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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
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
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 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
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 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).
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 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
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)
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).
Steps in the formation of a long bone from a hyaline cartilage model
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
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 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.
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 theepiphysis
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 5.10 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
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 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)
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