5
The Skeletal System:
Osseous Tissue and
Skeletal Structure
Introduction
• The skeletal system is made of:
•
•
•
•
Skeletal bones
Cartilage
Ligaments
Connective tissue to stabilize the skeleton
• Bones are dynamic organs, which consist of several
tissue types
Introduction
• Functions of the skeletal system
• Support
• Provides the framework for the attachment of other
organs
• Storage of minerals
• Calcium ions: 98% of the body’s calcium ions are in the bones
• Phosphate ions
• Blood cell production
• Bone marrow produces erythrocytes, leukocytes, and platelets
Introduction
• Functions of the skeletal system (continued)
• Leverage
• Muscles pull on the bones to produce movement
• Protection
•
•
•
•
Ribs protect heart and lungs
Skull protects the brain
Vertebrae protect the spinal cord
Pelvic bones protect the reproductive organs
Anatomy of Skeletal Elements
• There are seven broad categories of bones according
to their shapes
•
•
•
•
•
•
•
Sutural bones
Irregular bones
Short bones
Pneumatized bones
Flat bones
Long bones
Sesamoid bones
Classification of Bones
• Long Bones: longer than they are wide, upper and lower
limbs.
• Short Bones: Broad as they are long, cube shaped, round,
i.e. tarsals & carpals
• Flat Bones: Thin, flat and curved, skull (parietal), sternum,
scapulae.
• Irregular Bones: Don’t fit in the others, face, vertebrae
• Sesamoid: Sesame seed, patella, small and flat
• Sutural bones: Wormian bones, borders like a jigsaw
puzzle.
Figure 5.11 Shapes of Bones
Sutural Bones
Flat Bones
Pneumatized Bones
External table
Parietal bone
Sutures
Sutural
bone
Ethmoid
Diploë
Internal
table (spongy bone)
Air cells
Long Bones
Irregular Bones
Vertebra
Humerus
Short Bones
Carpal
bones
Sesamoid Bones
Patella
Structure of Bone
• Bones (osseous tissue)
• Supporting connective tissue
• Specialized cells
• Solid matrix
• Outer lining
• Called the periosteum
• Inner lining
• Called the endosteum
Structure of Bone
• The Histological Organization of Mature Bone
• The matrix
• Calcium phosphate eventually converts to
hydroxyapatite crystals
• Hydroxyapatite crystals resist compression
Structure of Bone
• The Histological Organization of Mature Bone
• Collagen fibers
• Make up 2/3 of the bone matrix
• Contribute to the tensile strength of bones
• Collagen and hydroxyapatite make bone tissue extremely strong
• Bone cells
• Contribute only 2% of the bone mass
Structure of Bone
• The Cells of Mature Bone
• Osteocytes
• Mature bone cells
• Maintain the protein and mineral content of the matrix
• Osteoblasts
•
•
•
•
Immature bone cells
Found on the inner and outer surfaces of bones
Produce osteoid, which is involved in making the matrix
Osteoblasts are involved in making new bone. This is a process called
osteogenesis
• Osteoblasts can convert to osteocytes
Structure of Bone
• The Cells of Mature Bone (continued)
• Osteoprogenitor cells
• Found on the inner and outer surfaces of bones
• Differentiate to form new osteoblasts
• Heavily involved in the repair of bones after a break
• Osteoclasts
• Secrete acids, which dissolve the bones thereby causing the
release of stored calcium ions and phosphate ions into the blood
• This process is called osteolysis
Figure 5.1a Histological Structure of a Typical Bone
Canaliculi
Osteocyte
Matrix
Endosteum
Osteoprogenitor cell
Medullary
cavity
Osteocyte: Mature bone cell
that maintains the bone matrix
Osteoblast
Osteoid
Osteoprogenitor cell:
Stem cell whose divisions produce
osteoblasts
Osteoclast
Matrix
Matrix
Medullary
cavity
Osteoblast: Immature bone
cell that secretes organic
components of matrix
The cells of bone
Osteoclast: Multinucleate cell
that secretes acids and enzymes
to dissolve bone matrix
Structure of Bone
• The Osteon
• It is the basic unit of skeletal bones
• Consists of:
•
•
•
•
•
Central canal
Canaliculi
Osteocytes
Lacunae
Lamellae
Figure 5.1c Histological Structure of a Typical Bone
Canaliculi
Concentric
lamellae
Central canal
Osteon
Lacunae
Osteons
LM  220
A thin section through
compact bone; in this
procedure the intact matrix
and central canals appear
white, and the lacunae and
canaliculi are shown in black.
Figure 5.1d Histological Structure of a Typical Bone
Canaliculi
Concentric
lamellae
Central canal
Osteon
Lacunae
Osteon
LM  343
A single osteon at higher
magnification
Figure 5.1b Histological Structure of a Typical Bone
Osteon
Lacunae
Central
canals
Lamellae
Osteons
SEM  182
A scanning electron
micrograph of several osteons
in compact bone
Structure of Bone
• Two types of osseous tissue
• Compact bone (dense bone)
• Compact bones are dense and solid
• Forms the walls of bone outlining the medullary cavity
• Medullary cavity consists of bone marrow
• Spongy bone (trabecular bone)
• Open network of plates
Figure 5.2a-c The Internal Organization in Representative Bones
Concentric
lamellae
Spongy bone
Blood vessels
Collagen fiber
orientation
Compact bone
Medullary cavity
Central
canal
Endosteum
Endosteum
The organization of collagen
fibers within concentric lamellae
Periosteum
Compact Spongy
bone
bone
Medullary
cavity
Capillary
Small vein
Gross anatomy
of the humerus
Circumferential
lamellae
Concentric
lamellae
Osteons
Periosteum
Interstitial
lamellae
Artery Vein
Trabeculae of
spongy bone
Perforating
canal
Central
canal
Diagrammatic view of the histological
organization of compact and spongy bone
Figure 5.2d The Internal Organization in Representative Bones
Trabeculae of
spongy bone
Canaliculi
opening
on surface
Endosteum
Lamellae
Location and structure of spongy bone. The photo
shows a sectional view of the proximal end of the femur.
Structure of Bone
• Structural Differences
• Compact bone
• Consists of osteons
• Makes up the dense, solid portion of bone
• Spongy bone
•
•
•
•
Trabeculae are arranged in parallel struts
Trabeculae form branching plates
Trabeculae form an open network
Creates the lightweight nature of bones
Structure of Bone
• Functional Differences
• Compact bone
• Conducts stress from one area of the body to another area of the
body
• Generates tremendous strength from end to end
• Weak strength when stress is applied to the side
• Spongy bone
• Trabeculae create strength to deal with stress from the side
Figure 5.3a Anatomy of a Representative Bone
Spongy bone
Epiphysis
Articular
surface of
head of femur
Metaphysis
Compact bone
Diaphysis
(shaft)
Medullary
cavity
Metaphysis
Epiphysis
Posterior view
Sectional view
The femur, or thigh bone, in superficial and sectional views. The femur has a
diaphysis (shaft) with walls of compact bone and epiphyses (ends) filled with
spongy bone. A metaphysis separates the diaphysis and epiphysis at each
end of the shaft. The body weight is transferred to the femur at the hip joint.
Because the hip joint is off center relative to the axis of the shaft, the body
weight is distributed along the bone so that the medial portion of the shaft is
compressed and the lateral portion is stretched.
Figure 5.3b Anatomy of a Representative Bone
An intact femur chemically cleared
to show the orientation of the
trabeculae in the epiphysis
Figure 5.3c Anatomy of a Representative Bone
Articular surface
of head of femur
Trabeculae
of spongy
bone
Cortex
Medullary cavity
Compact bone
A photograph showing the
epiphysis after sectioning
Structure of Bone
• Organization of Compact and Spongy Bone
• Epiphysis
• Each end of the long bones
• Diaphysis
• Shaft of the long bones
• Metaphysis
• Narrow growth zone between the epiphysis and the diaphysis
Figure 5.3a Anatomy of a Representative Bone
Spongy bone
Epiphysis
Articular
surface of
head of femur
Metaphysis
Compact bone
Diaphysis
(shaft)
Medullary
cavity
Metaphysis
Epiphysis
Posterior view
Sectional view
The femur, or thigh bone, in superficial and sectional views. The femur has a
diaphysis (shaft) with walls of compact bone and epiphyses (ends) filled with
spongy bone. A metaphysis separates the diaphysis and epiphysis at each
end of the shaft. The body weight is transferred to the femur at the hip joint.
Because the hip joint is off center relative to the axis of the shaft, the body
weight is distributed along the bone so that the medial portion of the shaft is
compressed and the lateral portion is stretched.
Structure of Bone
• The Periosteum and Endosteum
• Periosteum
• Outer surface of the bone
• Isolates and protects the bone from surrounding tissue
• Provides a route and a place for attachment for circulatory and
nervous supply
• Actively participates in bone growth and repair
• Attaches the bone to the connective tissue network of the deep
fascia
Structure of Bone
• The Periosteum and Endosteum
• Periosteum and Tendons
• Tendons are cemented into the lamellae by osteoblasts
• Therefore, tendons are actually a part of the bone
Figure 5.4c Anatomy and Histology of the Periosteum and Endosteum
Zone of
tendonbone
attachment
Tendon
Periosteum
Medullary cavity
Endosteum
Spongy bone
of epiphysis
Epiphyseal
cartilage
LM  100
A tendonbone junction
Structure of Bone
• The Periosteum and Endosteum
• Endosteum
•
•
•
•
Inner surface of bone
Lines the medullary cavity
Consists of osteoprogenitor cells
Actively involved in repair and growth
Bone Development and Growth
• Factors Regulating Bone Growth
•
•
•
•
•
•
•
•
Nutrition
Calcium ions
Phosphate ions
Magnesium ions
Citrate
Carbonate ions
Sodium ions
Vitamins A, C, D (calcitriol)
Bone Development and Growth
• Factors Regulating Bone Growth (continued)
• Hormones: Parathyroid gland
•
•
•
•
Releases parathyroid hormone
Stimulates osteoclasts
Stimulates osteoblasts
Increases calcium ion absorption from the small intestine to the
blood
Bone Development and Growth
• Factors Regulating Bone Growth (continued)
• Hormones: Thyroid gland
Releases calcitonin
• Inhibits osteoclasts
• Removes calcium ions from blood and adds it to bone
Bone Development and Growth
• Factors Regulating Bone Growth (continued)
• Hormones: Thyroid gland
• Releases thyroxine (T4)
• Maintains normal activity of the epiphyseal cartilage
Bone Development and Growth
• Factors Regulating Bone Growth (continued)
• Hormones: Pituitary gland
• Releases growth hormone (somatotropin)
• Maintains normal activity of the epiphyseal cartilage
Bone Development and Growth
• There are four major sets of blood vessels associated
with the long bones
• Nutrient vessels
• Enter the diaphysis and branch toward the epiphysis
• Re-enter the compact bone, leading to the central
canals of the osteons
• Metaphyseal vessels
• Supply nutrients to the diaphyseal edge of the
epiphysis
Bone Development and Growth
• Four major sets of blood vessels (continued)
• Epiphyseal vessels
• Supply nutrients to the medullary cavities of the
epiphysis
• Periosteal vessels
• Supply nutrients to the superficial osteons
Figure 5.4ab Anatomy and Histology of the Periosteum and Endosteum
Joint capsule
Cellular layer
of periosteum
Fibrous layer
of periosteum
Endosteum
Compact bone
Circumferential
lamellae
Cellular layer
of periosteum
Fibrous layer
of periosteum
Canaliculi
Lacuna
Osteocyte
Perforating
fibers
Bone
matrix
Giant
multinucleate
osteoclast
Endosteum
Osteoprogenitor
cell
Osteocyte
Osteoid
Osteoblasts
The endosteum is an incomplete
cellular layer containing osteoblasts,
osteoprogenitor cells, and osteoclasts.
The periosteum contains outer
(fibrous) and inner (cellular) layers.
Collagen fibers of the periosteum are
continuous with those of the bone,
adjacent joint capsules, and attached
tendons and ligaments.
Bone Development and Growth
• Before six weeks of development, the skeleton is
cartilage
• Cartilage cells will be replaced by bone cells
• This is called ossification
• Osteogenesis
• Bone formation
• Calcification
• The deposition of calcium ions into the bone tissue
Nutrition and hormones
• Bone growth requires Osteoblasts & Chondroblasts to proliferate.
• Calcium is critical for growth
• Vitamin D is necessary for Calcium absorption in the small
intestine….. Vitamin D is soluble in fat
• Lack of calcium results in Rickets in kids and osteomalacia in
adults.
• Vitamin C is critical for collagen synthesis.
• Scurvy is the malady of deficient Vitamin C.
• This causes bones deficient in collagen.
• scurvy causes poor wound healing and easy hemorrhage.
Hormones
• Human Growth hormone secreted from the anterior pituitary increases
general tissue growth, specifically appositional bone growth and interstitial
cartilage growth
• Thyroid hormones: Calcitonin & Thyroxine.
• Calcitonin: promotes Ca loss in the kidneys & reduces blood levels, also
inhibits osteoclast activity.
• Thyroxine: stimulates osteoblast activity, bone matrix synthesis.
• Sex Hormones; Estrogen & testosterone stimulates osteoblasts activity.
Bone Formation, Growth, and Remodeling
• most bones develop using hyaline cartilage structures as their "models.“
• In embryos, the skeleton is primarily made of hyaline cartilage
• In childhood most of the cartilage has been replaced by bone
• Cartilage remains only in the bridge of the nose, parts of the ribs, and surfaces of the
joints.
• flat bones form on fibrous membranes,
Bone cell types
•Osteocytes: These are the most common of the 4 types of bone cells. Each
of these occupies a lacuna. They have 2 primary functions, 1) maintain and
monitor protein & mineral content of bone matrix 2) participate in bone
repair.
•Osteoblasts: bone producing cells
•Osteoclasts: cells that break down bone
•Both of these cells are found in the inner layer of the periosteum.
•Osteoprogenitor Cells; stem cells that differentiate into osteoblasts
Bone Ossification
Ossification: the formation of bone by osteoblasts and synthesis of
extracellular matrix and addition of minerals to matrix. The process of
replacing other tissue with bone.
• Calcification: deposition of calcium salts during ossification.
ossification
Hyaline
cartilage
model
Ossification

Bone
starting
to
replace
cartilage
New
center
of bone
growth
process of bone formation in fetus
1.
the hyaline cartilage model is completely covered with
bone matrix by bone-forming cells called osteoblasts.
2.
Bone
Hyaline
cartilage
the fetus has cartilage "bones" enclosed by "bony" bones.
Yellow
marrow cavity
forming
3.
the enclosed hyaline cartilage model is digested away,
opening up a medullary cavity within the newly formed bone.Artic
ular
4.
By birth or shortly after, most hyaline cartilage models
have been converted to bone
cartil
age
Blood
vessels
articular cartilages that cover the bone ends, and the
epiphyseal plates remain.
Articular cartilage
articular
Bone
Epiphyseal plate
cartilage forming
Bone forming

cartilages persist for life, reducing friction at the
joint surfaces.
epiphyseal
plates provide for longitudinal growth of the long
bones during childhood.
Growth in bone width
Bone forming
Epiphyseal plate
cartilage forming
Bone Development and Growth
• There are two types of ossification
• Intramembranous ossification
• Involved in the development of clavicle, mandible,
skull, and face
• Endochondral ossification
• Involved in the development of limbs, vertebrae, and hips
Intramembranous Ossification:
(dermal ossification) bone that develops from fibrous connective tissue.
This occurs in deep layers of dermis (dermal bones), i.e. skull
(fontanels), mandible, clavicle. The 3 steps of intramembranous
ossification!
• Step 1: Mesenchymal cells (stem cells that are present in many
connective tissues) aggregate, and begin the ossification process. The
bone expands out from the ossification center as a series of spicules
(small struts). The mesenchymal cells differentiate and produce
osteoblasts.
• Step 2: Bone growth is active & requires oxygen and food.
Blood vessels form and become trapped in the growing bone.
• Step 3: Formed intramembranous is only spongy bone, as the bone
matures spongy bone is removed for marrow cavities, also spongy bone
formed in this process can become compact.
Figure 5.6 Fetal Intramembranous and Endochondral Ossification
Temporal
bone
Parietal bone
Mandible
Intramembranous
ossification
produces the
roofing bones of
the skull
Clavicle
Frontal bone
Scapula
Humerus
Ribs
Metacarpal bones
Phalanges
Radius
Endochondral
ossification
replaces cartilages
of embryonic skull
Primary ossification
centers of the
diaphyses (bones
of the lower limb)
Future
hip bone
At 10 weeks the fetal skull clearly shows
both membrane and cartilaginous bone,
but the boundaries that indicate the limits
of future skull bones have yet to be
established.
Vertebrae
Hip bone
(ilium)
Femur
Ulna
Cartilage
Fibula
Tibia
Phalanx
Metatarsal bones
At 16 weeks the fetal skull shows the irregular
margins of the future skull bones. Most
elements of the appendicular skeleton form
through endochondral ossification. Note the
appearance of the wrist and ankle bones at 16
weeks versus at 10 weeks.
• Endochondral Ossification:
• Is responsible for the increase in length of bones.
• In long bones endochondral growth at the epiphyseal plate results in the
increase of the diaphysis. Most bone starts as hyaline cartilage. The
cartilage is a miniature model of what the bone will look like in adulthood.
These models become bone in endochondral ossification.
• The six steps of endochondral growth
• Step 1: cartilage enlarges, by appositional growth, chondrocytes at center of
cartilage grow in size. The matrix reduces in size and spicules calcify.
Chondrocytes die and leave cavities in cartilage.
• Step 2: blood vessels grow around edges of cartilage, osteoblasts form in
the perichondrium form and the cartilage becomes encased in bone.
• Step 3: The perichondrium needs oxygen & food, so capillaries begin to grow
where the cartilage has died off.
Endochondral Bone Ossification
• Fibroblasts become osteoblasts and replace cartilage with spongy bone. This
happens in an area called Primary Center of Ossification here bone grows
towards the ends of the bone, the entire diaphysis is spongy bone.
• Step 4; As the bone continues to grow osteoclasts appear breaking down the
trabeculae of spongy bone starting a marrow cavity. Now bone grows two
ways, 1 in length and 2 in diameter by appositional growth.
• Step 5: Capillaries and osteoblasts migrate to the epiphysis creating
secondary ossification centers.
• Step 6: The epiphysis fills with spongy bone, a small layer of cartilage
remains and becomes articular cartilage.
Bone Growth
• Bones cannot grow by interstitial growth like cartilage,
ligaments, & tendons.
• bones grow by two methods: Appositional growth or
Endochondral growth.
• Appositional Growth is responsible for the increase in
diameter of long bones and most growth of other bones.
• cells on the inner layer of periosteum differentiate into
osteoblasts and cause growth of the bone matrix. They
become surrounded with matrix and become osteocytes.
• On the surface of the bone, appositional growth adds layers
of bone that become lamellae.
• Remember, Bone is deposited by osteoblasts on the surface
of the bone and reabsorbed by osteoclasts on the inner
surface of the bone, so marrow cavity enlarges as bone
grows.
Longitudinal growth
• 1."New" cartilage is formed continuously
on the external face of the articular
cartilage and on the epiphyseal plate
surface that is farther away from the
medullary cavity.
Articular
cartilage
• 2. At the same time, the "old" cartilage
abutting the internal face of the articular Hyaline
cartilage grows
cartilage and the medullary cavity is
here
broken down and replaced by bony matrix
• This process of long-bone growth is
controlled by hormones,
• (growth hormone and, during
puberty, the sex hormones).
• It ends during adolescence, when the
epiphyseal plates are completely
converted to bone.
Hyaline
cartilage
replaced by
bone here
Epiphyseal
plate
Medullary
cavity
Bone Development and Growth
• Intramembranous ossification
•
•
•
•
•
•
Mesenchymal cells differentiate to form osteoblasts
Osteoblasts begin secreting a matrix
Osteoblasts become trapped in the matrix
Osteoblasts differentiate and form osteocytes
More osteoblasts are produced, thus move outward
Eventually, compact bone is formed
Figure 5.5 Histology of Intramembranous Ossification
Mesenchymal cells aggregate, differentiate into osteoblasts,
and begin the ossification process. The bone expands as a
series of spicules that spread into surrounding tissues.
Osteocyte in lacuna
As the spicules interconnect, they
trap blood vessels within the bone.
Bone matrix
Osteoblast
Osteoid
Embryonic connective tissue
Mesenchymal cell
Blood
vessel
Osteocytes
in lacunae
Blood
vessels
Osteoblast
layer
Over time, the bone
assumes the structure of
spongy bone. Areas of
spongy bone may later be
removed, creating
medullary cavities.
Through remodeling,
spongy bone formed in this
way can be converted to
compact bone.
Blood vessel
Blood vessel
Osteoblasts
Spicules
LM  32
LM  32
Bone Development and Growth
• Endochondral ossification
• The developing bone begins as cartilage cells
• Cartilage matrix grows inward
• Interstitial growth
• Cartilage matrix grows outward
• Appositional growth
• Blood vessels grow around the cartilage
Bone Development and Growth
• Endochondral ossification (continued)
• Perichondrial cells convert to osteoblasts
• Osteoblasts develop a superficial layer of bone around the
cartilage
• Blood vessels penetrate the cartilage
• Osteoblasts begin to develop spongy bone in the diaphysis
• This becomes the primary center of ossification
Figure 5.7a Anatomical and Histological Organization of Endochondral Ossification (Part 1 of 2)
As the cartilage enlarges,
chondrocytes near the
center of the shaft
increase greatly in size.
The matrix is reduced to a
series of small struts that
soon begin to calcify. The
enlarged chondrocytes
then die and disintegrate,
leaving cavities within the
cartilage.
Blood vessels grow
around the edges of the
cartilage, and the cells of
the perichondrium
convert to osteoblasts.
The shaft of the cartilage
then becomes ensheathed
in a superficial layer of
bone.
Enlarging
chondrocytes within
calcifying matrix
Epiphysis
Diaphysis
Bone
formation
Hyaline cartilage
Steps in the formation of a long bone from a hyaline cartilage model
Figure 5.7a Anatomical and Histological Organization of Endochondral Ossification (Part 2 of 2)
Blood vessels penetrate the
cartilage and invade the central
region. Fibroblasts migrating
with the blood vessels
differentiate into osteoblasts
and begin producing spongy
bone at a primary center of
ossification. Bone formation
then spreads along the shaft
toward both ends.
Medullary
cavity
Blood
vessel
Primary
ossification
center
Remodeling occurs as growth
continues, creating a medullary
cavity. The bone of the shaft
becomes thicker, and the cartilage
near each epiphysis is replaced by
shafts of bone. Further growth
involves increases in length (Steps 5
and 6) and diameter (see Figure 5.9).
Medullary
cavity
Superficial
bone
See
Figure 5.9
Spongy
bone
Metaphysis
Steps in the formation of a long bone from a hyaline cartilage model
Bone Development and Growth
• Endochondral ossification (continued)
•
•
•
•
The cartilage near the epiphysis converts to bone
Blood vessels penetrate the epiphysis
Osteoblasts begin to develop spongy bone in the epiphysis
Epiphysis becomes the secondary center of
ossification
Figure 5.7b Anatomical and Histological Organization of Endochondral Ossification (Part 1 of 2)
Capillaries and osteoblasts
migrate into the epiphyses,
creating secondary
ossification centers.
Hyaline cartilage
Soon the epiphyses are filled with
spongy bone. An articular cartilage
remains exposed to the joint cavity;
over time it will be reduced to a thin
superficial layer. At each
metaphysis, an epiphyseal cartilage
separates the epiphysis from the
diaphysis.
Articular cartilage
Epiphysis
Spongy
bone
Metaphysis
Periosteum
Compact
bone
Secondary
ossification
center
Epiphyseal
cartilage
Diaphysis
Bone Development and Growth
• Epiphyseal plate
•
•
•
•
Area of cartilage in the metaphysis
Also called the epiphyseal cartilage
Cartilage near the diaphysis is converted to bone
The width of this zone gets narrower as we age
Figure 5.7b Anatomical and Histological Organization of Endochondral Ossification (Part 2 of 2)
Epiphyseal
cartilage matrix
Cartilage cells
undergoing division
Zone of
proliferation
Zone of
hypertrophy
Medullary
cavity
Osteoblasts
Osteoid
Epiphyseal cartilage
Light micrograph showing the
zones of cartilage and the
advancing osteoblasts at an
epiphyseal cartilage
LM  250
Figure 5.7 Anatomical and Histological Organization of Endochondral Ossification
As the cartilage enlarges,
chondrocytes near the
center of the shaft
increase greatly in size.
The matrix is reduced to a
series of small struts that
soon begin to calcify. The
enlarged chondrocytes
then die and disintegrate,
leaving cavities within the
cartilage.
Blood vessels grow
around the edges of the
cartilage, and the cells of
the perichondrium convert
to osteoblasts. The shaft
of the cartilage then
becomes ensheathed in a
superficial layer of bone.
Blood vessels penetrate the
cartilage and invade the central
region. Fibroblasts migrating
with the blood vessels
differentiate into osteoblasts
and begin producing spongy
bone at a primary center of
ossification. Bone formation
then spreads along the shaft
toward both ends.
Remodeling occurs as growth
continues, creating a medullary
cavity. The bone of the shaft
becomes thicker, and the cartilage
near each epiphysis is replaced by
shafts of bone. Further growth
involves increases in length (Steps 5
and 6) and diameter (see Figure 5.9).
Capillaries and osteoblasts
migrate into the epiphyses,
creating secondary
ossification centers.
Hyaline cartilage
Articular cartilage
Cartilage cells
undergoing division
Epiphyseal
cartilage matrix
Spongy
bone
Epiphysis
Enlarging
chondrocytes within
calcifying matrix
Soon the epiphyses are filled with
spongy bone. An articular cartilage
remains exposed to the joint cavity;
over time it will be reduced to a thin
superficial layer. At each metaphysis,
an epiphyseal cartilage separates the
epiphysis from the diaphysis.
Zone of
proliferation
Metaphysis
Zone of
hypertrophy
Epiphysis
Medullary
cavity
Blood
vessel
Diaphysis
Primary
ossification
center
Superficial
bone
Spongy
bone
Bone
formation
Hyaline cartilage
Steps in the formation of a long bone from a hyaline cartilage model
Periosteum
Medullary
cavity
Compact
bone
Epiphyseal
cartilage
Diaphysis
See
Figure 5.9
Medullary
cavity
Metaphysis
Osteoblasts
Osteoid
Epiphyseal cartilage
Secondary
ossification
center
Light micrograph showing the
zones of cartilage and the
advancing osteoblasts at an
epiphyseal cartilage
LM  250
Figure 5.8 Epiphyseal Cartilages and Lines
X-ray of the hand of a young child. The arrows
indicate the locations of the epiphyseal cartilages.
X-ray of the hand of an adult. The arrows indicate the
locations of epiphyseal lines.
Bone Development and Growth
• Enlarging the diameter of bone
• Called appositional growth
• Blood vessels that run parallel to the bone becomes
surrounded by bone cells
• “Tunnels” begin to form
• Each “tunnel” has a blood vessel in it
Bone Development and Growth
• Enlarging the diameter of bone
• Osteoblasts begin to produce matrix, thus creating
concentric rings
• As osteoblasts are laying down more bone material,
osteoclasts are dissolving the inner bone, thus creating the
marrow cavity
Figure 5.9a Appositional Bone Growth (Part 1 of 2)
Bone formation at the surface
of the bone produces ridges
that parallel a blood vessel.
Ridge
The ridges enlarge and create
a deep pocket.
Periosteum
Perforating
canal
Artery
Three-dimensional diagrams illustrate the mechanism
responsible for increasing the diameter of a growing bone.
The ridges meet and fuse, trapping
the vessel inside the bone.
Figure 5.9a Appositional Bone Growth (Part 2 of 2)
Bone deposition proceeds inward toward
the vessel, beginning the creation of a
typical osteon.
Additional circumferential lamellae are
deposited and the bone continues to
increase in diameter.
Circumferential
lamellae
Three-dimensional diagrams illustrate the mechanism
responsible for increasing the diameter of a growing bone.
Osteon is complete with new central
canal around blood vessel. Second blood
vessel becomes enclosed.
Central canal
of new
osteon
Periosteum
Figure 5.9b Appositional Bone Growth
Bone resorbed
by osteoclasts
Infant
Child
Bone deposited
by osteoblasts
Young adult
A bone grows in diameter as new bone is added to the outer surface. At the same
time, osteoclasts resorb bone on the inside, enlarging the medullary cavity.
Adult
Figure 5.10 Circulatory Supply to a Mature Bone
Branches of
nutrient artery
and vein
Periosteum
Articular
cartilage
Epiphyseal
artery and vein
Metaphyseal
artery and
vein
Periosteum
Compact
bone
Periosteal
arteries and
veins
Connections to
superficial osteons
Medullary cavity
Nutrient artery
and vein
Nutrient foramen
Metaphyseal
artery and vein
Metaphysis
Epiphyseal
line
Bone Maintenance, Remodeling, and Repair
• Aging and the Skeletal System
• When we’re young, osteoblast activity balances with
osteoclast activity
• When we get older, osteoblast activity slows faster than
osteoclast activity
• When osteoclast activity is faster than osteoblast activity,
bones become porous
• Estrogen keeps osteoclast activity under control
Bone Maintenance, Remodeling, and Repair
• Aging and the Skeletal System
• As women age, estrogen levels drop
• Osteoclast control is lost
• Osteoclasts are overactive
• Bones become porous
• This is osteoporosis
Bone Maintenance, Remodeling, and Repair
• Injury and Repair
• When a bone is broken, bleeding occurs
• A network of spongy bone forms
• Osteoblasts are overly activated, thus resulting in enlarged
callused area
• This area is now stronger and thicker than normal bone
Clinical Note 5.3 Fractures and Their Repair (Part 3 of 4)
Repair
of a
fracture
Fracture
hematoma
Dead
bone
Bone
fragments
Immediately after the fracture,
extensive bleeding occurs.
Over a period of several
hours, a large blood clot, or
fracture hematoma, develops.
Spongy bone of
external callus
Periosteum
An internal callus forms as
a network of spongy bone
units the inner edges, and
an external callus of
cartilage and bone
stabilizes the outer edges.
Clinical Note 5.3 Fractures and Their Repair (Part 4 of 4)
External
callus
Internal
callus
External
callus
The cartilage of the external
callus has been replaced by bone,
and struts of spongy bone now
unite the broken ends. Fragments
of dead bone and the areas of
bone closest to the break have
been removed and replaced.
A swelling initially marks
the location of the fracture.
Over time, this region will
be remodeled, and little
evidence of the fracture
will remain.
Anatomy of Skeletal Elements
• Bone markings include:
•
•
•
•
•
Projections
Depressions
Fissures
Foramina
Canals (meatuses)
Figure 5.12a Examples of Bone Markings (Surface Features)
Trochanter
Head
Neck
Facet
Tubercle
Condyle
Femur
Figure 5.12b Examples of Bone Markings (Surface Features)
Fissure
Process
Foramen
Ramus
Skull, anterior view
Figure 5.12c Examples of Bone Markings (Surface Features)
Canal
Sinuses
Meatus
Skull, sagittal section
Figure 5.12d Examples of Bone Markings (Surface Features)
Tubercle
Head
Sulcus
Neck
Tuberosity
Fossa
Trochlea
Condyle
Humerus
Figure 5.12e Examples of Bone Markings (Surface Features)
Crest
Spine
Fossa
Line
Foramen
Ramus
Pelvis
Table 5.1 Common Bone Marking Terminology