5 The Skeletal System: Osseous Tissue and Skeletal Structure.

5The 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

Irregular Bones

Suturalbone

Sutures

Pneumatized BonesSutural Bones

Air cellsEthmoid

Vertebra

Carpalbones

Humerus

External tableParietal bone

Internaltable

Diploë(spongy bone)

Patella

Short Bones

Flat Bones

Long Bones

Sesamoid Bones

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

Osteocyte: Mature bone cellthat maintains the bone matrix

Osteoblast

Matrix

Matrix

Canaliculi Osteocyte

Osteoid

Osteoblast: Immature bonecell that secretes organiccomponents of matrix The cells of bone

Endosteum Osteoprogenitor cell

Medullarycavity

Osteoprogenitor cell:Stem cell whose divisions produceosteoblasts

MatrixOsteoclast

Medullarycavity

Osteoclast: Multinucleate cellthat secretes acids and enzymesto 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

Concentriclamellae

Osteon

Lacunae

Osteons

A thin section throughcompact bone; in thisprocedure the intact matrixand central canals appearwhite, and the lacunae andcanaliculi are shown in black.

LM 220

Central canal

Figure 5.1d Histological Structure of a Typical Bone

A single osteon at highermagnification

Osteon LM 343

Canaliculi

Concentriclamellae

Osteon

Lacunae

Central canal

Figure 5.1b Histological Structure of a Typical BoneOsteon

Lacunae

Centralcanals

Lamellae

A scanning electronmicrograph of several osteonsin compact bone

Osteons SEM 182

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

Spongy bone

Blood vessels

Compact bone

Medullary cavity

Endosteum

Periosteum

Gross anatomyof the humerus

Compactbone

Spongybone

Medullarycavity

Concentriclamellae

Interstitiallamellae

Capillary

Small veinCircumferentiallamellae

Osteons

Periosteum

Collagen fiberorientation

Concentriclamellae

Centralcanal

Endosteum

The organization of collagenfibers within concentric lamellae

Trabeculae ofspongy bone

Centralcanal

Diagrammatic view of the histologicalorganization of compact and spongy bone

Perforatingcanal

Artery Vein

Figure 5.2d The Internal Organization in Representative Bones

Canaliculiopening

on surface

Location and structure of spongy bone. The photo shows a sectional view of the proximal end of the femur.

Trabeculae ofspongy bone

LamellaeEndosteum

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

Articularsurface ofhead of femur

Epiphysis

Metaphysis

Diaphysis(shaft)

Metaphysis

Epiphysis

Medullary cavity

Posterior view Sectional view

The femur, or thigh bone, in superficial and sectional views. The femur has adiaphysis (shaft) with walls of compact bone and epiphyses (ends) filled withspongy bone. A metaphysis separates the diaphysis and epiphysis at eachend 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 bodyweight is distributed along the bone so that the medial portion of the shaft iscompressed and the lateral portion is stretched.

Spongy bone

Compact bone

Figure 5.3b Anatomy of a Representative Bone

An intact femur chemically clearedto show the orientation of thetrabeculae in the epiphysis

Figure 5.3c Anatomy of a Representative Bone

Articular surface of head of femur

A photograph showing theepiphysis after sectioning

Compact bone

Medullary cavity

Cortex

Trabeculaeof spongy

bone

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

Articularsurface ofhead of femur

Epiphysis

Metaphysis

Diaphysis(shaft)

Metaphysis

Epiphysis

Medullary cavity

Posterior view Sectional view

The femur, or thigh bone, in superficial and sectional views. The femur has adiaphysis (shaft) with walls of compact bone and epiphyses (ends) filled withspongy bone. A metaphysis separates the diaphysis and epiphysis at eachend 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 bodyweight is distributed along the bone so that the medial portion of the shaft iscompressed and the lateral portion is stretched.

Spongy bone

Compact bone

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 oftendonboneattachment

Tendon

Medullary cavity

Endosteum

Spongy boneof epiphysis

Epiphysealcartilage

A tendonbone junction

Periosteum

LM 100

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

Figure 5.4ab Anatomy and Histology of the Periosteum and Endosteum

The endosteum is an incompletecellular layer containing osteoblasts,osteoprogenitor cells, and osteoclasts.

Joint capsule

Osteoblasts

Osteoid

Osteocyte

Osteoprogenitorcell

Endosteum

Giantmultinucleate

osteoclast

Bone matrix

Compact bone

Fibrous layerof periosteum

Cellular layerof periosteum

Endosteum

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 attachedtendons and ligaments.

Circumferentiallamellae

Cellular layerof periosteum

Fibrous layerof periosteum

Canaliculi

Lacuna

Osteocyte

Perforatingfibers

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


• 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• 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 aseries of spicules that spread into surrounding tissues.

Bone matrixOsteoblast

Osteoid

Embryonic connective tissue

Mesenchymal cellBlood vessel

Osteocyte in lacuna

Blood vessel Osteoblasts Spicules LM 32

As the spicules interconnect, theytrap blood vessels within the bone.

Osteocytesin lacunae

Bloodvessels

Osteoblastlayer

Blood vessel

Over time, the boneassumes the structure ofspongy bone. Areas of spongy bone may later beremoved, creatingmedullary cavities.Through remodeling,spongy bone formed in thisway can be converted tocompact bone.

LM 32


• 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


• 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 thecenter of the shaftincrease greatly in size.The matrix is reduced to aseries of small struts thatsoon begin to calcify. Theenlarged chondrocytesthen die and disintegrate,leaving cavities within thecartilage.

Blood vessels growaround the edges of thecartilage, and the cells ofthe perichondrium convert to osteoblasts. The shaft of the cartilage then becomes ensheathed in a superficial layer of bone.

Epiphysis

Diaphysis

Boneformation

Enlargingchondrocytes within

calcifying matrix

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 thecartilage and invade the centralregion. Fibroblasts migratingwith the blood vesselsdifferentiate into osteoblastsand begin producing spongybone at a primary center ofossification. Bone formationthen spreads along the shafttoward both ends.

Blood vessel

Medullarycavity

Primaryossificationcenter

Superficialbone

Spongybone

Medullarycavity

Metaphysis

SeeFigure 5.9

Remodeling occurs as growthcontinues, creating a medullarycavity. The bone of the shaftbecomes thicker, and the cartilagenear each epiphysis is replaced byshafts of bone. Further growthinvolves increases in length (Steps 5and 6) and diameter (see Figure 5.9).



• 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 osteoblastsmigrate into the epiphyses,creating secondaryossification centers.

Soon the epiphyses are filled withspongy bone. An articular cartilageremains exposed to the joint cavity;over time it will be reduced to a thinsuperficial layer. At each metaphysis, an epiphyseal cartilage separates the epiphysis from the diaphysis.

Articular cartilage

Spongybone

Epiphysealcartilage

Diaphysis

Hyaline cartilage

Epiphysis

Metaphysis

Periosteum

Compactbone

Secondaryossification

center


• 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)

Epiphysealcartilage matrix

Cartilage cellsundergoing division

Zone of proliferation

Zone of hypertrophy

Medullarycavity

Osteoblasts Osteoid

Epiphyseal cartilage

Light micrograph showing thezones of cartilage and theadvancing osteoblasts at anepiphyseal cartilage

LM 250

Figure 5.7 Anatomical and Histological Organization of Endochondral Ossification

As the cartilage enlarges,chondrocytes near thecenter of the shaftincrease greatly in size.The matrix is reduced to aseries of small struts thatsoon begin to calcify. Theenlarged chondrocytesthen die and disintegrate,leaving cavities within thecartilage.

Blood vessels growaround the edges of thecartilage, and the cells ofthe perichondrium convertto osteoblasts. The shaftof the cartilage thenbecomes ensheathed in asuperficial layer of bone.

Blood vessels penetrate thecartilage and invade the centralregion. Fibroblasts migratingwith the blood vesselsdifferentiate into osteoblastsand begin producing spongybone at a primary center ofossification. Bone formationthen spreads along the shafttoward both ends.

Epiphysis

Diaphysis

Blood vessel

Medullarycavity

Primaryossificationcenter

Superficialbone

Spongybone

Boneformation

Medullarycavity

Metaphysis

SeeFigure 5.9

Enlargingchondrocytes within

calcifying matrix

Hyaline cartilage

Remodeling occurs as growthcontinues, creating a medullarycavity. The bone of the shaftbecomes thicker, and the cartilagenear each epiphysis is replaced byshafts of bone. Further growthinvolves increases in length (Steps 5and 6) and diameter (see Figure 5.9).


Epiphysealcartilage matrix

Cartilage cellsundergoing division

Zone of proliferation

Zone of hypertrophy

Medullarycavity

Osteoblasts Osteoid

Epiphyseal cartilage

Light micrograph showing thezones of cartilage and theadvancing osteoblasts at anepiphyseal cartilage

LM 250

Soon the epiphyses are filled withspongy bone. An articular cartilageremains exposed to the joint cavity;over time it will be reduced to a thinsuperficial layer. At each metaphysis,an epiphyseal cartilage separates theepiphysis from the diaphysis.

Articular cartilage

Spongybone

Epiphysealcartilage

Diaphysis

Capillaries and osteoblastsmigrate into the epiphyses,creating secondaryossification centers.

Hyaline cartilage

Epiphysis

Metaphysis

Periosteum

Compactbone

Secondaryossification

center

Figure 5.8 Epiphyseal Cartilages and Lines

X-ray of the hand of a young child. The arrowsindicate the locations of the epiphyseal cartilages.

X-ray of the hand of an adult. The arrows indicate thelocations of epiphyseal lines.

Figure 5.6 Fetal Intramembranous and Endochondral Ossification

Intramembranousossificationproduces theroofing bones ofthe skull

Temporalbone

Mandible

Clavicle

Scapula

Humerus

Femur

Vertebrae

Hip bone(ilium)

Endochondralossificationreplaces cartilagesof embryonic skull

Primary ossificationcenters of thediaphyses (bonesof the lower limb)

Futurehip bone

At 10 weeks the fetal skull clearly showsboth membrane and cartilaginous bone,but the boundaries that indicate the limitsof future skull bones have yet to beestablished.

At 16 weeks the fetal skull shows the irregularmargins of the future skull bones. Mostelements of the appendicular skeleton formthrough endochondral ossification. Note theappearance of the wrist and ankle bones at 16weeks versus at 10 weeks.

Parietal bone

Frontal bone

Metacarpal bones

Phalanges

Radius

UlnaCartilage

FibulaTibia

Phalanx

Metatarsal bones

Ribs


• 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


• 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 surfaceof the bone produces ridgesthat parallel a blood vessel.

The ridges enlarge and createa deep pocket.

Perforatingcanal

Ridge

Artery

Periosteum

Three-dimensional diagrams illustrate the mechanismresponsible for increasing the diameter of a growing bone.

The ridges meet and fuse, trappingthe vessel inside the bone.

Figure 5.9a Appositional Bone Growth (Part 2 of 2)

Central canalof newosteon

Periosteum

Osteon is complete with new centralcanal around blood vessel. Second bloodvessel becomes enclosed.

Additional circumferential lamellae aredeposited and the bone continues toincrease in diameter.

Circumferentiallamellae

Bone deposition proceeds inward towardthe vessel, beginning the creation of atypical osteon.

Three-dimensional diagrams illustrate the mechanismresponsible for increasing the diameter of a growing bone.

Figure 5.9b Appositional Bone Growth

Bone resorbed by osteoclasts

Bone depositedby osteoblastsChild

Infant

Young adult AdultA bone grows in diameter as new bone is added to the outer surface. At the sametime, osteoclasts resorb bone on the inside, enlarging the medullary cavity.


• 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 theepiphysis


• 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 5.10 Circulatory Supply to a Mature Bone

Branches ofnutrient artery

and vein

Periostealarteries and

veins

Periosteum

Connections tosuperficial osteons

Nutrient arteryand vein

Nutrient foramen

Metaphysealartery and vein

ArticularcartilageEpiphysealartery and vein

Metaphysealartery andvein

Periosteum

Compactbone

Medullary cavity

Metaphysis

Epiphysealline


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


• 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


• Factors Regulating Bone Growth (continued)• Hormones: Thyroid gland

Releases calcitonin• Inhibits osteoclasts• Removes calcium ions from blood and adds it to bone


• Factors Regulating Bone Growth (continued)• Hormones: Thyroid gland

• Releases thyroxine (T4)• Maintains normal activity of the epiphyseal cartilage


• Factors Regulating Bone Growth (continued)• Hormones: Pituitary gland

• Releases growth hormone (somatotropin)• Maintains normal activity of the epiphyseal cartilage

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


• Aging and the Skeletal System• As women age, estrogen levels drop• Osteoclast control is lost

• Osteoclasts are overactive• Bones become porous

• This is osteoporosis


• 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)

Fracturehematoma

Deadbone

Bone fragments

Spongy bone ofexternal callus

Periosteum

Immediately after the fracture,extensive bleeding occurs.Over a period of severalhours, a large blood clot, orfracture hematoma, develops.

Repair of afracture

An internal callus forms asa network of spongy boneunits the inner edges, andan external callus ofcartilage and bonestabilizes the outer edges.

Clinical Note 5.3 Fractures and Their Repair (Part 4 of 4)

Internalcallus

Externalcallus

Externalcallus

A swelling initially marksthe location of the fracture.Over time, this region willbe remodeled, and little evidence of the fracturewill remain.

The cartilage of the externalcallus has been replaced by bone,and struts of spongy bone nowunite the broken ends. Fragmentsof dead bone and the areas ofbone closest to the break havebeen removed and replaced.

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

Femur

Facet

Tubercle

Condyle

Figure 5.12b Examples of Bone Markings (Surface Features)

Fissure

Ramus

Process

Foramen

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)

Humerus

Condyle

Trochlea

Fossa

Tuberosity

Neck

SulcusHead

Tubercle

Figure 5.12e Examples of Bone Markings (Surface Features)

Crest

Pelvis

Spine

Line

Foramen

Fossa

Ramus

Table 5.1 Common Bone Marking Terminology

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