Chapter 6:
Osseous Tissue
and Bone Structure
1
The Skeletal System
• Skeletal system includes:
– bones of the skeleton
– cartilages, ligaments, and other
connective tissues that stabilize the bones
2
Skeletal System
Functions:
1. Support: framework & structure of body
2. Storage of minerals and lipids
Minerals: calcium and phosphate
- for osmotic regulation, enzyme function,
nerve impulses
Yellow marrow: triglycerides
3. Blood cell production: all formed elements
- red marrow: stem cells  hematopiesis
4. Protection: surround soft tissues
5. Leverage for movement:
- levers upon which skeletal muscles act
3
Classification of Bones
•
Bone are identified by:
–
–
–
shape
internal tissues
bone markings
SHAPE:
1. Long bones
2. Flat bones
3. Sutural bones
4. Irregular bones
5. Short bones
6. Sesamoid bones
4
Shape of Bones
1. Long Bones:
-
Longer than wide, consist of shaft
and 2 ends
e.g. bones of appendages
2. Short Bones:
-
Approx. equal in all dimensions
e.g. carpals, tarsals
3. Flat Bones:
-
Thin, 2 parallel surfaces
e.g. skull, sternum, ribs, scapula
5
Figure 6–1a
Shape of Bones
4. Irregular Bones:
-
Complex shapes
E.g. vertebrae, os coxa
5. Sesamoid Bones:
-
Seed shaped, form in tendon
E.g. patella, total number can vary
6. Sutural Bones:
- Extra bones in sutures of skull
6
Bone Structure
• A bone is an organ consisting of many tissue
types:
–
Osseous, nervous, cartilage, fibrous CT, blood, etc.
All bones consist of 2 types of bone tissue
1. Compact bone:
- solid, dense bone, makes up surfaces and shafts
2. Spongy Bone/Cancellous bone:
- meshy, makes up interior of bones, houses red
marrow in spaces
7
Bone Markings
• Bones are not flat on the surface:
– Have projections, depressions, and holes for
muscle attachment, blood & nerve supply
• Depressions or grooves:
– along bone surface
• Projections:
– where tendons and ligaments attach
– at articulations with other bones
• Tunnels:
– where blood and nerves enter bone
8
Bone Markings
9
Table 6–1 (2 of 2)
Long Bones Structure
1. Diaphysis:
- Hollow shaft of compact bone
2. Medullary (marrow) cavity:
-
Center of diaphysis, contains
yellow marrow
Triglycerides for energy reserve
3. Epiphysis:
-
Expanded end of bone, surface of
compact bone
Center filled with spongy bone with
red marrow in spaces
-
Produces blood cells
10
Figure 6–2a
Long Bones Structure
4. Epiphyseal line or plate:
-
Cartilage that marks connection of
diaphysis with epiphysis
Line: adults, narrow (aka metaphysis)
Plate: thick, allows growth during
childhood
5. Periosteum:
-
2 layer covering around outside of bone:
-
Outer Fibrous Layer
Inner Cellular Layer
6. Endosteum:
-
Cellular layers, covers all inside surfaces
11
12
7. Articular Cartilage:
-
Hyaline cartilage on end where bone contacts another,
no periosteum or perichondrium
Joint/Articulation:
- connection between two bones, surrounded by
CT capsule, lined with synovial membrane
Joint cavity filled with synovial fluid to reduce
friction on articular cartilage
13
Flat Bone Structure
• Thin layer of spongy bone with red marrow
between two layers of compact bone
• Covered by periosteum and endosteum
• Site of most hematopoiesis
– Production of blood cells and cell fragments
that are suspended in plasma (RBC, WBC, and
platelets
14
Characteristics of Bone Tissue
• Periosteum:
– covers outer surfaces of bones
– consist of outer fibrous and inner cellular
layers
• Endosteum:
– Inner, cellular layer of periosteum
15
Bone Histology
• Bone = osseous tissue, supporting CT
• Consists of specialized cells in a matrix of
fibers and ground substance
• Characteristics of bone:
1.
2.
3.
4.
Dense matrix packed with calcium salts
Osteocytes in lacunae
Canaliculi for exchange of nutrients and waste
Two layer periosteum, covers bone except at
articular surfaces
16
Bone Histology
• Matrix = 98% of bone tissue
–
1/3 = osteoid; organic part:
• Collagen fibers + ground substance
• Tough and flexible
–
2/3 = densely packed crystals of hydroxyapatite
(calcium salts, mostly calcium phosphate)
• Hard but brittle
• Cells = only 2% of bone
1.
2.
3.
4.
Osteocytes
Osteoblasts
Osteoprogenitor cells
Osteoclasts
17
Cells located in Bones
1. Osteocytes = mature bone cells
-no cell division
-located in lacunae between layers of matrix called lamellae
-canaliculi link lacunae to each other and blood supply
-osteocytes linked to each other via gap junctions on cell
projections in canaliculi:
- allow exchange of nutrients and wastes
-Function
1. To maintain protein and mineral content of matrix
2. Can also participate in bone repair:
-become stem cell like when broken free of lacuna
18
Canaliculi
Osteocytes
in lacunae
Blood
vessels
Central canal
PERIOSTEUM
Fibrous
layer
Cellular
layer
Matrix
LM X 362
19
Cells located in Bones
2. Osteoblasts - Immature bone cells
-
Perform osteogenesis:
-
-
Produce osteoid
-
-
Formation of new bone matrix
Organic components of matrix that is not yet
calcified to form bone
Promote deposit of calcium salts which
spontaneously form hydroxyapatite
Once enclosed in lacuna by matrix,
osteoblast differentiates into osteocyte and
20
no longer produces new matrix
21
Cells located in Bones
3. Osteoprogenitor Cells – mesenchymal cells
- bone stem cell that produces daughters
- daughters become osteoblasts for
repair and growth
- located in endosteum and inner periosteum
22
23
Cells located in Bones
4. Osteoclasts
- large, multinuclear
- derived from monocytes (macrophages)
- perform osteolysis =
- digest and dissolve bone matrix
- release minerals:
1. For use in blood or
2. Recycling during bone remodeling
24
25
Cells located in Bones
Canaliculi Osteocyte
Matrix
Osteoid
Osteoblast
Osteoprogenitor
cell Matrix
Osteoclast Matrix
Marrow
cavity
Osteocyte: Mature bone
cell that maintains the
bone matrix
Osteoblast: Immature
bone cell that secretes
organic components
of matrix
Osteoprogenitor cell:
Stem cell whose divisions
produce osteoblasts
Osteoclast: Multinucleate
cell that secretes acids and
enzymes to dissolve bone matrix
26
Homeostasis
• Bone building (by osteocytes) and bone
recycling (by osteoclasts) must
balance:
– more breakdown than building, bones
become weak
– exercise causes osteocytes to build bone
27
How would the strength of a bone be
affected if the ratio of collagen to
hydroxyapatite increased?
1. Strength increases, flexibility
increases.
2. Strength increases, flexibility
decreases.
3. Strength decreases, flexibility.
decreases.
4. Strength decreases, flexibility
increases.
28
If the activity of osteoclasts exceeds the
activity of osteoblasts in a bone, how will
the mass of the bone be affected?
1. stable mass, but re-positioned
matrix
2. mass will not be affected
3. more mass
4. less mass
29
The difference between
compact bone
and spongy bone.
30
Structure of Compact Bone
• Consists of osteons:
– Parallel to surface
• Each osteon is around a central canal:
– Contains blood vessels and nerves
• Perforating canals perpendicular to osteons act to
connect the osteons
• Osteon is built of layers of matrix secreted by
osteoblasts
– Each layer = concentric lamella
• Osteocytes are located in lacunae between
lamellae
• Ostocytes are connected to neighboring cells and
central canal via canaliculi
31
Structure of Compact Bone
• Interstitial lamellae fill spaces between
osteons
• Circumferiential lamellae run perimeter
inside and out in contact with:
– endosteum and periosteum
• Compact bone is designed to receive stress
from one direction
– Very strong parallel to osteons
– Weak perpendicular to osteons
32
Compact Bone
33
Figure 6–5
Structure of Spongy Bone
• Lamellae = meshwork called trabeculae (no
osteons)
• Red marrow fills spaces around trabeculae
• Osteocytes in lacunae are linked by canaliculi
• No direct blood supply (no central canals)
• Nutrients diffuse into canaliculi in trabeculae from
red marrow
• Spongy bone make up:
– low stress bones
– Areas of bone where stress comes from multiple
directions
• Provide light weigh strength
34
Bone Marrow
• Red Marrow:
–
–
–
–
Located in space between trabeculae
Has blood vessels
Forms red blood cells
Supplies nutrients to osteocytes
• Yellow Marrow:
– In some bones, spongy bone holds yellow
bone marrow:
• is yellow because it stores fat
35
Structure of Spongy Bone
36
Periosteum and Endosteum
• Compact bone is covered with
membrane:
– periosteum on the outside
– endosteum on the inside
37
Periosteum
1. Fibrous outer layer:
-
Dense irregular CT
2. Cellular Inner layer:
-
Osteoprogenitor cells
Functions:
1. Isolate bone from surrounding tissues
2. Site for attachment for tendons and ligaments
3. Route for nerves and blood vessels to enter bone
4. Participates in bone growth and repair
38
39
Endosteum
• Thin cellular layer
• Lines medullary cavity, central canals, and
covers trabeculae
• Consists of:
– osteoblasts, osteoprogenitor cells, and
osteoclasts
• Cells become active during bone growth
and repair
40
Endosteum
41
Figure 6–8b
Bone Growth
• Begins 6-8 weeks post fertilization
• Continues through puberty (18-25 y)
• Osteogenesis = ossification = formation of
bone
• Not calcification
– Hardening of matrix or cytoplasm with calcium
– Can happen to many tissues
• Two types of Ossification:
1. Intramembranous: forms flat bones
2. Endochondrial: forms long bones
42
Bone Development
• Human bones grow until about age 25
• Osteogenesis:
– bone formation
• Ossification: Deposition of calcium salts
– the process of replacing other tissues with
bone
43
The difference between
intramembranous ossification
and endochondral ossification.
44
Intramembranous Ossification
• Bone develops from mesenchyme or
fibrous CT in deep layers of dermis
• Also called dermal ossification:
– because it occurs in the dermis
– produces dermal bones such as mandible and
clavicle
– Produces skull bones
• There are 4 main steps in
intramembranous ossification
45
Intramembranous
Ossification: Step 1
• Ossification center appears in the fibrous CT
membrane
– Mesenchymal cells aggregate
– Differentiate into osteoblasts
– Begin ossification at the ossification center
46
Intramembranous
Ossification: Step 2
• Bone matrix (osteoid) is secreted within the fibrous
membrane
– Osteoblasts begin to secrete osteoid, which is mineralized
within a few days
– Trapped osteoblasts become osteocytes
47
Intramembranous
Ossification: Step 3
• Woven bone and periosteum form
– Accumulating osteoid is laid down between embryonic
blood vessels, which form a random network
– Vascularized mesenchyme condenses on the external face
of the woven bone and becomes periosteum around spongy
bone
48
Intramembranous
Ossification: Step 4
• Bone collar of compact bone forms and red marrow
appears
– Trabeculae just deep to the periosteum thickens, forming
a woven bone collar that is later replaced with mature
lamellar bone
– Spongy bone, consisting of distinct trabeculae, persists
internally and its vascular tissue becomes red marrow
49
Endochondral Ossification
• Ossifies bones that originate as hyaline
cartilage
• Most bones originate as hyaline
cartilage
– Cartilage grows by interstitial and
appositional growth
– Cartilage is slowly replaced from the
inside out
50
Endochondral Ossification
• Growth and ossification of long bones
occurs in 6 steps
51
Endochondral
Ossification: Step 1
• Primary ossification center
begins to form:
– Chondrocytes in the center of
hyaline cartilage:
• Enlarge in diaphysis
• Surrounding matrix calcifies
killing the enclosed chondrocytes
• die, leaving cavities in cartilage
52
Figure 6–9 (Step 1)
Endochondral
Ossification: Step 2
• Blood vessels grow around the edges
of the cartilage
– Cells in the perichondrium change to
osteoblasts:
• Secrete osteoid
– Osteiod is mineralized and produces a
layer of superficial bone around the
shaft which will continue to grow
around the diaphysis and become
compact bone (appositional growth)
53
Figure 6–9 (Step 2)
Endochondral
Ossification: Step 3
• Capillaries and fibroblast
migrate into the primary
ossification center:
– Blood vessels enter the cartilage
– Bringing fibroblasts that become
osteoblasts and secrete osteoid
• Mineralized into rebeculae
– Spongy bone develops at the
primary ossification center and
continues to growth toward the
epiphysis
54
Figure 6–9 (Step 3)
Endochondral
Ossification: Step 4
• Remodeling creates a marrow
cavity:
– Osteoclasts degrade trabeculae in
the center to create the marrow
cavity
– Bone increases in length by
interstital growth of the epiphyseal
plate followed by replacement of
plate cartilage by spongy bone
• Cartilage continues to grow on
epiphyseal side and is replaced
by bone on diaphysis side
– Bone increases in diameter by
appositional growth from cellular
55
layers of peristeum
Figure 6–9 (Step 4)
Endochondral
Ossification: Step 5
• Secondary ossification
centers form in epiphyses:
– Capillaries and osteoblasts
enter the epiphyses:
• creating secondary
ossification centers
56
Figure 6–9 (Step 5)
Endochondral
Ossification: Step 6
• Epiphyses become ossified
with spongy bone
– Hyaline cartilage remains on
articular surfaces (not
calcified or ossified)
– Ossification continues at
both 1°and 2° ossification
centers until all epiphyseal
cartilage has been replaced
with bone  epiphyseal
closure
– Adult bone retains the
epiphyseal line
57
Figure 6–9 (Step 6)
Endochondral
Ossification
• Appositional growth:
– compact bone thickens and
strengthens long bone with
layers of circumferential
lamellae
58
Figure 6–9 (Step 2)
During intramembranous ossification,
which type(s) of tissue is/are
replaced by bone?
1.
2.
3.
4.
hyaline cartilage
fibrous connective tissue
mesenchymal connective tissue
osteoid tissue
59
In endochondral ossification, what is the
original source of osteoblasts?
1. de novo synthesis
2. cells brought with via the
nutrient artery
3. cells of the inner layer of the
perichondrium
4. chondrocytes from the original
model
60
The characteristics of adult
bones.
61
Epiphyseal Lines
62
Figure 6–10
Epiphyseal Lines
• When long bone stops growing, after
puberty:
– epiphyseal cartilage disappears
– is visible on X-rays as an epiphyseal line
63
A child who enters puberty several years
later than the average age is generally
taller than average as an adult. Why?
1. Epiphyseal plates fuse during
puberty.
2. Bone growth continues throughout
childhood.
3. Growth spurts usually occur at the
onset of puberty.
4. All of the above.
64
The skeletal system remodels
and maintains homeostasis.
The effects of nutrition,
hormones, exercise, and aging
on bone.
65
Bone Remodeling
• Bones are not static: constantly recycled
and renewed
• 5-7% of skeleton is recycled/week
• Osteoclasts secrete:
1. Lysosomal enzymes: digest osteoid
2. Hydrochloric acid: solubilize calcium salts
• Osteoblasts secrete:
1. Osteoid (organic matrix)
2. Alkaline phosphatase: induces mineralization
of osteoid
- Complete mineralization takes ~1 week
66
Bone Remodeling
• Bones Adapt:
– Stressed bones grow thicker
– Bumps and ridges for muscle attachment enlarge
when muscles are used heavily
– Bones weaken with inactivity: up to 1/3 or mass
is lost with few weeks of inactivity
– Heavy metals can get incorporated
• Condition of bones depends on interplay
between osteoclast and osteoblast activity
67
Skeleton as a Calcium Reserve
• Calcium is important for normal function
of neurons and muscle
• Blood calcium: 9-11 mg/100ml
• If blood levels are too high:
– Nerve and muscle cells are non responsive
• If blood levels are too low:
– Nerve and muscle cells are hyper-excitable
 convulsions, death
68
The Skeleton as Calcium Reserve
• Bones store calcium and other minerals
• Calcium is the most abundant mineral in
the body
• Calcium ions are vital to:
– membranes
– neurons
– muscle cells, especially heart cells
69
Skeleton as a Calcium Reserve
• Calcium homeostasis depends on:
1. Storage in the Bones
2. Absorption in the GI
3. Excretion at the Kidneys
** These factors are controlled by hormones
to regulate blood calcium levels
70
If blood calcium levels Low:
•
Parathyroid hormone
(from parathyroid
gland) triggers:
1. Increase osteoclast
activity
- decrease storage
2. Enhanced calcitriol
action
- increase absorption
3. Decreased calcium
excretion at the
kidneys
71
If Blood Calcium levels High
•
Calcitonin (from
thyroid gland)
triggers:
1. Inhibition of
osteoclast activity
2. Increased calcium
excretion at the
kidneys
72
Nutritional and Hormone
Effects on Bone
•
Many nutrients and hormones are required for
normal bone growth and maintenance:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Calcium and phosphate salts
Calcitriol
Vitamin C
Vitamin A
Vitamin K and B12
Growth Hormones
Thyroxin
Estrogens and Androgens
Calcitonin
Parathyroid Hormone
73
Nutritional and Hormone
Effects on Bone
1. Calcium and phosphate salts
- From food, for mineralization of matrix
2. Calcitriol
- From kidneys, for absorption of calcium and phosphate
3. Vitamin C
- From food, for collagen synthesis and osteoblast
differentiation
4. Vitamin A
- From carotene in food, for normal bone growth in
children
5. Vitamin K and B12
- From food, for synthesis of osteoid proteins
74
Nutritional and Hormone
Effects on Bone
6. Growth Hormones
- From pituitary gland, for protein synthesis and cell
growth
7. Thyroxin
- From thyroid gland, for cell metabolism and osteoblast
activity
8. Estrogens and Androgens
- From gonads, for epiphyseal closure
9. Calcitonin
- From thyroid gland AND
10. Parathyroid Hormone
- From parathyroid gland, to regulate calcium and
phosphate levels in body fluids
75
- Affects bone composition
Hormones for Bone Growth
and Maintenance
76
Table 6–2
Abnormalities
• Genetic/Physiological Abnormalities
1. Giantism:
– too much Growth hormone prior to epiphyseal
closure, bones grow excessively large
2. Acromegaly:
- too much GH after closure, bones don’t
grow but all cartilage does
- ribs, nose, ears, articular cartilage
3. Pituitary Dwarfism:
- not enough GH, bones fail to elongate
77
Abnormalities
• Diet Related Abnormalities:
1. Scurvy:
- lack of Vit. C
- causes low collagen content, reduced bone
mass, bones brittle
2. Osteomalacia:
- lack calcitriol, osteoid produced but
not mineralized, bones flexible
-Called Rickets in children and leads to
permanent deformity
78
A seven-year-old child has a pituitary tumor involving
the cells that secrete growth hormone (GH), resulting
in increased levels of GH. How will this condition
affect the child’s growth?
1. The individual will be taller.
2. The individual will be shorter.
3. Growth of the individual will be
erratic and slow.
4. Excessive growth will be limited
to axial skeleton.
79
Why does a child who has rickets have
difficulty walking?
1. Joints become fused, preventing
movement.
2. Bones are brittle and break
under body weight.
3. Bones are flexible and bend
under body weight.
4. Motor skills are impaired.
80
What effect would increased PTH
secretion have on blood calcium levels?
1.
2.
3.
4.
higher level of calcium
lower level of calcium
uncontrolled level of calcium
no effect on blood calcium, PTH
effects calcium in the bones
81
How does calcitonin help lower the
calcium ion concentration of blood?
1. by inhibiting osteoclast activity
2. by increasing the rate of calcium
excretion at the kidneys
3. by increasing the rate of calcium
uptake by intestinal cells
4. 1 and 2
82
Types of fractures and
how do they heal.
83
Fractures
• Fractures:
– cracks or breaks in bones
– caused by physical stress
• Bones break in response to excessive
stress
• Bones are designed to heal
• Fractures are repaired in 4 steps
84
Fracture Repair: Step 1
• Bleeding:
– produces a clot (fracture
hematoma)
– Seals off dead osteocytes
and broken blood vessels
85
Figure 6–15 (Step 1)
Fracture Repair: Step 2
• Cells of the endosteum and
periosteum:
– Divide and migrate into fracture
zone
– Cells of Periosteum:
• create external callus of fibrocartilage
– Cells of Endosteum:
• create internal callus of spongy bone
• Calluses stabilize the break:
– external callus of cartilage and
bone surrounds break
– internal callus develops in marrow
cavity
86
Figure 6–15 (Step 2)
Fracture Repair: Step 3
• Osteoblasts:
– replace cartilage with spongy
bone
• Fracture gap is now filled
with all spongy bone
87
Figure 6–15 (Step 3)
Fracture Repair: Step 4
• A bulge from the callus marks the
fracture point
• Osteoblasts and osteocytes
remodel the fracture for up to a
year:
– Spongy bone is replaced with
compact bone and excess callus
material is removed
88
Figure 6–15 (Step 4)
The effects of aging on the
skeletal system.
89
Effects of Aging
• Bones become thinner and weaker with
age
1. Osteopenia = reduction in bone mass
– All adults suffer in some degree
– Osteoclasts out-work osteoblast
• sex hormones in youth inhibit osteoclasts
– Women: 8%/decade after 40
– Men: 3%/decade after 40
90
Effects of Aging
2. Osteoporosis = reduction in bone mass
that compromises function
• More common in women:
– Over age 45, occurs in:
• 29% of women
• 18% of men
– Thinner bones to start
– Greater rate of osteopenia
91
(a) Normal spongy bone
SEM X 25
92
(b) Spongy bone in osteoporosis
SEM X 21
Effects of Bone Loss
• The epiphyses, vertebrae, and jaws are
most affected:
– resulting in fragile limbs
– reduction in height
– tooth loss
93
Hormones and Bone Loss
• Estrogens and androgens help maintain
bone mass
• Bone loss in women accelerates after
menopause
94
Why is osteoporosis more common in
older women than in older men?
1. Testosterone levels decline in
post-menopausal women.
2. Older women tend to be more
sedentary than older men.
3. Declining estrogen levels lead to
decreased calcium deposition.
4. In males, androgens increase with
95
age.
SUMMARY (1 of 2)
•
•
•
•
•
•
•
•
•
Bone shapes, markings, and structure
The matrix of osseous tissue
Types of bone cells
The structures of compact bone
The structures of spongy bone
The periosteum and endosteum
Ossification and calcification
Intramembranous ossification
Endochondrial ossification
96
SUMMARY (2 of 2)
•
•
•
•
•
•
•
Blood and nerve supplies
Bone minerals, recycling, and remodeling
The effects of exercise
Hormones and nutrition
Calcium storage
Fracture repair
The effects of aging
97