5

Introduction
•
• Skeletal bones
• Cartilage
• Ligaments
• Connective tissue to stabilize the skeleton
•

Introduction
•
• 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
•
• 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
•
• Sutural bones
• Irregular bones
• Short bones
• Pneumatized bones
• Flat bones
• Long bones
• Sesamoid 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
Sutures
Sutural bone
Pneumatized Bones
Ethmoid
Air cells
Irregular Bones
Vertebra
Short Bones
Carpal bones
Flat Bones
Parietal bone External table
Internal table
Diploë
(spongy bone)
Long Bones
Sesamoid Bones
Humerus
Patella

Structure of Bone
•
• Supporting connective tissue
• Specialized cells
• Solid matrix
• Outer lining
• Called the periosteum
• Inner lining
• Called the endosteum

Structure of Bone
•
• The matrix
• Calcium phosphate eventually converts to
hydroxyapatite crystals
• Hydroxyapatite crystals resist compression

Structure of 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
•
• 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
•
• 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
Osteocyte: Mature bone cell that maintains the bone matrix
Osteoblast Osteoid Matrix
Osteoblast: Immature bone cell that secretes organic components of matrix
The cells of bone
Endosteum Osteoprogenitor cell
Medullary cavity
Osteoprogenitor cell:
Stem cell whose divisions produce osteoblasts
Osteoclast Matrix
Medullary cavity
Osteoclast: Multinucleate cell that secretes acids and enzymes to dissolve bone matrix

Structure of Bone
•
• 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
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.
Canaliculi
Concentric lamellae
Central canal
Osteon
Lacunae

Figure 5.1d Histological Structure of a Typical Bone
Canaliculi
Concentric lamellae
Central canal
Osteon
Lacunae
Osteon
A single osteon at higher magnification
LM

343

Figure 5.1b Histological Structure of a Typical Bone
Osteon
Lacunae
Central canals
Osteons SEM

182
A scanning electron micrograph of several osteons in compact bone
Lamellae

Structure of Bone
•
• 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
Spongy bone
Blood vessels
Compact bone
Medullary cavity
Endosteum
Collagen fiber orientation
Concentric lamellae
The organization of collagen fibers within concentric lamellae
Periosteum
Gross anatomy of the humerus
Compact bone
Spongy bone
Medullary cavity
Concentric lamellae
Interstitial lamellae
Capillary
Small vein
Central canal
Endosteum
Circumferential lamellae
Osteons
Periosteum
Trabeculae of spongy bone
Artery Vein
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
Location and structure of spongy bone. The photo shows a sectional view of the proximal end of the femur.
Endosteum
Lamellae

Structure of Bone
•
• 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
•
• 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
Epiphysis
Metaphysis Articular surface of head of femur
Spongy bone
Compact bone
Diaphysis
(shaft)
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.
Medullary cavity

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
•
• 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
Epiphysis
Metaphysis Articular surface of head of femur
Spongy bone
Compact bone
Diaphysis
(shaft)
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.
Medullary cavity

Structure of Bone
•
• 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
•
• 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
LM

100
Zone of tendon  bone attachment
Tendon
Periosteum
Medullary cavity
Endosteum
Spongy bone of epiphysis
Epiphyseal cartilage
A tendon
 bone junction

Structure of Bone
•
• Endosteum
• Inner surface of bone
• Lines the medullary cavity
• Consists of osteoprogenitor cells
• Actively involved in repair and growth

Figure 5.4ab Anatomy and Histology of the Periosteum and Endosteum
Endosteum
Joint capsule
Cellular layer of periosteum
Fibrous layer of periosteum
Compact bone
Circumferential lamellae
Cellular layer of periosteum
Fibrous layer of periosteum
Canaliculi
Lacuna
Osteocyte
Perforating fibers
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 matrix
Giant multinucleate osteoclast
Endosteum
Osteoprogenitor cell
Osteocyte
Osteoid
Osteoblasts
The endosteum is an incomplete cellular layer containing osteoblasts, osteoprogenitor cells, and osteoclasts.

Bone Development and Growth
•
• Cartilage cells will be replaced by bone cells
• This is called ossification
•
• Bone formation
•
• The deposition of calcium ions into the bone tissue

Bone Development and Growth
•
• Intramembranous ossification
• Involved in the development of clavicle, mandible, skull, and face
• Endochondral ossification
• Involved in the development of limbs, vertebrae, and hips

Bone Development and Growth
•
• 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
Bone matrix
Osteoblast
Osteoid
Embryonic connective tissue
Mesenchymal cell
Blood vessel
As the spicules interconnect, they trap blood vessels within the bone.
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
•
• 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
•
• 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.
Enlarging chondrocytes within calcifying matrix
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.
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.
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).
Blood vessel
Medullary cavity
Primary ossification center
Superficial bone
Spongy bone
Medullary cavity
See
Figure 5.9
Metaphysis
Steps in the formation of a long bone from a hyaline cartilage model

Bone Development and Growth
•
• 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
Epiphysis
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
Spongy bone
Metaphysis
Periosteum
Compact bone
Epiphyseal cartilage
Diaphysis
Secondary ossification center

Bone Development and Growth
•
• 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 LM

250
Light micrograph showing the zones of cartilage and the advancing osteoblasts at an epiphyseal cartilage

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.
Enlarging chondrocytes within calcifying matrix
Hyaline 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.
Bone formation
Epiphysis
Diaphysis
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.
Blood vessel
Medullary cavity
Primary ossification center
Superficial bone
Spongy bone
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
Capillaries and osteoblasts migrate into the epiphyses, creating secondary ossification centers.
Hyaline cartilage
Epiphysis
Metaphysis
Periosteum
Compact bone
See
Figure 5.9
Metaphysis
Secondary ossification center
Steps in the formation of a long bone from a hyaline cartilage model
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
Spongy bone
Epiphyseal cartilage
Diaphysis
Epiphyseal cartilage matrix
Cartilage cells undergoing division
Medullary cavity
Osteoblasts
Epiphyseal cartilage
Osteoid
LM

250
Light micrograph showing the zones of cartilage and the advancing osteoblasts at an epiphyseal cartilage
Zone of proliferation
Zone of hypertrophy

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.

Figure 5.6 Fetal Intramembranous and Endochondral Ossification
Intramembranous ossification produces the roofing bones of the skull
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.
Endochondral ossification replaces cartilages of embryonic skull
Primary ossification centers of the diaphyses (bones of the lower limb)
Future hip bone
Temporal bone
Mandible
Clavicle
Scapula
Humerus
Ribs
Vertebrae
Hip bone
(ilium)
Femur
Parietal bone
Frontal bone
Metacarpal bones
Phalanges
Radius
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.

Bone Development and Growth
•
• 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
•
• 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 Periosteum
The ridges enlarge and create a deep pocket.
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
Osteon is complete with new central canal around blood vessel. Second blood vessel becomes enclosed.
Periosteum
Central canal of new osteon
Three-dimensional diagrams illustrate the mechanism responsible for increasing the diameter of a growing bone.

Figure 5.9b Appositional Bone Growth
Bone resorbed by osteoclasts
Infant
Bone deposited by osteoblasts
Child
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.
Young adult Adult

Bone Development and Growth
•
• 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
•
• Epiphyseal vessels
• Supply nutrients to the medullary cavities of the epiphysis
• Periosteal vessels
• Supply nutrients to the superficial osteons

Figure 5.10 Circulatory Supply to a Mature Bone
Branches of nutrient artery and vein
Periosteum
Periosteal arteries and veins
Connections to superficial osteons
Nutrient artery and vein
Nutrient foramen
Metaphyseal artery and vein
Articular cartilage
Epiphyseal artery and vein
Metaphyseal artery and vein
Periosteum
Compact bone
Medullary cavity
Metaphysis
Epiphyseal line

Bone Development and Growth
•
• Nutrition
• Calcium ions
• Phosphate ions
• Magnesium ions
• Citrate
• Carbonate ions
• Sodium ions
• Vitamins A, C, D (calcitriol)

Bone Development and Growth
•
• 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
•
• Hormones: Thyroid gland
Releases calcitonin
• Inhibits osteoclasts
• Removes calcium ions from blood and adds it to bone

Bone Development and Growth
•
• Hormones: Thyroid gland
• Releases thyroxine (T
4
)
• Maintains normal activity of the epiphyseal cartilage

Bone Development and Growth
•
• Hormones: Pituitary gland
• Releases growth hormone (somatotropin)
• Maintains normal activity of the epiphyseal cartilage

Bone Maintenance, Remodeling, and Repair
•
• 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
•
• As women age, estrogen levels drop
• Osteoclast control is lost
• Osteoclasts are overactive
• Bones become porous
• This is osteoporosis

Bone Maintenance, Remodeling, 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)
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.
External callus
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
•
• Projections
• Depressions
• Fissures
• Foramina
• Canals (meatuses)

Figure 5.12a Examples of Bone Markings (Surface Features)
Trochanter
Head
Neck
Facet
Tubercle
Femur
Condyle

Figure 5.12b Examples of Bone Markings (Surface Features)
Fissure
Ramus
Skull, anterior view
Process
Foramen

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
Condyle
Humerus
Fossa
Trochlea

Figure 5.12e Examples of Bone Markings (Surface Features)
Spine
Line
Foramen
Pelvis
Ramus
Crest
Fossa

Table 5.1 Common Bone Marking Terminology