Bone as a Tissue
Bone Tissue
• osteology – the study of bone
• tissues and organs of the skeletal
system
• skeletal system - composed of bones,
cartilages, ligaments, tendons
• histology of osseous tissue
– strong flexible framework of the body
– cartilage – forerunner of most bones
• bone development
• covers many joint surfaces of mature bone
• physiology of osseous tissue
• ligaments
• tendons
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7-2
Shapes of Bones
Functions of the Skeleton
Flat bones
Irregular bones
• long bones
Sphenoid bone
• support –
• short bones
• protection –
• flat bones
• movement –
Scapula
• irregular bones
Vertebra
• electrolyte balance –
Sternum
• acid-base balance –
Short bones
Long bones
Femur
• blood formation –
Ulna
Capitate
(carpal) bone
Talus
Radius
Figure 7.1
7-3
Anatom of a Long Bone
Articular
cartilage
Epiphysis
Red bone
marrow
Epiphyseal
line
Marrow cavity
Yellow bone marrow
Periosteum
Nutrient foramen
Diaphysis
Site of endosteum
Compact bone
Spongy bone
Epiphyseal
line
Epiphysis
Anatomy of a Flat Bone
• epiphyses and
diaphysis
• compact and spongy
bone
• marrow cavity
• articular cartilage
• Periosteum
- Inner: osteogenic
stem cells
- Outer: collagen
fibers
• Endostium
• Nutrient Foramen
• diploe – spongy layer
in the cranium
Suture
Outer compact
bone
Periosteum
– absorbs shock
– marrow spaces lined
with endosteum
Spongy bone
(diploe)
Trabeculae
Inner compact
bone
Periosteum
Figure 7.3
Articular
cartilage
(a) Living
7-4
(b) Dried
Epiphyseal plate or growth plate = hyaline cartilage
7-5
7-6
1
Histology of Osseous Tissue
Osteogenic cell
Osteoblast
Cells of Osseous Tissue
2. osteoblasts – bone forming cells
– single layer of cells under endosteum and periosteum
– are nonmitotic
– synthesize soft matrix which hardens by mineral deposition
– stress and fractures stimulate osteogenic cells
Osteocyte
Rough
endoplasmic
reticulum
Secretory
vesicles
Nucleus
Mitochondrion
Figure 7.4a
(a) Osteocyte development
• Connective tissue!!!!!!! cells, fibers and ground substance
• four principal types of bone cells
–
–
–
–
osteogenic (osteoprogenator) cells
osteoblasts
osteocytes
osteoclasts
3. osteocytes – former osteoblasts that have become trapped in the
matrix
– lacunae – tiny cavities where osteocytes reside
– canaliculi – little channels that connect lacunae
– cytoplasmic processes reach into canaliculi
1. osteogenic (osteoprogenator) cells - stem cells found in
endosteum, periosteum, and in central canals
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7-8
Cells of Osseous Tissue
The Matrix
• matrix of osseous tissue (by dry weight), ~ one-third
organic and ~ two-thirds inorganic matter
Osteocyte
Osteoclast
Stem cells
Osseous
tissue
Resorption
bay
Periosteum
Nuclei
• organic matter – synthesized by osteoblasts
– collagen, carbohydrate – protein complexes, such as
glycosaminoglycans, proteoglycans, and glycoproteins
Osteoclast
Fusion
Lysosomes
Ruffled
border
(b) Osteoclast development
3. 4.
• inorganic matter
– 85% hydroxyapatite (crystallized calcium phosphate
salt)
– 10% calcium carbonate
– other minerals (fluoride, sodium, potassium,
magnesium)
osteoclasts
– Arise from bone marrow stem cells
– unusually large cells
– ruffled border – side facing bone surface
– remodeling – results from combined action of the bone-dissolving
osteoclasts and the bone-depositing osteoblasts
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7-10
Compact Bone
• nutrient foramina – on
bone surface
Osteon is the functional unit of compact bone
Nerve
Blood vessel
• perforating (Volkmann’s)
canals – transverse or
diagonal canals
Spicules
Lamella
Trabeculae
Lacunae
Spongy bone
Endosteum
• central canals – vertical
canals
Periosteum
Perforating fibers
Perforating canal
Canaliculi
• circumferential lamellae
Central
canal
Central
canal
• interstitial lamellae
Osteon
Lacuna
Collagen
fibers
Concentric
lamellae
Circumferential
lamellae
20 m
(b)
Figure 7.5b
7-12
2
Bone Marrow
Spongy Bone
• red marrow (myeloid tissue)
– in nearly every bone in a child
– hemopoietic tissue - produces
• sponge-like appearance
blood cells and is composed of
multiple tissues
– in adults, found in skull, vertebrae,
ribs, sternum, part of pelvic girdle,
and proximal heads of humerus
and femur
• spongy bone consists of:
– slivers of bone called spicules
– thin plates of bone called trabeculae
– spaces filled with red bone marrow
• yellow marrow found in adults
• few osteons and no central canals
• provides strength with minimal weight
– trabeculae develop along bone’s lines of stress
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7-14
Intramembranous Ossification
Bone Development
Mesenchymal cell
Osteocyte
Osteoblasts
• ossification or osteogenesis – the formation of
bone
Trabecula
Sheet of condensing
mesenchyme
Calcified bone
Osteoid tissue
Blood capillary
Fibrous periosteum
2 Deposition of matrix by osteoblasts
on mesenchymal surface; entrapment of first
osteocytes; formation of periosteum
1 Condensation of mesenchyme cells
permeated with blood capillaries – turn
Into osteoblast cells
• human fetus and infant, bone develops by two
methods:
1. intramembranous ossification
Osteoblasts
Fibrous periosteum
Trabeculae
Osteoblasts
Osteocytes
Spongy bone
Marrow cavity
Compact bone
2. endochondral ossification
Figure 7.8
3 bony trabeculae formed by continued
mineral deposition; creation of
spongy bone
7-15
Endochondral Ossification
4 Surface bone filled in by bone deposition,
converting spongy bone to compact bone.
Persistence of spongy bone in the middle layer.
produces flat bones of skull and clavicle
7-16
Fetal Skeleton at 12 Weeks
Articular
cartilage
Spongy bone
Epiphyseal
line
Cranial
bones
Perichondrium
Secondary
marrow cavity
Hyaline
cartilage
1 Early cartilage model
Nutrient
foramen
Epiphysis
Secondary
ossification
center
Periosteum
Epiphyseal
plate
Mandible
Marrow cavity
Metaphysis
Compact bone
Enlarging
chondrocytes
Bony collar
Primary
ossification
center
Blood
vessel
Radius
Ulna
Scapula
Metaphysis
Secondary
ossification
center
Periosteum
2 Formation of
primary
ossification center,
bony collar, and
periosteum
Vertebrae
Humerus
Diaphysis
Primary
marrow
cavity
3 Vascular invasion,
formation of primary
marrow cavity, and
appearance of
secondary
ossification center
4 Bone at birth, with
enlarged primary
marrow cavity and
appearance of
secondary marrow
cavity in one epiphysis
Ribs
Cartilage
5 Bone of child, with
epiphyseal plate at
distal end
6 Adult bone with a
single marrow
cavity and closed
epiphyseal plate
Femur
7-17
Pelvis
Figure 7.11
7-18
© Biophoto Associates/Photo Researchers, Inc.
3
Interstitial Growth: Zones of the Metaphysis
Bone Growth and Remodeling
• interstitial growth - bones grow in length – cartilage!
• appositional growth - bones increase in width
throughout life
– the deposition of new bone at the surface
Zone 1
Zone 5
• bone remodeling occurs throughout life - 10% per year
– Wolff’s law of bone –
– Osteoclasts & osteoblasts
– Lines of stress
1
Zone of reserve cartilage
Typical histology of resting
hyaline cartilage
2
Zone of cell proliferation
Chondrocytes multiplying
(lining up in rows of small
flattened lacunae)
Enlarging
chondrocytes
3
Zone of cell hypertrophy
Cessation of mitosis;
enlargement of chondrocytes
& thinning of lacuna walls
Breakdown
of lacunae
4
Zone of calcification
Temporary calcification of
cartilage matrix
5
Zone of bone deposition
Breakdown of lacuna walls,
leaving open channels; death
of chondrocytes; bone
deposition by osteoblasts,
forming trabeculae of spongy
bone
Multiplying
chondrocytes
Calcifying
cartilage
Bone
marrow
Osteoblasts
Osteocytes
7-19
Trabeculae of
spongy bone
7-20
Victor Eroschenko
Bone Growth in Width – Appositional growth
Cartilaginous Epiphyseal Plates
Vs. Epiphyseal Lines
Diaphysis
Epiphysis
Epiphyseal
plate
Metacarpal
bone
At bone surface – periosteum cells
osteoblasts (create matrix)
Osteoblasts in Endostium deposit matrix and OSTEON is made
Epiphyseal
plates
7-21
6-22
Bone Growth in Width
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4
Mineral Deposition
Physiology of Osseous Tissue
• mineral deposition (mineralization) - calcium phosphate, and other ions
are taken from the blood plasma and deposited in bone tissue
• a mature bone remains a metabolically active organ
– osteoblasts produce collagen fibers that spiral the length of the osteon
– fibers become encrusted with minerals that harden the matrix
– involved in its own maintenance of growth and
remodeling
• calcium and phosphate (hydroxyapatite) from blood plasma are deposited along the
fibers
• the calcium and phosphate ion concentration must reach a critical value called the
solubility product for crystal formation to occur
• most tissues have inhibitors
• osteoblasts neutralize these inhibitors and allow salts to precipitate in the bone
matrix
– exerts a profound influence over the rest of the body
by exchanging minerals with tissue fluid
• disturbance of calcium homeostasis in skeleton disrupts function
of other organ systems
• abnormal calcification (ectopic ossification)
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7-25
Control of Calcium
Mineral Resorption
• mineral resorption – the process of dissolving bone
and releasing minerals into the blood
Calcium Intake and Excretion
Blood
Bone
Dietary requirement
1,000 mg/day
– performed by osteoclasts
– hydrogen pumps in membrane secrete H+ into space between
osteoclast and bone surface
– chloride ions follow by electrical attraction
– hydrochloric acid (pH 4) dissolves bone minerals
– acid phosphatase enzyme digests the collagen
Deposition by
osteoblasts
Absorption by
digestive tract
Digestive tract
Calcitriol
Calcitonin – Thyroid Gland
Ca2+
(9.2–10.4 mg/dL)
Hydroxyapatite
Ca10(PO4)6(OH)2
Kidneys
Filtration
by kidneys
Resorption by
osteoclasts
Calcium carbonate
CaCO3
PTH
Reabsorption
by kidneys
Calcitriol
Fecal loss
350 mg/day
7-27
Calcitriol Synthesis and Action (Vit. D)
Urinary loss
650 mg/day
Figure 7.17
calcitriol, calcitonin, and PTH maintain normal blood
calcium concentration
7-28
Correction for Hypercalcemia
7-dehydrocholesterol
HO
Blood Ca2+
excess
Ultraviolet
light
Blood Ca2+
returns to
normal
Vitamin D3
(cholecalciferol)
Calcitonin
(Thyroid Gland)
secretion
CH2
HO
Bone
resorption
Calcidiol
OH
CH2
HO
Reduced
excretion
of Ca2+
Calcitriol
OH
CH2
HO
Absorption
of Ca2+ and
phosphate
Reduced
osteoclast
activity
Less bone
resorption
Increased
osteoblast
activity
More bone
deposition
OH
7-30
5
Phosphate Homeostasis
Correction for Hypocalcemia
• Important molecules with P?
Blood Ca2+
deficiency
Blood Ca2+
returns to
normal
– no immediate functional disorders if low or high
• calcitriol promotes its absorption by small intestine &
promotes bone deposition
Parathyroid
hormone
secretion
Increased
osteoclast
activity
More bone
resorption
Reduced
osteoblast
activity
Less bone
deposition
More urinary
phosphate
excretion
Prevention of
hydroxyapatite
formation
Less urinary
calcium
excretion
Conservation
of calcium
• PTH lowers blood phosphate level by promoting its
urinary excretion
Figure 7.18b
7-31
7-32
(b) Correction for hypocalcemia
Fractures and Their Repairs
Other Factors Affecting Bone
• 20 or more hormones, vitamins, and growth factors affect
osseous tissue
•
• bone growth especially rapid in puberty & adolescence
stress fracture – break caused by
abnormal trauma to a bone
• pathological fracture – break in a
bone weakened by some other
disease
– surges of growth hormone, estrogen, and testosterone occur and
promote ossification
– girls grow faster than boys
– males grow for a longer time and taller
• closed reduction – bone fragments
are manipulated into normal
positions
• open reduction – involves surgical
exposure of the bone
7-33
Healing of Fractures
Fibrocartilage
Hard
callus
Hematoma
Soft callus
Spongy
bone
New blood
vessels
Compact bone
1 Hematoma formation
The hematoma is converted
to granulation tissue by invasion
of cells and blood capillaries.
Stress fracture
2 Soft callus formation
Deposition of collagen and
fibrocartilage converts granulation
tissue to a soft callus.
3 Hard callus formation
Osteoblasts deposit a temporary
bony collar around the fracture to
unite the broken pieces while
ossification occurs.
4 Bone remodeling
Small bone fragments are
removed by osteoclasts, while
osteoblasts deposit spongy
bone and then convert it to
compact bone.
Figure 7.20
6-35
7-36
6
Osteoporosis
Spinal Osteoporosis
• osteoporosis – most common bone disease
– severe loss of bone density
• bones loses mass and becomes brittle due to loss of
organic matrix and minerals
– affects spongy bone the most since it is most metabolically
active
• estrogen maintains density in both sexes inhibits
resorption by osteoclasts
(a)
7-37
(b)
7-38
7