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Basics of the
Skeletal
System and
Articulations
Learn and Understand
• Skeleton is more than just bone
• Functions go beyond support
• Bone grows upon existing bone or upon existing
cartilage, bone never grows by expanding existing
bone from within
• The pattern of bone growth in the fetus aids in birth
• Bones exhibit a trade off between strength and
weight
• Many joints do not allow movement
• Synovial joints incorporate numerous adaptations
which protect them from damage even when
frequently used
Functions of Bones
Seven important functions
– Support
– Protection
– Movement
– Mineral storage
– Blood cell formation
– Triglyceride (fat) storage
– Hormone production
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Components of Skeletal System
• Bone
– Compact or cancellous
(spongy)
• Cartilage
– Hyaline
– Fibrocartilage
– Elastic
• Tendons, ligaments
Figure 6.1 The bones and cartilages of the
human skeleton.
Cartilage inexternal ear
Cartilage inintervertebraldisc
Pubicsymphysis
Meniscus(padlike cartilagein knee joint)
Articularcartilage of a joint
Costalcartilage
Articularcartilageof a joint
Cartilages innose
EpiglottisThyroidcartilageCricoidcartilage
Larynx
TracheaLung
Bones of skeleton
Axial skeleton
Appendicular skeleton
Hyaline cartilages
Elastic cartilages
Fibrocartilages
Cartilages
Respiratory tube cartilagesin neck and thorax
Hyaline Cartilage and Interstitial Growth
Interstitial Growth:
An increase in the size of a tissue by cell
division within the interior of a part or
structure that is already formed
Daughter cells
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Cells of Bone Tissue
Cell Type Location Function
Osteo-
progenitor
cells
Periosteum,
endosteum
When stimulated, divide into
osteoblasts or bone lining cells
– “osteogenic”
Osteoblasts Initially on edge
of existing
bone, then in
matrix (become
osteocytes)
Form bone by secreting matrix
components which assemble
outside of blasts and
eventually entrap them in hard
matrix
Osteocytes Lacunae of
bone matrix
Monitor and maintain bone
matrix, stimulate bone
remodeling in response to
physical stress
Osteoclasts(derived from
macrophages)
Edges of
existing bone,
temporarily
occupy an area
Summoned to an area by
osteocytes or signaling
chemicals, have ruffled border
that uses H+ and proteolytic
enzymes to dissolve bone
matrix
Figure 6.7 Microscopic anatomy of compact bone. Compact bone Spongy bone
Central(Haversian) canal
Osteon(Haversian system)
Circumferentiallamellae
Perforating (Volkmann’s) canal
Endosteum lining bony canalsand covering trabeculae
Perforating (Sharpey’s) fibers
Periosteal blood vesselPeriosteum
Lamellae
NerveVeinArtery
CanaliculiOsteocytein a lacuna
LamellaeCentralcanalLacunae
Interstitiallamella
Lacuna(with osteocyte)
Blood & Nervous Supply:
good throughout compact bone,
indirect supply to cancellous trabeculae
Figure 6.6 A single osteon.
Structuresin thecentralcanal
Artery withcapillaries
Vein
Nerve fiber
Collagenfibersrun indifferentdirections
Twistingforce
Lamellae
Structural unit of
compact bone
Hollow tubes of bone
matrix called
lamellae
• Collagen fibers in
adjacent rings run
in different
directions
– Withstands
stress – resist
twisting
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Figure 6.4c The structure of a long bone (humerus of arm).
Endosteum
Yellowbone marrow
Compact bone
Periosteum
Perforating(Sharpey’s)fibers
Nutrientarteries
Periosteum
• White, double-layered
membrane
• Covers external surfaces
except joint surfaces
• Outer fibrous layer of
dense irregular connective
tissue
– Sharpey's fibers secure to
bone matrix
• Many nerve fibers and
blood vessels
• Anchoring points for
tendons and ligaments
Figure 6.4a The structure of a long bone (humerus of arm).Articularcartilage
Spongy bone
Epiphysealline
Periosteum
Compact bone
Medullarycavity (linedby endosteum)
Proximalepiphysis
Diaphysis
Distalepiphysis
Epiphyseal line
• Remnant of
childhood bone
growth at
epiphyseal plate
Figure 6.3 Flat bones consist of a layer of spongy bone sandwiched between two thin layers of compact bone.
Spongy bone(diploë)
Compact bone
Trabeculae ofspongy bone
Spongy Bone
• Trabeculae– Align along lines of stress to
help resist it
– No osteons
– Contain irregularly arranged
lamellae and osteocytes
interconnected by canaliculi
– Capillaries in endosteum
supply nutrients
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Bone Development
• Ossification (osteogenesis)
– Process of bone tissue formation
– Formation of bony skeleton• Embryonic skeleton ossifies predictably
• Begins in 2nd month of fetal development
– Most long bones begin ossifying by 8 weeks
– Primary ossification centers by 12 weeks
• At birth, most long bones well ossified (except epiphyses)
• At age 25 ~ all bones completely ossified and skeletal growth
ceases
– Bone remodeling and repair
• Lifelong
Approximate size of a human conceptus from
fertilization to the early fetal stage
Fertilization 1-week
conceptus
3-week
embryo
(3 mm)
5-week embryo
(10 mm)
Embryo
8-week embryo
(22 mm)
12-week fetus
(90 mm)
Bone Development: Two Types of Ossification
Intramembranous
ossificationBone develops from fibrous connective
tissue membranes 1. Ossification centers appear
2. Osteoid is secreted
3. Woven bone and periosteum form
4. Lamellar bone replaces woven bone &
red marrow appears
Forms flat bones, e.g. clavicles and
cranial bones
Endochondral
ossificationBone forms by replacing cartilage
(endochondral) bones made of hyaline
cartilage
Forms most of skeleton
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Figure 6.8 Endochondral
ossification in a long bone.
Week 9
Month 3
Birth Childhood toadolescence
Hyalinecartilage
Bone collar
Primaryossificationcenter
Area ofdeterioratingcartilage matrix
Spongyboneformation
Bloodvessel ofperiostealbud
Epiphysealblood vessel
Secondaryossificationcenter
Articularcartilage
Spongybone
Epiphysealplatecartilage
Medullarycavity
Bone collarforms around thediaphysis of thehyaline cartilagemodel.
Cartilage in thecenter of thediaphysis calcifiesand then developscavities.
The periostealbud invades theinternal cavitiesand spongy boneforms.
The diaphysiselongates and amedullary cavityforms. Secondaryossificationcenters appear inthe epiphyses.
The epiphysesossify. Whencompleted, hyalinecartilage remainsonly in theepiphyseal platesand articularcartilages.
1 2 3 4 5
Lengthening of Long Bones in Childhood
and Adolescence
• Requires presence of epiphyseal cartilage
• Epiphyseal plate maintains constant thickness
– Rate of cartilage growth on one side balanced by bone replacement on other
• Concurrent remodeling of epiphyseal ends to maintain proportion
• Result of five zones within cartilage
– Resting (quiescent) zone
– Proliferation (growth) zone
– Hypertrophic zone
– Calcification zone
– Ossification (osteogenic) zone
Figure 6.10 Growth in length of a long bone occurs at the epiphyseal plate.
Resting zone
1 Proliferation zoneCartilage cells undergomitosis.
2 Hypertrophic zoneOlder cartilage cellsenlarge.
3 Calcification zoneMatrix calcifies; cartilagecells die; matrix beginsdeteriorating; bloodvessels invadecavity.
4 Ossification zoneNew bone forms.
Calcifiedcartilage spicule
Osteoblastdepositing bonematrix
Osseous tissue (bone) coveringcartilage spicules
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Growth in Length of Long Bones
• Near end of adolescence chondroblasts divide less often
• Epiphyseal plate thins then is replaced by bone
• Epiphyseal plate closure
– Bone lengthening ceases• Requires presence of cartilage
– Bone of epiphysis and diaphysis fuses
– Females – about 18 years
– Males – about 21 years
Appositional Growth: Growth in Width
• Allows lengthening bone to widen
• Occurs throughout life
• Osteoblasts beneath periosteum secrete
bone matrix on external bone
• Osteoclasts remove bone on endosteal
surface
• Usually more building up than breaking
down
Thicker, stronger bone but not too heavy
Bone Homeostasis
• Recycle 5-7% of bone mass each week
– Spongy bone replaced ~ every 3-4 years
– Compact bone replaced ~ every 10 years
• Older bone becomes more brittle
– Calcium salts crystallize
– Fractures more easily
• Consists of bone remodeling and bone repair
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Figure 6.15 Stages in the healing of a bone fracture.
Hematoma
Internalcallus(fibroustissue andcartilage)
Externalcallus
Newbloodvessels
Spongybonetrabecula
Bonycallus ofspongybone
Healedfracture
1 A hematoma forms. 2 Fibrocartilaginous
callus forms.
3 Bony callus
forms.4 Bone
remodeling
occurs.
Stages of Bone Repair
Joints (Articulations)
• Site where two or more bones meet
• Functions of joints
– Give skeleton mobility
– Hold skeleton together
• Two classifications
– Functional – based on amount of movement
• Synarthroses—immovable joints
• Amphiarthroses—slightly movable joints
• Diarthroses—freely movable joints
– Structural – based on binding materials and
presence/absence of joint cavity
• Fibrous joints
• Cartilaginous joints
• Synovial joints – has a joint cavity
Fibrous Joints
• Bones joined by dense fibrous connective
tissue
• No joint cavity
• Most synarthrotic (immovable)
– Depends on length of connective tissue fibers
• Three types:
– Sutures
– Syndesmoses
– Gomphoses
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Figure 8.1a Fibrous joints.Suture
Joint held together with very short,
interconnecting fibers, and bone edges
interlock. Found only in the skull.
Densefibrousconnectivetissue
Sutureline
• Rigid, interlocking joints
• Immovable joints for
protection of brain
• Contain short connective
tissue fibers
• Allow for growth during
youth
• In middle age, sutures
ossify and fuse
– Called Synostoses
Figure 8.1b Fibrous joints.
• Bones connected by
ligaments
• Fiber length varies so
movement varies
– Little to no movement at
inferior tibiofibular joint
– Large amount of movement
at interosseous membrane
connecting radius and ulna
• Interosseous
membrane Radioulnar Syndesmosis
Figure 8.1c Fibrous joints.
Gomphosis
“Peg in socket” fibrous joint.
Periodontal ligament holds
tooth in socket.
Socket ofalveolarprocess
Periodontalligament
Root oftooth
• Peg-in-socket joints of
teeth in alveolar sockets
• Fibrous connection is the
periodontal ligament
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Cartilaginous Joints
• Bones united by cartilage
• No joint cavity
• Not highly movable
• Two types:
– Synchondroses
– Symphyses
Synchondroses
Bones united by hyaline cartilage
Epiphysealplate (temporaryhyaline cartilagejoint)
Sternum(manubrium)
Joint between firstrib and sternum(immovable)
Bar/plate of hyaline cartilage unites bones
– Temporary epiphyseal plate joints
• Become synostoses after plate closure
– Cartilage of 1st rib with manubrium
– Many are synarthrotic
Cartilaginous Joints: Synchondroses
Symphyses
Bones united by fibrocartilage
Body of vertebra
Fibrocartilaginous intervertebral disc (sandwiched between hyaline cartilage)
Pubic symphysis
• Fibrocartilage unites bone
– Hyaline cartilage present as articular cartilage
• Strong, flexible amphiarthroses
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Synovial Joints: Six Distinguishing Features
1. Articular cartilage: hyaline cartilage
– Prevents crushing of bone ends
2. Joint (synovial) cavity
– Small, fluid-filled potential space
3. Articular (joint) capsule
– Two layers
• External Fibrous layer
– Dense irregular connective tissue
• Inner Synovial membrane
– Loose connective tissue
– Makes synovial fluid
Figure 8.3 General structure of a synovial joint.
Ligament
Joint cavity(containssynovial fluid)
Articular (hyaline)cartilage
Fibrouslayer
Synovialmembrane(secretes synovial fluid)
Articularcapsule
Periosteum
Synovial Joints: Six Distinguishing Features
4. Synovial fluid
– Viscous, slippery filtrate of plasma and
hyaluronic acid
– Lubricates and nourishes articular cartilage
– Contains phagocytic cells to remove microbes
and debris
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Synovial Joints: Six Distinguishing Features
5. Different types of reinforcing ligaments
– Capsular
• Thickened part of fibrous layer
– Extracapsular
• Outside the capsule
– Intracapsular
• Deep to capsule; covered by synovial membrane
6. Nerves and blood vessels
– Nerve fibers detect pain, monitor joint position
and stretch
– Capillary beds supply filtrate for synovial fluid
Other Features of Some Synovial Joints
• Fatty pads
– For cushioning between fibrous layer and
synovial membrane or bone
• Articular discs (menisci)
– Fibrocartilage separates articular surfaces to
improve "fit" of bone ends, stabilize joint, and
reduce wear and tear
Structures Associated with Synovial Joints
• Bursae
– Sacs lined with synovial membrane
• Contain synovial fluid
– Reduce friction where ligaments, muscles,
skin, tendons, or bones rub together
• Tendon Sheaths
– Elongated bursa wrapped completely around
tendon subjected to friction
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Three Stabilizing Factors at Synovial Joints
• Shapes of articular surfaces (minor role)
• Ligament number and location (limited
role)
• Muscle tendons that cross joint (most
important)
– Muscle tone keeps tendons taut
• Extremely important in reinforcing shoulder and
knee joints and arches of the foot
• Largest, most complex joint of body
• Three joints surrounded by a single joint cavity
– Femoropatellar joint
• Plane joint
• Allows gliding motion during knee flexion
– Lateral and medial tibiofemoral joints
• Femoral condyles with lateral and medial menisci of tibia
• Allow flexion, extension, and some rotation when knee partly flexed
Knee Joint
Figure 8.8c The knee joint.
Anterior view of right knee
Quadricepsfemorismuscle
Lateralpatellarretinaculum
Tibialcollateralligament
Fibula
Fibularcollateralligament
Patella
Tendon ofquadricepsfemorismuscle
Patellarligament
Medialpatellarretinaculum
Tibia
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Figure 8.8d The knee joint.
Tibia
Arcuatepoplitealligament
Obliquepoplitealligament
Bursa
Popliteusmuscle(cut)
Lateralhead ofgastrocnemiusmuscle
Articularcapsule
Medial head ofgastrocnemiusmuscle
Fibularcollateralligament
Tibialcollateralligament
Tendon ofsemimembranosusmuscle
Posterior view of the joint capsule, including ligaments
FemurTendon ofadductormagnus
Figure 8.8a The knee joint.
Patellar ligament
Sagittal section through the right knee joint
Anteriorcruciateligament
Posteriorcruciateligament
Tibia
Femur
Lateralmeniscus
Articularcapsule
Lateral meniscus
Synovial cavity
Infrapatellarfat pad
Subcutaneousprepatellar bursa
Patella
Suprapatellarbursa
Tendon ofquadricepsfemoris
Deep infrapatellerbursa
Figure 8.8e The knee joint.
Fibularcollateralligament
Lateralcondyleof femurLateralmeniscus
Tibia
Fibula
Anterior view of flexed knee, showingthe cruciate ligaments (articularcapsule removed, and quadricepstendon cut and reflected distally)
Posteriorcruciateligament
Medialcondyle
Tibialcollateralligament
Medialmeniscus
Patellarligament
Patella
Anteriorcruciateligament
Quadricepstendon
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Figure 8.8b The knee joint.
Articularcartilage on lateral tibialcondyle
Posteriorcruciateligament
Lateralmeniscus
Superior view of the right tibia in the knee joint, showingthe menisci and cruciate ligaments
Anteriorcruciateligament
Medialmeniscus
Articularcartilageon medialtibial condyle
Anterior
Anterior Knee
https://www.youtube.com/watch?feature=pl
ayer_detailpage&v=_q-Jxj5sT0g
Posterior Knee
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Optional Slides:
• Stages in bone repair that accompany a
Figure 6.15
• Structural and functional differences
between cartilage and bone
• Structural and functional differences
between cancellous bone and compact
bone
Stages of Bone Repair: HEMATOMA Forms
• Torn blood vessels hemorrhage
• Clot (hematoma) forms
• Site swollen, painful, and inflamed
Stages of Bone Repair:
Fibrocartilaginous Callus Forms
• Capillaries grow into hematoma
• Phagocytic cells clear debris
• Fibroblasts secrete collagen fibers to span break and connect broken ends
• Fibroblasts, cartilage, and osteogenic cells begin reconstruction of bone
– Create cartilage matrix of repair tissue
– Osteoblasts form spongy bone within matrix
• Mass of repair tissue called fibrocartilaginous callus
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Stages of Bone Repair:
Bony Callus Forms
• Within one week new trabeculae appear in
fibrocartilaginous callus
• Callus converted to bony (hard) callus of
spongy bone
• ~2 months later firm union forms
Stages of Bone Repair:
Bone Remodeling Occurs
• Begins during body callus formation
• Continues for several months
• Excess material on diaphysis exterior and
within medullary cavity removed
• Compact bone laid down to reconstruct
shaft walls
• Final structure resembles original because
responds to same mechanical stressors
Cartilage Characteristics Bone Characteristics
living
components
chondroblasts, chondrocytes osteogenic cells, osteoblasts, osteoclasts,
osteocytes
matrix proteinaceous:
packed collagen fibers,
proteoglycan, water
inorganic calcium hydroxyapatite and
protein:
CaPO4 – 65% (and related Ca minerals)
collagen, proteoglycan – 35%
sheath perichondrium, except articular
surfaces
periosteum, endosteum
Not at articular surfaces
growth interstitial and appositional Appositional: endochondral and
intramembranous
blood &
nervous
supply
good blood supply at perichondrium,
none within cartilage
good throughout compact bone,
indirect supply to cancellous trabeculae
types hyaline, fibrocartilage, elastic Woven (temporary) and lamellar
cancellous and compact
benefits smooth articulation surfaces,
flexible, resilient, strong but lighter
than bone
rigid strength, protection, reservoir
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Cancellous
(Trabecular, Spongy)
Compact
Trabeculae covered with endosteum,
encased in compact bone
Osteons cemented together by
interstitial lamellae – endosteum
internally, periosteum externally
Creates a network of spaces for bone
marrow
Essentially a solid mass
No direct blood supply to the bone
matrix, rich blood supply to the
marrow-containing cavities
Network of blood vessels throughout
Strong yet lighter than compact,
orientation reflects lines of stress
Strongest, densest bone available
Epiphyses of long bones, internal
portions of short, flat, and irregular
bones
Shafts of long bones, thin outer
covering of most bones