skeletal tissues

8/20/2019 Skeletal Tissues

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The Skeletal System

Tuesday, 23 October 12


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The Skeletal System

• Functions of the Skeletal System

• Support against gravity

• Leverage for muscle action - movement

• Protection of soft internal organs

• Blood cell production

• Storage - calcium, phosphorous, fat



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The Skeletal System

• The skeletal system includes:• Cartilages

• Bones

• Joints• Ligaments

• Other connective tissues



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Cartilage

• Embryo

• More prevalent than in

adult• Skeleton initially mostly

cartilage

•

Bone replaces cartilagein fetal and childhoodperiods



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• Specialized connective tissue

• Rigid, Elastic, Resilient -

RESISTS COMPRESSION

• AVASCULAR –

nutrients diffuse throughmatrix



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Location of cartilage in adults

• External ear

• Nose

• “Articular” – coveringthe ends of most bonesand movable joints

• “Costal” – connectingribs to sternum

• Larynx - voice box



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• Epiglottis – flap

keeping food out oflungs

• Cartilaginous ringsholding open the airtubes of therespiratory system(trachea and bronchi)

• Intervertebral discs• Pubic symphysis

• Articular discs such as

meniscus in knee jointTuesday, 23 October 12


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Cartilage is connective tissue

• Cells calledchondrocytes

• Abundant extracellularmatrix• Fibers: collagen & elastin

• Jellylike groundsubstance of complexsugar molecules

• 60-80% water(responsible for theresilience)

• No nerves or vessels



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PERICHONDRIUM

• Dense irregularlyarranged connective

tissue (type Icollagen)

• Ensheaths thecartilage

• Houses the bloodvessels that nourishchondrocytes



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CHONDROBLAST

• Progenitor of

chondrocytes

• Lines borderbetween

perichondrium andmatrix

• Secretes type IIcollagen and other

ECM components



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CHONDROCYTE

• Mature cartilagecell

• Reside in a spacecalled the lacuna

• Clear areas =Golgi and lipiddroplets



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• Chondrocytescompletely fill theirlacunae

• RER andeuchromatic nuclei

• Synthetically

active, secretematrix Cartilage matrix

RER

N



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MATRIX• Provides the rigidity,elasticity, & resilience

• FIBERS• Collagenous and elastic

• GROUND SUBSTANCE

• Glycosaminoglycans

(chondroitin sulfates,keratin sulfate, hyaluronicacid)

• Proteoglycans: GAGs +core protein

• Water

• Basophilic

• Territorial matrix - high [ ]of sulfated


CARTILAGE GROWTH


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CARTILAGE GROWTH

• Appositional

•

“Growth from outside”• Chrondroblasts in perichondrium

• (external covering of cartilage) secretematrix

• Interstitial

• “Growth from within”

• Chondrocytes within divide and secretenew matrix

• Cartilage stops growing in late teens(chrondrocytes stop dividing)

• Regenerates poorly in adults



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TYPES OF CARTILAGE

• HYALINE

• ELASTIC

• FIBROUS


ypes o car age:


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ypes o car age:1. Hyaline cartilage: flexible and

resilient• Chondrocytes appear spherical

• Lacuna – cavity in matrix holdingchondrocyte

• Collagen the only fiber

2. Elastic cartilage: highly bendable• Matrix with elastic as well as

collagen fibers

• Epiglottis, larynx and outer ear

3. Fibrocartilage: resists

compression and tension• Rows of thick collagen fibers

alternating with rows ofchondrocytes (in matrix)

•

Knee menisci and annunulusfibrosis of intervertebral discsTuesday, 23 October 12


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Hyaline Cartilage


Elastic Cartilage


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Elastic Cartilage



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FIBROCARTILAGE

• FUNCTION• Support with great

tensile strength

• MATRIX• Type I collagen -

Oriented parallel tostress plane

• LOCATION• Intervertebral disks,

pubic symphysis



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Types ofCartilage



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Bone (Osseous Tissue)

• Specialized cells - 2% of bone weight• Strong flexible matrix

• Calcium phosphate crystals - two-thirds of boneweight (hydroxyapatite Ca10(PO4)6(OH)2)

• Collagen fibers



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General Shapes Of Bones• Long bones (e.g., humerus, femur)

•

Short bones (e.g., carpals, tarsals, patella• Flat bones (e.g., parietal bone, scapula, sternum)

• Irregular bones (e.g., vertebrae, hip bones)


S f T i l L B


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Structure of Typical Long Bone

• Diaphysis - tubular shaft forming the axis

of long bones.• Composed of compact bone

• Central medullary cavity

• Contains bone marrow

• Epiphysis – expanded end of long bones.

• Composed mostly of spongy bone

• Joint surface is covered with articular(hyaline) cartilage

• Epiphyseal lines separate the diaphysisfrom the epiphyses

• Metaphysis – where epiphysis anddiaphysis meet


Structure of Short Flat and irregular


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• Thin plates of periosteum-

covered compact bone on theoutside and endosteum-covered spongy bone within.

• Have no diaphysis or

epiphysis because they arenot cylindrical.

• Contain bone marrowbetween their trabeculae, but

no marrow cavity.• In flat bones, the internalspongy bone layer is knownas the diploë, and the wholearrangement resembles a

stiffened sandwich.

Structure of Short, Flat and irregularBones


Bone Membranes


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Bone Membranes

• Periosteum• Provides anchoring points for

tendons and ligaments• Double-layered protective

membrane, supplied with nervefibers, blood, and lymphaticvessels entering the bone vianutrient foramina.

• Inner osteogenic layer iscomposed of osteoblasts andosteoclasts

• Endosteum• Delicate CT membrane covering

internal surfaces of bone• Covers trabeculae of spongy

bone

• Lines canals in compact bone

• Also contains both osteoblastsand osteoclasts


G A t f B


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Gross Anatomy of Bones• External Features of Bones – projections, depressions, and

openings that serve as sites of muscle, ligament, and tendon

attachment, as joint surfaces, or conduits for blood vessels andnerves

• Compact Bone – dense outer layer

• Spongy Bone – (cancellous bone) honeycomb of trabeculae(needle-like or flat pieces) filled with bone marrow


G A t B M ki


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Gross Anatomy - Bone Markings

Table 6.1

• Superficial surfaces of bones reflect stresses on them

• There are three broad categories of bone markings

• Projections for muscle attachment

• Surfaces that form joints

• Depressions and openings


Hi t l f C t B


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Histology of Compact Bone• Osteon – the structural unit of compact bone

• Lamellae – column-like matrix tubes composed ofcollagen and crystals of bone salts

• Central canal - (Haversian canal) canal containing

blood vessels and nerves


Histology of Compact Bone


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Histology of Compact Bone

• Lacunae - cavities in bone containing osteocytes

• Canaliculi - hairlike canals that connect lacunae to each other andthe central canal

• Perforating canal (Volkmann’s) – channels lying at right angles to thecentral canal, connecting blood and nerve supply of the periosteumto the central canal


Osteons


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• Each osteon consists of a singlecentral canal, known as a haversian

canal, surrounded by concentriclayers of calcified bone matrix.

• Haversian canals allow thepassage of blood vessels,lymphatic vessels, and nervefibers.

• Each of the concentric matrix“tubes” that surrounds ahaversian canal is known as alamella.

• All the collagen fibers in aparticular lamella run in a singledirection, while collagen fibers inadjacent lamellae will run in theopposite direction. This allowsbone to better withstand twistingforces.

Osteons



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Cells in Bone


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Cells in Bone• Osteoprogenitor cells – precursors to osteoblasts

• Osteocytes - mature bone cells between lamellae• Osteoclasts - bone-destroying cells, break down

bone matrix for remodeling and release of calcium

• Source of acid, enzymes for osteolysis

• Calcium homeostasis

• Osteoblasts - bone-forming cells

• Responsible for osteogenesis (new bone)

• Source of collagen, calcium salts



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1. Osteoblasts• Bone-building cells.• Synthesize and

secrete collagenfibers and otherorganiccomponents ofbone matrix.

• Initiate the processof calcification.• Found in both the

periosteum and theendosteum



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2. Osteocytes

• Mature bonecells.

• Osteoblasts thathave become

trapped by thesecretion ofmatrix.

• No longer secrete

matrix.• Responsible for

maintaining thebone tissue.

Yellow arrows indicate

osteocytes – notice

how they aresurrounded by the

pinkish bone matrix.

Blue arrow shows an

osteoblast in the

process of becoming an

osteocyte.


3 Osteoclasts


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3. Osteoclasts

• Huge cells derived from the fusion of as many as 50monocytes (a type of white blood cell).

•

Cells that digest bone matrix – this process is called boneresorption and is part of normal bone growth, development,maintenance, and repair.

• Concentrated in the endosteum.

• On the side of the cell that faces the bone surface, the PM is

deeply folded into a ruffled border.



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The Structure of Spongy Bone


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The Structure of Spongy Bone

• No osteons

• Lamellae as trabeculae• Arches, rods, plates of

bone

•

Branching network ofbony tissue

• Strong in many

directions

• Red marrow (bloodforming) spaces


Short Irregular and Flat Bones


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Short, Irregular, and Flat Bones

• Plates of periosteum-covered compact boneon the outside withendosteum-coveredspongy bone, diploë,

on the inside• Have no diaphysis or

epiphyses

• Contain bone marrow

between the trabeculae


Bone Development


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Bone Development

• Osteogenesis or Ossification – theprocess of bone tissue formation thatleads to:

•

The formation of the skeleton in embryos• Bone growth until early adulthood

• Bone thickness, remodeling, and repair


Formation of the Skeleton


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Formation of the Skeleton

• Before week 8, the skeleton of a

human embryo consists offibrous membanes and hyalinecartilage

• Intramembranous ossification –bone develops from a fibrous

connective tissue membrane.The flat bones of the skull(frontal, parietal, temporal,occipital) and the clavicles areformed this way.

• Endochondral ossification – boneforms by replacing hyalinecartilage, uses hyaline cartilage“bones” as patterns

Bone Formation in 16-Week-Old Fetus


Bone Formation and Growth


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• Intramembranous Ossification

• Ossification—Process of converting othertissues to bone

• Forms flat bones of skull, mandible,

clavicle• Stem cells differentiate to osteoblasts

• Produces spongy bone, then compactbone

Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings


Intramembranous Ossification


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• An ossification center

appears in the fibrousconnective tissuemembrane

• Osteoblasts secrete

bone matrix within thefibrous membrane

• Osteoblasts matureinto osteocytes




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• The bone matrix developsinto trabeculae.

• The trabeculae formed from

various ossification centers

fuse with one another to

create spongy bone.

• Eventually the spacesbetween trabeculae fill with

red bone marrow.




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• Endochondral Ossification

• Most bones formed this way

• Cartilage model replaced by bone

• Replacement begins in middle (diaphysis)

• Replacement follows in ends (epiphyses)


Endochondral Ossification


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Enlargingchondrocytes within

calcifying matrix

Chondrocytes atthe center of the

growing cartilagemodel enlarge andthen die as thematrix calicifies.

Newly derivedosteoblasts cover

the shaft of thecartilage in a thinlayer of bone.

Blood vesselspenetrate the

cartilage. Newosteoblasts form aprimary ossificationcenter.

The bone of theshaft thickens, and

the cartilage neareach epiphysis isreplaced by shaftsof bone.

Blood vessels invade theepiphyses and osteo-

blasts form secondarycenters of ossification.

Cartilagemodel

Boneformation

Epiphysis

Diaphysis Marrowcavity

Primaryossificationcenter

Blood

vessel

Marrowcavity

Bloodvessel

Secondaryossificationcenter

Epiphysealcartilage

Articular cartilage

Figure 6-5

1 of 6


Longitudinal Bone Growth


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g• Longitudinal Growth (interstitial) – cartilage continually grows

and is replaced by bone

• Bones lengthen entirely by growth of the epiphyseal plates

• Cartilage is replaced with bone CT as quickly as it grows

• Epiphyseal plate maintains constant thickness


Epiphyseal Plate


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• Cartilage is organized forquick, efficient growth

• Cartilage cells form tall stacks• Chondroblasts at the top of

stacks divide quickly

• Pushes the epiphysis awayfrom the diaphysis

• Lengthens entire long bone

• Older chondrocytes signalsurrounding matrix to calcify,then die and disintegrate• Leaves long trabeculae

(spicules) of calcifiedcartilage on diaphysis side

• Trabeculae are partly erodedby osteoclasts

• Osteoblasts then covertrabeculae with bone tissue

• Trabeculae finally eaten awayfrom their tips by osteoclasts


At puberty, growth in bone lengthis increased dramatically by the


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•As a result osteoblasts begin producing bone faster than the

rate of epiphyseal cartilageexpansion. Thus the bone growswhile the epiphyseal plate getsnarrower and narrower andultimately disappears. A remnant(epiphyseal line) is visible on X-rays (do you see them in theadjacent femur, tibia, and fibula?)

y ycombined activities of growthhormone, thyroid hormone, and

the sex hormones.


Appositional Bone Growth


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• Growing bones widen as they lengthen

• Appositional growth – growth of a bone by addition of bone tissue to its

surface• Bone is resorbed at endosteal surface and added at periosteal surface

• Osteoblasts – add bone tissue to the external surface of the diaphysis

• Osteoclasts – remove bone from the internal surface of the diaphysis

Figure 6-6


Bones get thicker by


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appositional growth

• Osteoblasts formnew extracellularmatrix

• Blood vesselbecomes enclosed,supports new osteon

• Osteoblasts lay

down extracellularmatrix



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CLEIDOCRANIAL DYSPLASIA


Bone - Remodeling/Homeostasis


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• Role of Remodeling in Support

• Remodeling—Continuous breakdown and

reforming of bone tissue

• Shapes reflect applied loads

•

Mineral turnover enables adapting to newstresses

• What you don’t use, you lose. The stressesapplied to bones during exercise are

essential to maintaining bone strength andbone mass


Bone Remodeling


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• Bone is active tissue – small changes in bone architectureoccur continuously – 5 to 7% of bone mass is recycled weekly

– spongy bone is replaced every 3-4 years and compact bone

approximately every 10 years

• Remodeling Units – adjacent osteoblasts and osteoclasts

deposit and reabsorb bone at periosteal and endosteal

surfaces


Bone Remodeling


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• Bone Depostition• Occurs when bone is injured or extra strength is needed

• Requires a healthy diet - protein, vitamins C, D, and A, andminerals (calcium, phosphorus, magnesium, manganese, etc.)

• Bone Resorption• Accomplished by Osteoclasts (multinucleate phagocytic cells)

• Resorption involves osteoclast secretion of:

• Lysosomal enzymes that digest organic matrix

• HCl that converts calcium salts into soluble forms

• Dissolved matrix is endocytosed and transcytosed into theinterstitial fluid! the blood


Bone - Remodeling/Homeostasis


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• Homeostasis and Mineral Storage

• Bones store calcium• Contain 99% of body calcium

• Store up to two kg calcium

• Hormones control storage/release• PTH, calcitriol release bone calcium

• Calcitonin stores bone calcium

• Blood levels kept constant


Process of fracture repair


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Process of fracture repair

Bone grows slowly; blood supply may beaffected


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Fractures

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• Despite its mineral strength, bone

may crack or even break ifsubjected to extreme loads,sudden impacts, or stresses fromunusual directions.

• The damage produced

constitutes a fracture.• The proper healing of a fracture

depends on whether or not, theblood supply and cellularcomponents of the periosteumand endosteum survive.


FractureRepair

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Repair

• Step 1: A. Immediately

after thefracture,extensivebleedingoccurs. Over a

period ofseveral hours,a large bloodclot, or fracturehematoma,develops.

B. Bone cells at

the site becomedeprived ofnutrients anddie. The sitebecomesswollen,

• Step 2:

A. Granulation tissue is formed as the hematoma isinfiltrated by capillaries and macrophages, whichbegin to clean up the debris.

B. Some fibroblasts produce collagen fibers that span

the break , while others differentiate intochondroblasts and begin secreting cartilagematrix.

C. Osteoblasts begin forming spongy bone.

D. This entire structure is known as afibrocartilaginous callus and it splints the broken

bone.Tuesday, 23 October 12

FractureRepair


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• Step 3: A. Bone trabeculae

increase innumber andconvert thefibrocartilaginouscallus into a bony

callus of spongybone. Typicallytakes about 6-8weeks for this tooccur.

Repair

• Step 4: A. During the next several months, the bony callus is continually

remodeled.

B. Osteoclasts work to remove the temporary supportive structureswhile osteoblasts rebuild the compact bone and reconstruct thebone so it returns to its original shape/structure.


Fracture Types


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• Fractures are often classified according to the

position of the bone ends after the break:Open (compound) ! bone ends penetrate the

skin.

Closed (simple) ! bone ends don’t penetrate theskin.

Comminuted ! bone fragments into 3 or morepieces. Common in theelderly (brittle bones).

Greenstick ! bone breaks incompletely. Oneside bent, one side broken.Common in children

whose bone contains more collagen and

are less mineralized.Tuesday, 23 October 12

Osteoporosis


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This bone:

a. Has been demineralized

b. Has had its organic component removed


Joints


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• Rigid elements of the skeleton meet at joints

or articulations• Greek root “arthro” means joint

• Functions of joints

•

Hold bones together • Allow for mobility

• Articulations can be

• Bone to bone

• Bone to cartilage

• Teeth in bony sockets


Classification of Joints


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• Joints can be classified by function or

structure• Functional

• Synarthroses – immovable joints

• Amphiarthroses – slightly moveable joints

• Diarthroses – freely moveable joints

• Structural• Fibrous joints - generally immovable

•

Cartilaginous joints - immovable or slightlymoveable

• Synovial joints - freely moveable


Functional Classification


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• Functional classification – based on amount ofmovement

• Synarthroses – immovable joints

• Suture – very short CT fibers, e.g. between cranial bones

• Gomphosis – teeth in sockets

• Synchondrosis – hyaline cartilage unites bones, e.g.

epiphyseal plate, costal cartilage of 1st

rib and manubrium• Amphiarthroses – slightly moveable joints

• Syndesmosis – bones connected by ligaments, e.g. betweentibia and fibula

• Symphysis - bones are covered by hyaline cartilage fused

with fibrocartilage, e.g. between vertebrae, pubic bones ofthe hip

• Diarthroses - freely moveable; knee, elbow, etc


Classifications of Joints


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• Structural classification based on

• Material that binds bones together • Presence or absence of a joint cavity

• Structural classifications include

• Fibrous• Cartilaginous

• Synovial


Fibrous Joints

B d b fib


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• Bones are connected by fibrous

connective tissue• Primarily dense regular CT

• Do not have a joint cavity

• Most are immovable or slightly movable

• Types

• Sutures


Fibrous Joints - Sutures• Bones are tightly bound by a minimal amount of fibrous tissue


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• Bones are tightly bound by a minimal amount of fibrous tissue

• Only occur between the bones of the skull

• Allow bone growth so the skull can expand with brain during

childhood

• Fibrous tissue ossifies in middle age

• Synostoses – closed sutures


Fibrous Joints - Syndesmoses• Bones are connected exclusively by ligaments


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y y g

• Amount of movement depends on length of fibers

• Tibiofibular joint –immovable synarthrosis

• Interosseous membrane between radius and ulna

• Freely movable diarthrosis


Fibrous Joints - Gomphoses

T th i


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• Tooth in a

socket• Connecting

ligament – the

periodontalligament

Figure 9.1c


Cartilaginous Joints• Bones are united by cartilage


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• Lack a joint cavity

• Two types

• Synchondroses - hyaline cartilage unites bones

• Epiphyseal plates

• Rib and sternum

• Symphyses


Symphyses• Fibrocartilage unites bones – resists tension and


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• Fibrocartilage unites bones – resists tension andcompression

• Hyaline cartilage – also present as articular cartilage

• Slightly movable joints that provide strength with flexibility

• Intervertebral discs

• Pubic symphysis


Synovial Joints

• Most movable type of joint


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• Most movable type of joint

• All are diarthroses• Each contains a fluid-filled joint cavity


General Structure of Synovial Joints


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• Articular cartilage

• Ends of opposing bones arecovered with hyaline cartilage

• Absorbs compression

• Joint cavity (synovial cavity)

• Unique to synovial joints

• Cavity is a potential space that

holds a small amount of

synovial fluid



• Articular capsule – joint


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p jcavity is enclosed in a two-

layered capsule• Fibrous capsule – dense

irregular connective tissue,which strengthens joint

• Synovial membrane – looseconnective tissue

• Lines joint capsule andcovers internal joint surfaces

• Functions to make synovialfluid



• Synovial fluid


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Synovial fluid

• A viscous fluid similar toraw egg white

• A filtrate of blood

• Arises from capillaries insynovial membrane

• Contains glycoproteinmolecules secreted byfibroblasts


General Structure of Synovial Joints• Reinforcing ligaments


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• Often are thickened parts

of the fibrous capsule

• Sometimes are

extracapsular ligaments –

located outside the

capsule

• Sometimes are

intracapsular ligaments –

located internal to the

capsule


Structures Associated with the Synovial Joint

• Tendon sheath - elongated bursa that wraps around a tendon

• Bursae flattened fibrous sacs


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• Bursae – flattened fibrous sacs

•

Lined with synovial membranes• Filled with synovial fluid

• Not actually part of the joint

• Menisci

• Fat pads


Structural Classification of Synovial Joints

Glidi ( l j i t t b t b )


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• Gliding (plane joint, e.g., vertebra–vertebra)

• Hinge (e.g., knee)

• Pivot (e.g., atlas–axis)

• Ellipsoidal (condyloid plant, e.g., distal

radius)• Saddle (e.g., thumb)

• Ball-and-Socket (e.g., hip)


Types of Synovial Joints Based on Shape


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SlideCopyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Figure 5.29a–c

(Gliding)


Types of Synovial Joints Based on Shape


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SlideCopyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Figure 5.29d–f

(Ellipsoidal)


Summary of Joint Classes


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skeletal tissues

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