The Skeletal
System:
Bone Tissue
Chapter 6
The Skeletal System: Bone Tissue
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Functions of Bone and Skeletal System
Structure of Bone
Histology of Bone Tissue
Blood and Nerve Supply of Bone
Bone Formation
Bone’s Role in Calcium Homeostasis
Exercise and Bone Tissue
Aging and Bone Tissue
Functions of Bone and Skeletal System
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Support
Protection
Assistance in Movement
Mineral Homeostasis
Blood Cell Production
Triglyceride Storage
Functions of Bone and Skeletal System
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Support
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Structural framework of the body
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Supports soft tissues
Provides attachment points for tendons of skeletal
muscle
Protection
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Protects important internal organs
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Cranium protects brain
Vertebrae protects spinal cord
Ribs protect lungs and heart
Functions of Bone and Skeletal System
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Assistance in Movement
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Skeletal muscle attaches to bone
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Skeletal muscle contraction pulls on bone producing
movement
Mineral Homeostasis
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Bone tissue stores several minerals
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Acts to serve as a reservoir of critical minerals
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Calcium (99% of body’s content)
Phosphorus
Functions of Bone and Skeletal System
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Blood Cell Production
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Red bone marrow produces (Hemopoiesis)
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Red blood cells
White blood cells
Platelets
Triglyceride Storage
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Yellow bone marrow
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Triglycerides stored in adipose cells
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Serves as a potential chemical energy reserve
Structure of Bone
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Long Bone Anatomy
(Humerus)
Diaphysis
Epiphysis
Metaphysis
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Articular cartilage
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Epiphyseal growth plate
Perforating fibers
Periosteum
Medullary cavity
Endosteum
Histology of Bone Tissue
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Extracellular matrix surrounding widely
separated cells
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Matrix
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15% water
30% collagen fibers
55% crystallized mineral salts
The most abundant mineral salt is
calcium phosphate
Histology of Bone Tissue
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A process called calcification is initiated
by bone-building cells called osteoblasts
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Mineral salts are deposited and crystallize
in the framework formed by the collagen
fibers of the extracellular matrix
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Bone’s flexibility depends on collagen
fibers
Histology of Bone Tissue
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Four types of cells are present in bone tissue
Osteogenic cells
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Osteoblasts
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Undergo cell division (stem cells); the resulting
cells develop into osteoblasts
Bone-building cells
Synthesize extracellular matrix of bone tissue
Osteocytes
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Mature bone cells
Exchange nutrients and wastes with the blood
Histology of Bone Tissue
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Osteoclasts
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Release enzymes that digest the mineral
components of bone matrix (resorption)
Regulate blood calcium level
Histology of Bone Tissue
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Bone may be categorized as:
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Compact
Spongy
Histology of Bone Tissue
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Compact Bone
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Resists the stresses produced by weight and
movement
Components of compact bone are arranged
into repeating structural units called osteons
or Haversian systems
Osteons consist of a central (Haversian) canal
with concentrically arranged lamellae, lacunae, osteocytes, and canaliculi
Histology of Bone Tissue
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Osteon
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Central canals run longitudinally through bone
Around the central canals are concentric
lamellae
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Rings of calcified matrix (like the rings of a tree
trunk)
Between the lamellae are small spaces called
lacunae which contain osteocytes
Radiating in all directions from the lacunae are
tiny canaliculi filled with extracellular fluid and
the plasma membrane extensions of the
osteocyte
Histology of Bone Tissue
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Osteon
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Canaliculi connect
lacunae, forming a
system of
interconnected
canals
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Providing routes for
nutrients and
oxygen to reach the
osteocytes
The organization of
osteons changes in
response to the
physical demands
placed on the
skeleton
Histology of Bone Tissue
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Spongy Bone
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Lacks osteons
Lamellae are arranged in a lattice of thin
columns called trabeculae
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Spaces between the trabeculae make bones
lighter
Trabeculae of spongy bone support and protect
the red bone marrow
Hemopoiesis (blood cell production) occurs in red
bone marrow of spongy bone
Histology of Bone Tissue
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Spongy Bone
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Within each trabecula are lacunae that
contain osteocytes
Osteocytes are nourished from the blood
circulating through the trabeculae within the
red bone marrow.
Interior bone tissue is made up primarily of
spongy bone
The trabeculae of spongy bone are oriented
along lines of stress
 helps bones resist stresses without breaking
while keeping them light.
Blood and Nerve Supply of Bone
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Bone is richly supplied
with blood
 Periosteal arteries
accompanied by
nerves supply the
periosteum and
compact bone
 Epiphyseal veins
carry blood away
from long bones
Nerves accompany
the blood vessels that
supply bones
 The periosteum is
rich in sensory
nerves sensitive to
tearing or tension
Bone Formation
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The process by which bone forms is
called ossification
Bone formation occurs in four situations:
 1) Formation of bone in an embryo
 2) Growth of bones until adulthood
 3) Remodeling of bone
 4) Repair of fractures
Bone Formation
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Formation of Bone in an Embryo
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Bone formation follows one of two patterns
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Intramembranous ossification
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Flat bones of the skull and mandible are formed in this
way
“Soft spots” that help the fetal skull pass through the
birth canal later become ossified forming the skull
Endochondral ossification
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The replacement of cartilage by bone
Most bones of the body are formed in this way including
long bones
Bone Formation
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Formation of Bone in an Embryo
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Cartilage formation and ossification occurs
during the sixth week of embryonic
development
Bone Formation - Intramembranous ossification
Bone Formation - Intramembranous ossification
Bone Formation - Intramembranous ossification
Bone Formation - Intramembranous ossification
Bone Formation - Endochondral ossification
Bone Formation - Endochondral ossification
Bone Formation - Endochondral ossification
Bone Formation - Endochondral ossification
Bone Formation - Endochondral ossification
Bone Formation - Endochondral ossification
Bone Growth During
Infancy, Childhood and
Adolescence
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Growth in Length
The growth in length of
long bones involves two
major events:
 1) Growth of cartilage
on the epiphyseal
plate
 2) Replacement of
cartilage by bone
tissue in the
epiphyseal plate
Bone Growth During Infancy, Childhood
and Adolescence
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Osteoclasts dissolve the
calcified cartilage, and
osteoblasts invade the area
laying down bone matrix
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The activity of the epiphyseal
plate is the way bone can
increase in length
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At adulthood, the epiphyseal
plates close and bone replaces
all the cartilage leaving a bony
structure called the epiphyseal
line
Bone Growth During Infancy, Childhood
and Adolescence
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Growth in Thickness
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Bones grow in thickness at the outer surface
Remodeling of Bone
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Bone forms before birth and continually renews
itself
The ongoing replacement of old bone tissue by
new bone tissue
Old bone is continually destroyed and new bone is
formed in its place throughout an individual’s life
Bone Growth During Infancy, Childhood
and Adolescence
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A balance must exist between the actions of
osteoclasts and osteoblasts
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If too much new tissue is formed, the bones become
abnormally thick and heavy
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Excessive loss of calcium weakens the bones, as
occurs in osteoporosis
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Or they may become too flexible, as in rickets and
osteomalacia
Factors Affecting Bone Growth and Bone
Remodeling
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Normal bone metabolism depends on several factors
Minerals
 Large amounts of calcium and phosphorus and
smaller amounts of magnesium, fluoride, and
manganese are required for bone growth and
remodeling
Vitamins
 Vitamin A stimulates activity of osteoblasts
 Vitamin C is needed for synthesis of collagen
 Vitamin D helps build bone by increasing the
absorption of calcium from foods in the gastrointestinal
tract into the blood
 Vitamins K and B12 are also needed for synthesis of
bone proteins
Factors Affecting Bone Growth and Bone
Remodeling
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Hormones
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During childhood, the hormones most important to
bone growth are growth factors (IGFs), produced by
the liver and bone tissue (under influence of hGH)
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IGFs stimulate osteoblasts, promote cell division at the
epiphyseal plate, and enhance protein synthesis
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Thyroid hormones (T3 and T4) also promote bone
growth by stimulating osteoblasts
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Insulin promotes bone growth by increasing the
synthesis of bone proteins
Factors Affecting Bone Growth and Bone
Remodeling
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Hormones
 Estrogen and testosterone cause a dramatic
effect on bone growth
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Cause of the sudden “growth spurt” that occurs
during the teenage year
Promote changes in females, such as widening of
the pelvis
Ultimately-Shut down growth at epiphyseal plates
Parathyroid hormone, calcitriol, and calcitonin
are other hormones that can affect bone
remodeling
Bone’s Role in Calcium Homeostasis
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Bone is the body’s major calcium reservoir
Levels of calcium in the blood are maintained
by controlling the rates of calcium resorption
from bone into blood and of calcium deposition
from blood into bone
 Both nerve and muscle cells depend on
calcium ions (Ca2+) to function properly
 Blood clotting also requires Ca2+
 Many enzymes require Ca2+ as a cofactor
Bone’s Role in Calcium Homeostasis
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Actions that help elevate blood Ca2+ level
 Parathyroid hormone (PTH) regulates
Ca2+ exchange between blood and bone
tissue
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PTH increases the number and activity of
osteoclasts (i.e., resorption)
PTH acts on the kidneys to decrease loss of
Ca2+ in the urine
PTH stimulates formation of calcitriol
(active form of Vitamin D) a hormone that
promotes absorption of calcium from foods
in the gastrointestinal tract
Bone’s Role in
Calcium
Homeostasis
Bone’s Role in Calcium Homeostasis
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Actions that work to decrease blood Ca2+
level
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The thyroid gland (i.e., parafollicular cells)
secretes calcitonin (CT) which inhibits
activity of osteoclasts
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The result is that CT promotes bone
formation and decreases blood Ca2+ level
See Table 6.2
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Fracture and Repair of Bone
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Fracture Types
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Open (compound) fracture
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Closed (simple) fracture
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Fracture of the fibula, with injury of the tibial articulation
Colles’ fracture
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One end of the fractured bone is forcefully driven into another
Pott’s fracture
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A partial fracture in which one side of the bone is broken and the other side
bends
Impacted fracture
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The bone is splintered, crushed, or broken into pieces
Greenstick fracture
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Does not break the skin
Comminuted fracture
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The broken ends of the bone protrude through the skin
A fracture of the radius in which the distal fragment is displaced
Stress fracture
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A series of microscopic fissures in bone
Fracture and Repair
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Anatomical appearance – like breaking a green twig
Greenstick
Fracture and Repair
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Anatomical appearance – the distal part is
shoved up into the proximal part.
Impacted
Fracture and Repair
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Anatomical appearance – though not seen
here, one or both bones are “open” to the
outside.
Open (compound)
Fracture and Repair
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Eponyms – Colles’ is a fracture of the distal
radius ± ulna.
Colles’
Fracture and Repair of Bone
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Calcium and phosphorus needed to strengthen and
harden new bone after a fracture are deposited only
gradually and may take several months
The repair of a bone fracture involves the following
steps
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1) Formation of fracture hematoma
 Blood leaks from the torn ends of blood vessels, a clotted
mass of blood forms around the site of the fracture
2) Fibrocartilaginous callus formation
 Fibroblasts invade the fracture site and produce collagen
fibers bridging the broken ends of the bone
3) Bony callus formation
 Osteoblasts begin to produce spongy bone trabeculae joining
portions of the original bone fragments
4) Bone remodeling
 Compact bone replaces spongy bone
Osteon
Periosteum
Compact bone
Spongy bone
Fracture hematoma
Blood vessel
Fibroblast
Phagocyte
Phagocyte
Fracture
hematoma
Red blood
cell
Bone
fragment
Osteocyte
1
Fibrocartilaginous
callus
Osteoblast
Collagen fiber
Chondroblast
Cartilage
Formation of fracture hematoma
Bony callus
2 Fibrocartilaginous callus formation
New compact
bone
Osteoblast
Spongy bone
Osteoclast
Osteocyte
3 Bony callus formation
4 Bone remodeling
Fracture and Repair
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Once a bone is fractured, repair proceeds in
a predictable pattern:
The first step, which occurs 6-8 hours after
injury, is the formation of a fracture
hematoma, as a result of
blood vessels breaking in
the periosteum and
in osteons.
Fracture and Repair
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The second and third steps involve the
formation of a callus (takes a few weeks, to as
many as six months).
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Phagocytes remove cellular debris and fibroblasts
deposit collagen to
form a fibrocartilaginous callus...
Fracture and Repair
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... which is followed by osteoblasts forming a bony
callus of spongy bone.
Fracture and Repair
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The final step takes several months and is
called remodeling :
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Spongy bone is replaced by
compact bone.
The fracture line
disappears, but
evidence of the break
remains.
Exercise and Bone Tissue
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Bone tissue alters its strength in response to
changes in mechanical stress
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Under stress, bone tissue becomes stronger through
deposition of mineral salts and production of collagen
fibers by osteoblasts and fibroblasts
Unstressed bones diminishes because of the loss of
bone minerals and decreased numbers of collagen
fibers
The main mechanical stresses on bone are
those that result from the pull of skeletal muscles
and the pull of gravity
Weight-bearing activities help build and retain
bone mass
Aging and Bone Tissue
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The level of sex hormones diminishes during
middle age, especially in women after
menopause
 A decrease in bone mass occurs
 Bone resorption by osteoclasts outpaces bone
deposition by osteoblasts
Female bones generally are smaller and less
massive than males
 Loss of bone mass in old age has a greater
adverse effect in females
Aging and Bone Tissue
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There are two principal effects of aging on bone tissue:
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1) Loss of bone mass
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Results from the loss of calcium from bone matrix
The loss of calcium from bones is one of the symptoms in
osteoporosis
2) Brittleness
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Results from a decreased rate of protein synthesis
Collagen fibers gives bone its tensile strength
The loss of tensile strength causes the bones to become very brittle
and susceptible to fracture
Compact Bone
Spongy Bone (Cancellous)
Hyaline Cartilage
Adipose Tissue