically growth ends during adolescence
- Calcification – calcium salts deposited in matrix
-
Old age, cause to harden
Classification of Bone
• 206 bones
• 2 groups
1. Axial Skeleton – long axis of body, includes
bones of skull, vertebral column, and rib cage
- Protection, support, carrying
2. Appendicular Skeleton - bones of upper and
lower limbs and girdles (shoulder bones and hip
bones) that attach limbs to axial skeleton
-
Locomotion and manipulation
Cartilage in
external ear
Cartilage in
Intervertebral
disc
Cartilages in
nose
Articular
Cartilage
of a joint
Costal
cartilage
Pubic
symphysis
Meniscus
(padlike
cartilage in
knee
joint)
Articular
cartilage
of a joint
Figure 6.1
Classification of Bone
•
•
•
•
•
Many shapes and sizes
Pisiform – wrist – pea sized
Femur – approx 2 feet long in some people
Shape fulfills need
Classified according to shape – long, short,
flat, irregular
Classification of Bone
1. Long Bones – much longer than they are wide
• Shaft and 2 ends
• All limb bones except patella, wrist and ankle
bones
• Named for elongated shape, NOT overall size
• 3 bones in fingers are long bones.
Figure 6.2
Classification of Bone
2. Short – roughly cube shaped
• Ex. Wrist and ankle bones
• Sesamoid bones – shaped like a sesame seed
– Special type of short bone that forms a tendon
– Ex. Patella
• Vary in size and number in different
individuals
• Act to alter the direction of pull of a tendon
Figure 6.2
Classification of Bone
3. Flat Bones
• Thin, flattened and usually a bit curved
• Sternum – breast bone
• Scapulae – shoulder blades
• Ribs
• Skull bones
Figure 6.2
Classification of Bone
4. Irregular bones –
• Complicated shapes that fit none of preceding
cases
• Include vertebrea and hip bones
Figure 6.2
Functions of Bones
• Contribute to body shape and form
• Also
1. Support – framework that supports the body
and cradles its soft organs
-
ex. Lower limbs support the trunk
2. Protection – fused bones of the skull – protect
brain
-
Vertebrea – protect spinal cord
Ribcage – vital organs
Functions of Bones
3. Movement – skeletal muscles attach to bones
by tendons
- Use bones as levers to move body and its parts
- Walking, grasping, breath
4. Mineral and Growth Factor Storage –
reservoir for minerals – calcium and
phosphates
- Growth factors – IGF, TGF, BMP, etc.
Functions of Bones
5. Blood Cell Formation – hematopoiesis –
blood cell formation occurs in cavities of
certain bones
6. Triglyceride (fat) storage – fat stored in bone
cavities, stored energy for body
Bone Structure
• Bones are organs – several different tissues
• Primary tissue – osseous (bone) tissue
• Also
– nervous system tissue
– Cartilage – articular cartilage
– Fibrous connective tissue in cavities
– Muscle and epithelial tissue in blood vessels
Gross Anatomy
• Bone Markings –
• Projections, depressions, and opening serve as
sites of muscle, ligament, and tendon attachment
• Named in different ways
• Projections – bulges
–
–
–
–
Grow outward from bone surface
Heads, trochanters, spines, etc.
Indications of stress created by muscles
Modified surfaces where bones meet joints
Table 6.1
Table 6.1
Table 6.1
Gross Anatomy
• Depressions & Opening – fossae, sinuses,
foramina and groves
• Allow passage of nerves and blood vessels
Bone Textures
• Dense Bone Layer – looks smooth and solid
• External layer – compact bone
• Internal layer – spongy bone
– Cancellous bone
– Honeycomb of small need like or flat pieces called
trabeculae
– Living bone spaces filled with red or yellow bone
marrow
Structure of Typical Bone
• Long Bones – same general
structures – shaft, bone ends,
and membrane
1. Diaphysis – tubular
• Shaft
• Long axis of bone
• Thick collar of compact bone
that surrounds central
medullary cavity – marrow
cavity
• Adults – contains fat – yellow
marrow cavity
Long Bone
2. Epiphyses - Bone ends
• More expanded than diaphysis
• Exterior – compact bone
• Interior – spongy bone
• Joint surface covered with thin layer of articular
(hyaline) cartilage – cushions bone ends and absorbs
stress
• Epiphyseal line – reminant of epiphyseal plate – disc of
hyaline cartilage that grows during childhood
• Region also called metaphysis
Articular
cartilage
Proximal
epiphysis
Compact bone
Spongy bone
Epiphyseal
line
Periosteum
Compact bone
Medullary
cavity (lined
by endosteum)
(b)
Diaphysis
Distal
epiphysis
(a)
Figure 6.3a-b
Long Bone
3. Membranes – covers entire external surface
except at joint surfaces
• Periosteum – double layered membrane
• Fibrous layer – outer layer – dense irregular
tissue
• Osteogenic layer – inner layer – consists of bone
forming cells – osteoblasts – secrete bone matrix
– Also Osteoclasts – bone destroying cells
– Also osteogenic cells – primitive stem cells, give rise to
osteoblasts
Long Bone
• 3. Membranes cont
• Richly survived with nerve fibers, lymphatic
vessels and blood vessels
• Enter diaphysis – nutrient foramina
• Secured to bone by perforating (Sharpey’s) fibers
– tuffs of collagen fibers that extend into bone
matrix
• Internal Bone Surfaces – covered with delicate
connective tissue membrane – endosteum –
covers trabeculae of spongy bones
• Lines cavities that pass through compact bone
Endosteum
Yellow
bone marrow
Compact bone
Periosteum
Perforating
(Sharpey’s) fibers
Nutrient
arteries
(c)
Figure 6.3c
Short, Irregular, And Flat Bones
• Thin plates of periosteum – covered with
compact bone on outside
• Inside – endosteum – covered spongy bone
• Not cylindrical – no shaft or epiphyses
• Contain bone marrow but no significant
marrow cavity
• Spongy bone called – diploë
Spongy bone
(diploë)
Compact
bone
Trabeculae
Figure 6.5
Hematopoietic Tissue
• Red marrow
• Found with in trabecular cavities of spongy bone of
long bones and in the diploë of flat bones
• Cavities – called red bone marrow cavities
• In infants – medullary cavity of diaphysis and all areas
of spongy bone contain red bone marrow
• Adult – red in spongy extends into epiphysis
• Blood cell proliferation – heads of femurs and
humerous
• Diploë of flat bones (sternum) and irregular bone (hip
bone) red bone marrow very active
Microscopic Anatomy
• 4 major cell types –
1. Osteogenic cells
2. Osteoblasts
3. Osteocytes
4. Osteoclasts
- All surrounded by an extracellular matrix
- Osteogenic cells – osteoprogenitor cells,
mitotically active stem cells
- Some differentiate into osteoblasts
(a) Osteogenic cell
Stem cell
(b) Osteoblast
Matrix-synthesizing
cell responsible
for bone growth
Figure 6.4a-b
(c) Osteocyte
Mature bone cell
that maintains the
bone matrix
(d) Osteoclast
Bone-resorbing cell
Figure 6.4c-d
Compact Bone
• Looks dense and solid
• Microscope – riddled with passageways
• Conduits for nerves blood vessels and lymph
vessels
• Structural unit – Osteon – Haversian System –
elongated cylinders oriented parallel to long
bone axis
• Tiny weight bearing pillars
Structures
in the
central
canal
Artery with
capillaries
Vein
Nerve fiber
Lamellae
Collagen
fibers
run in
different
directions
Twisting
force
Figure 6.6
Compact Bone
• Osteon – hallow tubes of bone matrix –
lamella – laminar bone
• Collagen fibers run in one direction, then the
next run in a different direction
• Alternating pattern – designed to withstand
torsion stresses
• Bone salts align with collagen fibers and also
alternate directions
Compact Bone
•
•
•
•
Central canal – runs through core of osteon
Haversian canal
Small blood vessels and nerve fibers
Proliferating canals – Volkmann’s canals – lie
at right angles to long axis, connect it to blood
and nerve supply
Spongy bone
Compact
bone
Central
(Haversian) canal
Perforating
(Volkmann’s) canal
Endosteum lining bony canals
and covering trabeculae
Osteon
(Haversian system)
Circumferential
lamellae
(a)
Perforating (Sharpey’s) fibers
Lamellae
Nerve
Vein
Artery
Canaliculi
Osteocyte
in a lacuna
(b)
Periosteal blood vessel
Periosteum
Lamellae
Central
canal
Lacunae
Lacuna (with
osteocyte)
(c)
Interstitial lamellae
Figure 6.7a-c
Compact Bone
• Osteocytes – occupy lacunae at junctions of lamellae
• Hair like canals – cancaliculi – connects lacunae to each
other and central canal
• Tie all osteocytes together – pathway for nutrients and
wastes
• Canaliculi and cell –to-cell relays allow bone cells to be
nourished
• Maintains the bone matrix
• If die – matrix is reabsorbed
• Also act as stress or stain sensors – in case of bone
deformation or damaging stimuli
Compact Bone
• Interstitial lamellae – incomplete – lie in
between intact osteons
• Either fill gaps or are remnants of osteons
• Circumferential lamellae – just deep to
periosteum and superficial to endosteum
• Extend around circumference of diaphysis
Spongy Bone
• Poorly organized
• Haphazard
• Align precisely along lines of stress and help resist
bone stress
• Irregularly arranged lamellae and osteocytes
interconnected by canaliculi
• No osteons
• Nutrients – diffusion from capillaries in
endosteum
Nerve
Vein
Artery
Canaliculus
Osteocyte
in a lacuna
Lamellae
Central
canal
Lacunae
(b)
Figure 6.3b
Chemical Composition of Bone
• Organic Components – Cells
–
–
–
–
Osteogenic cells
Osteoblasts
Osteoclasts
Osteocytes
• Osteoid – organic part of matrix
– 1/3 of matrix
– Ground substance and collagen fibers
• Collagen fibers – contribute to structure but also
flexibility and tensile strength
• Resilience – from sacrificial bonds – in or between
collagen fibers
Chemical Composition of Bone
•
•
•
•
Inorganic hydroxyapatites – mineral salts
65 % of bone tissue
Largely calcium and phosphates
Tiny, tightly packed needle like crystals in and
around collagen fibers
• Healthy bones – ½ as strong as steel
• Salts – enable bones to last a long time after
death
Bone Development
• Ossification and osteogenesis – bone
formation
• Embryo formation of bony skeleton
• Bone growth – till early adulthood
• Adults – remodeling and repair only
Formation of Bony Skeleton
• Before week 8 – skeleton of embryo constructed
from fibrous membrane and hyaline cartilage
• Bone begins to develop and eventually replaces
most of existing fibrous or cartilage structures
• Intramembraneous ossification – bone from
fibrous membrane – Membrane Bone
• Endochondrial ossification – bone from hyaline
cartilage – cartilage or endochondrial bone
• Flexible – accommodates mitosis
Formation of Bony Skeleton
• Intramembraneous Ossification – results in
formation of cranial bones of skull – frontal,
parietal, occipital, and temporal bones and
clavicles
• Approximately week eight of development –
ossification begins on fibrous connective
tissue
• Membranes formed by mesenchymal cells
Mesenchymal
cell
Collagen
fiber
Ossification
center
Osteoid
Osteoblast
Ossification centers appear in the fibrous
connective tissue membrane.
• Selected centrally located mesenchymal cells cluster
and differentiate into osteoblasts, forming an
ossification center.
1
Figure 6.8, (1 of 4)
Osteoblast
Osteoid
Osteocyte
Newly calcified
bone matrix
2 Bone matrix (osteoid) is secreted within the
fibrous membrane and calcifies.
• Osteoblasts begin to secrete osteoid, which is calcified
within a few days.
• Trapped osteoblasts become osteocytes.
Figure 6.8, (2 of 4)
Mesenchyme
condensing
to form the
periosteum
Trabeculae of
woven bone
Blood vessel
Woven bone and periosteum form.
• Accumulating osteoid is laid down between embryonic
blood vessels in a random manner. The result is a network
(instead of lamellae) of trabeculae called woven bone.
• Vascularized mesenchyme condenses on the external face
of the woven bone and becomes the periosteum.
3
Figure 6.8, (3 of 4)
Fibrous
periosteum
Osteoblast
Plate of
compact bone
Diploë (spongy
bone) cavities
contain red
marrow
4 Lamellar bone replaces woven bone, just deep to
the periosteum. Red marrow appears.
• Trabeculae just deep to the periosteum thicken, and are later
replaced with mature lamellar bone, forming compact bone
plates.
• Spongy bone (diploë), consisting of distinct trabeculae, persists internally and its vascular tissue becomes red marrow.
Figure 6.8, (4 of 4)
Formation of Bony Skeleton
•
•
•
•
Endochondrial Ossification –
Essentially all bones
2nd month of embryonic development
Hyaline cartilage “bones” formed as patterns
for bone construction
• Hyaline cartilage must be broken down as
ossification proceeds
Endochondrial Ossification
• Long Bone –
• Begins at center of hyaline cartilage shaft at
region – primary ossification center
• Pericardium covering hyaline cartilage is
infiltrated with blood vessels
• Converted to vascularized periosteum
• Mesenchymal cells specialize into osteoblasts
Month 3
Week 9
Birth
Childhood to
adolescence
Articular
cartilage
Secondary
ossification
center
Epiphyseal
blood vessel
Area of
deteriorating
cartilage matrix
Hyaline
cartilage
Spongy
bone
formation
Bone
collar
Primary
ossification
center
1 Bone collar
Spongy
bone
Epiphyseal
plate
cartilage
Medullary
cavity
Blood
vessel of
periosteal
bud
2 Cartilage in the
3 The periosteal
forms around
center of the
hyaline cartilage diaphysis calcifies
model.
and then develops
cavities.
bud inavades the
internal cavities
and spongy bone
begins to form.
4 The diaphysis elongates
and a medullary cavity
forms as ossification
continues. Secondary
ossification centers appear
in the epiphyses in
preparation for stage 5.
5 The epiphyses
ossify. When
completed, hyaline
cartilage remains only
in the epiphyseal
plates and articular
cartilages.
Figure 6.9
Week 9
Hyaline cartilage
Bone collar
Primary
ossification
center
1
Bone collar forms around
hyaline cartilage model.
Figure 6.9, step 1
Area of deteriorating
cartilage matrix
2 Cartilage in the center
of the diaphysis calcifies
and then develops cavities.
Figure 6.9, step 2
Month 3
Spongy
bone
formation
Blood
vessel of
periosteal
bud
3
The periosteal bud inavades
the internal cavities and
spongy bone begins to form.
Figure 6.9, step 3
Birth
Epiphyseal
blood vessel
Secondary
ossification
center
Medullary
cavity
4
The diaphysis elongates and a medullary cavity forms
as ossification continues. Secondary ossification centers
appear in the epiphyses in preparation for stage 5.
Figure 6.9, step 4
Childhood to adolescence
Articular cartilage
Spongy bone
Epiphyseal plate
cartilage
5
The epiphyses ossify. When completed, hyaline cartilage
remains only in the epiphyseal plates and articular cartilages.
Figure 6.9, step 5
Month 3
Week 9
Birth
Childhood to
adolescence
Articular
cartilage
Secondary
ossification
center
Epiphyseal
blood vessel
Area of
deteriorating
cartilage matrix
Hyaline
cartilage
Spongy
bone
formation
Bone
collar
Primary
ossification
center
1 Bone collar
Spongy
bone
Epiphyseal
plate
cartilage
Medullary
cavity
Blood
vessel of
periosteal
bud
2 Cartilage in the
3 The periosteal
forms around
center of the
hyaline cartilage diaphysis calcifies
model.
and then develops
cavities.
bud inavades the
internal cavities
and spongy bone
begins to form.
4 The diaphysis elongates
and a medullary cavity
forms as ossification
continues. Secondary
ossification centers appear
in the epiphyses in
preparation for stage 5.
5 The epiphyses
ossify. When
completed, hyaline
cartilage remains only
in the epiphyseal
plates and articular
cartilages.
Figure 6.9
Formation of Bony Skeleton
• Now ossification begins
1. Bone Collar is laid down around diaphysis of
hyaline cartilage model
- Osteoblasts secrete osteoid against hyaline
cartilage encasing it in bone
- Periosteal bone collar
Formation of Bony Skeleton
2. Cartilage in center of diaphysis calcifies and
then develops cavities
- Collar forms – chondrocytes enlarge
- Shaft hypertrophy and signal to surrounding
cartilage to calcify
- Calcified cartilage impermeable to nutrients –
chondrocytes die and matrix begins to
deteriorate
- Opens up cavities
- Elsewhere cartilage remains healthy and begins
to grow
Formation of Bony Skeleton
3. Periosteal bud invades internal cavities and
spongy bones form
- 3 months – forming cavities are invaded by a
collection of elements called periosteal bud –
contains nutrient artery and views, lymph vessels,
nerve fibers, red bone marrow, osteoblasts, and
osteoclasts
- Osteoclasts – erodes calcified cartilage matrix
- Osteoblasts – secrete osteoid around remaining
fragments of hyaline cartilage
- Forms bone covered cartilage trabeculae
Formation of Bony Skeleton
4. The Diaphysis elongates and a medullary cavity forms.
- Primary ossification center enlarges, osteoclasts break
down newly formed spongy bone and open medullary
cavity in center of diaphysis
- Week 9 to births – rapid growing
- Epiphysis consist of only of cartilage and hyaline
cartilage models continue toe elongate by division of
viable cells
- Ossification “chases” cartilage formation along the
length of the shaft
- As cartilage calcifies, is eroded, and replaced by bony
spicules
Formation of Bony Skeleton
5. The epiphysis ossify shortly before or after birth
- Secondary ossification canters appear in 1 or
both epiphysis
- Gain bony tissue
- Cartilage center calcifies in epiphysis and
deteriorates – opens up cavities that allow
postiosteal bud to enter
- Reproduces events of primary ossification except
spongy bone – interior is retained and no
medullary cavity formed
Formation of Bony Skeleton
• When complete – hyaline at only 2 places
1. Epiphyseal surfaces – articular cartilage
2. At junction of diaphysis and epiphysis – forms
ephysygeal plates
Postnatal Bone Growth
• Infancy and youth – bone lengthens
• Interstitial growth of epiphyseal plate
• Thickness – appositional growth
• Most growths tops during adolescence
• Facial bones – nose and jaw – continue to
grow
Postnatal Bone Growth
• Growth in Length of Long Bones
• Mimics endochonrial ossification
• Cartilage inactive on side of epiphyseal placte
facing epiphysis – resting or quiescent zone
• Other side – organizes into pattern that allows
fast, efficient growth
• Cartilage – tail column, tall columns cells at top –
proliferating or growth zone
• Cells divide – quickly pushing epiphysis away
from daiphysis
Growth in Length of Long Bones
• At same time – older chondrocytes –
hypertrophy and lacunea erode and enlarge –
leaving interconnected spaces
• Matrix calcifies and chondrocytes die and
deteriorate – calcification zone
• Leaves long slender spicules of calcified
cartilage
• Become part of ossification or osteogenic zone
Resting zone
Proliferation zone
Cartilage cells undergo
mitosis.
1
Hypertrophic zone
Older cartilage cells
enlarge.
2
Calcified cartilage
spicule
Osteoblast depositing
bone matrix
Osseous tissue
(bone) covering
cartilage spicules
Calcification zone
Matrix becomes calcified;
cartilage cells die; matrix
begins deteriorating.
3
4 Ossification zone
New bone formation is
occurring.
Figure 6.10
Growth in Length of Long Bones
• Ossification zone – invaded by marrow
elements
• Cartilage eroded by osteoclasts and ultimately
replaced by spongy bone
• Epiphyseal plate remains constant thickness
because the rate of cartilage growth is
balanced by its replacement with bony tissue
Growth in Length of Long Bones
• Longitudinal growth accompanied by continuous
remodeling to maintain proper proportions
• Remodeling – new bone formation and bone
reabsorption (destruction)
• Adolescence ends - chondroblasts of epiphyseal
plates divide less
• Plates become thinner and thinner until replaced
by bone tissue
• Epiphyseal plate closure – 18 years female
• 21 years male
Growth in Width (thickness)
• Widen as they lengthen
• Increase in thickness or in long bones –
diameter
• Osteoblasts beneath the periosteum secrete
bone matrix on external bone surface as
osteoclasts remove bone on endosteal surface
• Less breaking down than building up – uneven
– creates thicker, stronger bone
Hormonal Regulation of Bone Growth
• Symphony of hormones
• Infancy and childhood – growth hormones modulated
by thyroid hormones
• At puberty – male and female sex hormones –
testosterone and progesterone released in increasing
amounts
• Beginning promote bone growth, then seal the plate at
the end of puberty
• Excess or Defects –
• Hypersecretion of growth hormone – gigantism
• Hyposecretion of growth hormone - dwarfism
Bone Homeostasis – Remodeling and
Repair
•
•
•
•
Bone – appears as a lifeless organ
Really dynamic and active tissue
Small scale changes continuously
Recycle ~5-7% of its mass everyday, ~0.1 g
calcium
• Spongy bone replaced every 3-4 years
• Compact bone replaced every 10 years
• Remains in place - long periods – calcium salts
crystallize – bone becomes brittle
Bone Remodeling
•
•
•
•
•
•
•
Bone appears to be a lifeless organ
Actually dynamic and active tissue
Small scale changes continuously
Recycle ~ 5-7% of mass each day, ~0.1 g Calcium
Spongy bone replaced every 3-4 years
Compact bone replaced every 10 years
Remains in place – long periods – Calcium salts
crystallize bone becomes brittle
Remodeling
• Bone deposit and reabsorption occur at surface
of periosteum and endosteum
• 2 processes – bone remodeling – coupled and
coordinated by “packets” of adjacent osteoblasts
and osteoclasts called remodeling units
• Healthy adults – total bone mass remains
constant
• Deposit = reabsorption
• Remodeling not uniform
Bone Deposit
• Occurs when a bone is injured or added bone is
required
• For optimal bone deposit – healthy diet rich in
proteins, vitamin C, D, A, and several minerals –
calcium, magnesium, phosphorus, and
manganese
• New matrix deposits by osteocytes marked by the
presence of osteiod seam – unmineralized band
of gauzy-looking bone matrix 10-12 µm wide
Bone Deposit
• Calcification front – between osteoid seam and
older bone – changes unmineralized to
mineralized
• Calcification –
• Precise trigger unknown
• Critical factor – local concentration of calcium
and phosphate
• Ca * P product – crystallization
• Alkaline phosphatase – shed by osteoblasts,
essential for mineralization
Bone Reabsorption
• Accomplished by osteoclats
• Arise from hemapoitic stem cells that
differentiate into macrophages
• Move along bone surface digging grooves as
they go
• Part that touches bone – highly folded, clings
to bone, seals off area of bone destruction
Bone Reabsorption
• Ruffled Border Secretes –
1. Lysosomal enzymes – digest organic matrix
2. Hydrochloric acid – converts calcium salts
into soluble forms
- Also phagocytise demineralized matrix and
dead osteocytes
- Products released into interstitial fluid and
blood
Control of Remodeling
• Regulated by 2 control loops
1. Negative feedback that maintains calcium homeostasis in
the blood
2. Other acts in response to mechanical and gravitational
forces acting on skeleton
Calcium- required for nerve impulses
- Muscle contraction
- Blood coagulation
- Secreted by glands and nerve cells
- Cell division
Daily requirements – birth  10 years old - ~400-800 mg/day
Ages 11  24 ~ 1200 – 1500 mg/day
Hormonal Control of Blood Ca2+
• May be affected to a lesser extent by calcitonin
 Blood Ca2+ levels

Parafollicular cells of thyroid release calcitonin

Osteoblasts deposit calcium salts

 Blood Ca2+ levels
• Leptin has also been shown to influence bone density by
inhibiting osteoblasts
Hormonal Control
• Parathyroid hormone (PTH)
• Calcitonin
– PTH released when blood levels of calcium are low
– Stimulated reabsorption of bone  increases
blood calcium levels
– Blood calcium level low for long periods – bone
demineralized
• Leptin – hormone from adipose tissue – role
in bone density – inhibits osteoblasts
Calcium homeostasis of blood: 9–11 mg/100 ml
BALANCE
BALANCE
Stimulus
Falling blood
Ca2+ levels
Thyroid
gland
Osteoclasts
degrade bone
matrix and
release Ca2+
into blood.
Parathyroid
glands
PTH
Parathyroid
glands release
parathyroid
hormone (PTH).
Figure 6.12
Mechanical Stress
• Response to stress (muscle pull) and gravity
• Wolff’s Law – bone grows or remodels in
response to demand placed on it
• Mechanical sensors – ex. Long bone thickest
midway through where bending stresses are
the greatest
Load here (body weight)
Head of
femur
Tension
here
Compression
here
Point of
no stress
Figure 6.13
Mechanical Stress
• Wolff’s law –
1. Handedness – results in bone of upper limb
being thicker
2. Curved bones - thickest where they are most
likely to buckle
3. Trabeculae of spongy bones – forms trusses
along lines of compression
4. Large bony projections form where heavy active
muscles attach
- Suggested that electrical signals – direct bone
remodeling
Bone Repair
•
•
•
•
•
Susceptible to fractures or breaks
Most result from trauma –
Increased risk –
Excessive vitamin A intake
Excessive amino acid derivative –
homocysteine
• Old age
Bone Repair
• Fractures classified by
1. Position of bone after fracture
-
Non displaced – bone ends at normal position
Displaced – out of alignment
2. Completeness
-
Complete – bone broken through
Incomplete – not broken through
3. Orientation of break relative to long axis of bone
-
Linear – parallel to axis
Transverse – perpendicular to axis
4. Whether bone ends penetrate skin
-
Open (complete) fracture – penetrates skin
Closed (simple) fracture – does not penetrate skin
Table 6.2
Table 6.2
Table 6.2
Bone Repair
• Treated by reduction –
• Realignment of bone
– Closed (external) reduction – ends coaxed into
position
– Open (internal) reduction – secured together
surgically
• Cast/traction – allow healing
• Simple fracture – 6 -8 weeks needed for healing
• Longer for large weight bearing bones and bones
of elderly
Repair
1. Hematoma forms – mass of clotted blood
forms at fracture site
- Tissue swollen and painful
A hematoma forms.
Repair
2. Fibrocartilagous callus forms – within a few days
- Events lead to the formation of soft granulation
tissue – soft callus
- Capillaries grow in hematoma and phagocytic
cells invade area
- Fibroblasts and osteoblasts – invade and begin
reconstructing area
- Produce collage fibers
- Osteblast convert collagen to spongy bone
- Fibrocartilaginous cells – splint bone
External
callus
Internal
callus
(fibrous
tissue and
cartilage)
New
blood
vessels
Spongy
bone
trabecula
2 Fibrocartilaginous
callus forms.
Figure 6.15, step 2
Repair
3. Bony callus Forms –
- New trabeculae begins to appear and
gradually convert to bone (hard) callus
- Continued until firm union formed ~ 2 months
Bony
callus of
spongy
bone
3 Bony callus forms.
Figure 6.15, step 3
Repair
4. Bone Remodeling –
- Bony callus remodeled
- Excess material removed
- Compact bone laid down
- Resembles original bone
Healed
fracture
4 Bone remodeling
occurs.
Figure 6.15, step 4
Hematoma
Internal
callus
(fibrous
tissue and
cartilage)
External
callus
New
blood
vessels
Bony
callus of
spongy
bone
Healed
fracture
Spongy
bone
trabecula
1 A hematoma forms. 2 Fibrocartilaginous
3 Bony callus forms.
callus forms.
4 Bone
remodeling
occurs.
Figure 6.15
Homeostatic Imbalances of Bone
• Osteomalacia
• Number of disorders in which bones are
inadequately mineralized
• Osteiod produced but calcium salts not
deposited
• Leaves bone brittle
Homeostatic Imbalances of Bone
• Rickets – osteomalacia in children
• You bones growing bow legs, deformities in
pelvis, skull and ribcage
• Caused by insufficient calcium in diet or
vitamin D deficiency
• Can be treated by vitamin D in milk and sun
exposure
Homeostatic Imbalances of Bone
• Osteoporesis –
• Diseases where bone reabsorption outpaces
bone deposit
• Bones fragile
• Composition of matrix normal but mass is
reduced
• Bones porous and light
• More in elderly women , men also but less
• 30 % women 60-70, 70% over 80
• 30% will have a fracture
Figure 6.16
Homeostatic Imbalances of Bone
• Osteoporesis –
• Sex hormones – estrogen help maintain healthy density of
bone
• After menopause, estrogen decreases
• Treated with calcium and vitamin D supplements, exercise,
and drugs
• Drugs – HRT-estrogen
• New drugs – alendronate – decreases osteoclast activity
– Raloxifene – mimic estrogen
– Statins – meant to decrease cholesterol, also increase bone
mineralization
• Prevention/Delay – increase calcium while bone are
increasing in density, fluorinated water  increases bone
hardness, exercise
Homeostatic Imbalances of Bone
• Pagat’s Disease – excessive and haphazard bone
deposit and reabsorption
• New bone – hastily made – high ratio of spongy
to compact bone
• Reduced mineralization
• Most often – spine, pelvis, femur, and skull
• Rarely occurs before age 40
• ~ 3% of elderly in North America
• Drug therapies – prevent breakdown
Developmental Aspects
•
•
•
•
•
•
Develops from the mesoderm
Each bone has its own schedule
Begin ossifying ~ 8 weeks
At birth most bones are ossified
Long bone growth continues till adulthood at plates
Children/Adolescent – bone formation exceeds
reabsorption
• Adult – balance
• Old Age – reabsorption greater
• ~40 – bone mass starts to decease with age
Parietal bone
Occipital bone
Mandible
Frontal bone
of skull
Clavicle
Scapula
Radius
Ulna
Ribs
Humerus
Vertebra
Ilium
Tibia
Femur
Figure 6.17