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Lecture 6: Osseous Tissue and Bone Structure

Topics:

Skeletal cartilageStructure and function of bone tissuesTypes of bone cellsStructures of the two main bone tissuesBone membranesBone formation Minerals, recycling, and remodelingHormones and nutritionFracture repairThe effects of aging

The Skeletal System

Skeletal system includes:bones of the skeletoncartilages, ligaments, and connective tissues

Skeletal Cartilage

Contains no blood vessels or nervesSurrounded by the perichondrium (dense irregular connective tissue) that resists outward expansionThree types – hyaline, elastic, and fibrocartilage

Hyaline Cartilage

Provides support, flexibility, and resilienceIs the most abundant skeletal cartilageIs present in these cartilages:

Articular – covers the ends of long bonesCostal – connects the ribs to the sternumRespiratory – makes up larynx, reinforces air passagesNasal – supports the nose

Elastic Cartilage

Similar to hyaline cartilage, but contains elastic fibersFound in the external ear and the epiglottis

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Fibrocartilage

Highly compressed with great tensile strengthContains collagen fibersFound in menisci of the knee and in intervertebral discs

Growth of Cartilage

Appositional – cells in the perichondrium secrete matrix against the external face of existing cartilageInterstitial – lacunae-bound chondrocytes inside the cartilage divide and secrete new matrix, expanding the cartilage from withinCalcification of cartilage occurs

During normal bone growthDuring old age

Bones and Cartilages of the Human Body

Figure 6.1

Functions of the Skeletal System1. Support2. Storage of minerals (calcium)3. Storage of lipids (yellow marrow) 4. Blood cell production (red marrow)5. Protection6. Leverage (force of motion)

Bone (Osseous) Tissue

Supportive connective tissue Very denseContains specialized cellsProduces solid matrix of calcium salt deposits and collagen fibers

Characteristics of Bone Tissue

Dense matrix, containing:deposits of calcium saltsosteocytes within lacunae organized around blood vessels

Canaliculi: form pathways for blood vesselsexchange nutrients and wastes

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Osteocyte and canaliculi Characteristics of Bone Tissue

Periosteum: covers outer surfaces of bones consist of outer fibrous and inner cellularlayersContains osteblasts responsible for bone growth in thickness

EndosteumCovers inner surfaces of bones

Bone Matrix

Solid ground is made of mineral crystals2/3 of bone matrix is calcium phosphate, Ca3(PO4)2:

reacts with calcium hydroxide, Ca(OH)2 to form crystals of hydroxyapatite, Ca10(PO4)6(OH)2 which incorporates other calcium salts and ions

Bone Matrix

Matrix Proteins:1/3 of bone matrix is protein fibers (collagen)

Question: why aren’t bones made of ALL collagen if it’s so strong?

Bone Matrix

Mineral salts make bone rigid and compression resistant but would be prone to shatteringCollagen fibers add extra tensile strengthbut mostly add tortional flexibility to resist shattering

Chemical Composition of Bone: Organic

Cells:Osteoblasts – bone-forming cellsOsteocytes – mature bone cellsOsteoprogenitor cells – grandfather cellsOsteoclasts – large cells that resorb or break down bone matrix

Osteoid – unmineralized bone matrix composed of proteoglycans, glycoproteins, and collagen; becomes calcified later

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There are four major types of cells

in matrix only

endosteumonly

periosteum+ endo

1. Osteoblasts

Immature bone cells that secrete matrix compounds (osteogenesis)Eventually become surrounded by calcified bone and then they become osteocytes

Figure 6–3 (2 of 4)

2.Osteocytes

Mature bone cells that maintain the bone matrix

Figure 6–3 (1 of 4)

Osteocytes

Live in lacunaeFound between layers (lamellae) of matrixConnected by cytoplasmic extensions through canaliculi in lamellae (gap junctions)Do not divide (remember G0?)Maintain protein and mineral content of matrixHelp repair damaged bone

3. Osteoprogenitor CellsMesenchymestem cells that divide to produce osteoblastsAre located in inner, cellular layer of periosteumAssist in fracture repair

4. Osteoclasts

Secrete acids and protein-digesting enzymes

Figure 6–3 (4 of 4)

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Osteoclasts

Giant, mutlinucleate cellsDissolve bone matrix and release stored minerals (osteolysis)Often found lining in endosteum lining the marrow cavity Are derived from stem cells that produce macrophages

Homeostasis

Bone building (by osteocytes and -blasts) and bone recycling (by osteoclasts) must balance:

more breakdown than building, bones become weakexercise causes osteocytes to build bone

Bone cell lineage summary

Osteoprogenitor cells

osteoblasts

osteocytes

Osteoclasts are related to macrophages (blood cell derived)

Gross Anatomy of Bones: Bone Textures

Compact bone – dense outer layerSpongy bone – honeycomb of trabeculae filled with yellow bone marrow

Compact Bone

Figure 6–5

Osteon

The basic structural unit of mature compact boneOsteon = Osteocytes arranged in concentric lamellae around a central canalcontaining blood vessels

Lamella – weight-bearing, column-like matrix tubes composed mainly of collagen

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Three Lamellae Types

Concentric LamellaeCircumferential Lamellae

Lamellae wrapped around the long bone line tree ringsBinds inner osteons together

Interstitial LamellaeFound between the osteons made up of concentric lamellaThey are remnants of old osteons that have been partially digested and remodeled by osteoclast/osteoblast activity

Compact Bone

Figure 6–5

Microscopic Structure of Bone: Compact Bone

Figure 6.6a, b

Microscopic Structure of Bone: Compact Bone

Figure 6.6a

Microscopic Structure of Bone: Compact Bone

Figure 6.6b

Microscopic Structure of Bone: Compact Bone

Figure 6.6c

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Spongy Bone

Figure 6–6

Spongy Bone Tissue

Makes up most of the bone tissue in short, flat, and irregularly shaped bones, and the head (epiphysis) of long bones; also found in the narrow rim around the marrow cavity of the diaphysis of long bone

Spongy Bone

Does not have osteonsThe matrix forms an open network of trabeculaeTrabeculae have no blood vessels

Bone Marrow

The space between trabeculae is filled with marrow which is highly vascular

Red bone marrowsupplies nutrients to osteocytes in trabeculaeforms red and white blood cells

Yellow bone marrowyellow because it stores fat

Question: Newborns have only red marrow. Red changes into yellow marrow in some bones as we age. Why?

Location of HematopoieticTissue (Red Marrow)In infants

Found in the medullary cavity and all areas of spongy bone

In adultsFound in the diploë of flat bones, and the head of the femur and humerus

Bone MembranesPeriosteum – double-layered protective membrane

Covers all bones, except parts enclosed in joint capsules (continuois w/ synovium)Made up of:

outer, fibrous layer (tissue?)inner, cellular layer (osteogenic layer) is composed of osteoblasts and osteoclasts

Secured to underlying bone by Sharpey’s fibersEndosteum – delicate membrane covering internal surfaces of bone

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Sharpy’s (Perforating) Fibers

Collagen fibers of the outer fibrous layer of periosteum, connect with collagen fibers in boneAlso connect with fibers of joint capsules, attached tendons, and ligamentsTendons are “sewn” into bone via periosteum

Periosteum

Figure 6–8a

Functions of Periosteum

1. Isolate bone from surrounding tissues2. Provide a route for circulatory and

nervous supply3. Participate in bone growth and repair

Endosteum

Figure 6–8b

Endosteum

An incomplete cellular layer:lines the marrow cavitycovers trabeculae of spongy bonelines central canals

Contains osteoblasts, osteoprogenitorcells, and osteoclastsIs active in bone growth and repair

Bone Development

Human bones grow until about age 25Osteogenesis:

bone formationOssification:

the process of replacing other tissues with boneOsteogenesis and ossification lead to:

The formation of the bony skeleton in embryosBone growth until early adulthoodBone thickness, remodeling, and repair through life

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Calcification

The process of depositing calcium salts Occurs during bone ossification and in other tissues

Formation of the Bony Skeleton

Begins at week 8 of embryo developmentOssification

Intramembranous ossification – bone develops from a fibrous membraneEndochondral ossification – bone forms by replacing hyaline cartilage

Intramembranous OssificationNote: you don’t have to know the steps of this process in detail

Also called dermal ossification (because it occurs in the dermis)

produces dermal bones such as mandible and clavicle

Formation of most of the flat bones of the skull and the claviclesFibrous connective tissue membranes are formed by mesenchymal cells

The Birth of Bone

When new bone is born, either during development or regeneration, it often starts out as spongy bone (even if it will later be remodeled into compact bone)

Endochondral OssificationNote: you DO have to know this one

Begins in the second month of developmentUses hyaline cartilage “bones” as models for bone construction then ossifies cartilage into bone Common, as most bones originate as hyaline cartilageThis is like a “trick” the body uses to allow long bones to grow in length when bones can only grow by appositional growth

Bone formation in a chick embryo

Stained to represent hardened bone (red) and cartilage (blue)

: This image is the cover illustration from The Atlas of Chick Development by Ruth Bellairs and Mark Osmond, published by Academic Press (New York) in 1998

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Fetal Primary Ossification Centers

Figure 6.15

Stages of Endochondral Ossification

Bone models form out of hyaline cartilageFormation of bone collarCavitation of the hyaline cartilageInvasion of internal cavities by the periosteal bud, and spongy bone formationFormation of the medullary cavity; appearance of secondary ossification centers in the epiphysesOssification of the epiphyses, with hyaline cartilage remaining only in the epiphyseal plates

Stages of Endochondral Ossification

Figure 6.8

Formation ofbone collararound hyalinecartilage model.

Hyalinecartilage

Cavitation ofthe hyaline carti-lage within thecartilage model.

Invasion ofinternal cavitiesby the periostealbud and spongybone formation.

Formation of themedullary cavity asossification continues;appearance of sec-ondary ossificationcenters in the epiphy-ses in preparationfor stage 5.

Ossification of theepiphyses; whencompleted, hyalinecartilage remains onlyin the epiphyseal platesand articular cartilages.

Deterioratingcartilagematrix

Epiphysealblood vessel

Spongyboneformation

Epiphysealplatecartilage

Secondaryossificatoncenter

Bloodvessel ofperiostealbud

Medullarycavity

Articularcartilage

Spongybone

Primaryossificationcenter

Bone collar

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EndochondralOssification: Step 1 (Bone Collar)

Blood vessels grow around the edges of the cartilage Cells in the perichondrium change to osteoblasts:

producing a layer of superficial bone (bone collar) around the shaft which will continue to grow and become compact bone (appositional growth) Figure 6–9 (Step 2)

EndochondralOssification: Step 2 (Cavitation)

Chondrocytes in the center of the hyaline cartilage of each bone model:

enlargeform struts and calcifydie, leaving cavities in cartilage

Figure 6–9 (Step 1)

EndochondralOssification: Step 3 (Invasion)

Periosteal bud brings blood vessels into the cartilage:

bringing osteoblasts and osteoclastsspongy bone develops at the primary ossification center

Figure 6–9 (Step 3)

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EndochondralOssification: Step 4a (Remodelling)

Figure 6–9 (Step 4)

Remodeling creates a marrow (medullary) cavity:

bone replaces cartilage at the metaphysesDiaphysis elongates

EndochondralOssification: Step 4b (2° Ossification)

Capillaries and osteoblastsenter the epiphyses:

creating secondary ossification centers (perinatal)

Figure 6–9 (Step 5)

EndochondralOssification: Step 5 (Elongation)

Epiphyses fill with spongy bone but cartilage remains at two sites:

ends of bones within the joint cavity = articularcartilagecartilage at the metaphysis = epiphysealcartilage (plate)

Figure 6–9 (Step 6)

Postnatal Bone Growth

Growth in length of long bonesCartilage on the side of the epiphyseal plate closest to the epiphysis is relatively inactiveCartilage abutting the shaft of the bone organizes into a pattern that allows fast, efficient growth Cells of the epiphyseal plate proximal to the resting cartilage form three functionally different zones: growth, transformation, and osteogenic

Functional Zones in Long Bone Growth

Growth zone – cartilage cells undergo mitosis, pushing the epiphysis away from the diaphysisTransformation zone – older cells enlarge, the matrix becomes calcified, cartilage cells die, and the matrix begins to deteriorateOsteogenic zone – new bone formation occurs

Growth in Length of Long Bone

Figure 6.9

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Postnatal bone growth

Remember that bone growth can only occur from the outside (appositional growth). So this type of endochondralgrowth is a way for bones to grow from the inside and lengthen because it is the cartilage that is growing, not the bone

Key Concept

As epiphyseal cartilage grows through the division of chondrocytes it pushes the ends of the bone outward in length. At the “inner” (shaft) side of the epiphysealplate, recently born cartilage gets turned into bone, but as long as the cartilage divides and extends as fast or faster than it gets turned into bone, the bone will grow longer

Long Bone Growth and Remodeling

Growth in length – cartilage continually grows and is replaced by bone as shown Remodeling – bone is resorbed and added by appositional growth as shown

compact bone thickens and strengthens long bones with layers of circumferential lamellae

Long Bone Growth and Remodeling

Figure 6.10

Appositional Growth Epiphyseal Lines

When long bone stops growing, between the ages of 18 – 25:

epiphyseal cartilage disappears epiphyseal plate closesvisible on X-rays as an epiphyseal line

At this point, bone has replaced all the cartilage and the bone can no longer grow in length

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Epiphyseal Lines

Figure 6–10

During infancy and childhood, epiphyseal plate activity is stimulated by growth hormoneDuring puberty, testosterone and estrogens:

Initially promote adolescent growth spurtsCause masculinization and feminization of specific parts of the skeletonLater induce epiphyseal plate closure, ending long bone growth

Hormonal Regulation of Bone Growth During Youth

RemodelingRemodeling continually recycles and renews bone matrixTurnover rate varies within and between bonesIf deposition is greater than removal, bones get strongerIf removal is faster than replacement, bones get weakerRemodeling units – adjacent osteoblasts and osteoclasts deposit and resorb bone at periosteal and endosteal surfaces

Bone DepositionOccurs where bone is injured or added strength is neededRequires a diet rich in protein, vitamins C, D, and A, calcium, phosphorus, magnesium, and manganeseAlkaline phosphatase is essential for mineralization of boneSites of new matrix deposition are revealed by the:

Osteoid seam – unmineralized band of bone matrixCalcification front – abrupt transition zone between the osteoid seam and the older mineralized bone

Effects of Exercise on Bone

Mineral recycling allows bones to adapt to stressHeavily stressed bones become thicker and stronger

Response to Mechanical Stress

Wolff’s law – a bone grows or remodels in response to the forces or demands placed upon itObservations supporting Wolff’s law include

Long bones are thickest midway along the shaft (where bending stress is greatest)Curved bones are thickest where they are most likely to buckle

Trabeculae form along lines of stressLarge, bony projections occur where heavy, active muscles attach

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Response to Mechanical Stress

Figure 6.12

Bone ResorptionAccomplished by osteoclastsResorption bays – grooves formed by osteoclasts as they break down bone matrixResorption involves osteoclast secretion of:

Lysosomal enzymes that digest organic matrixAcids that convert calcium salts into soluble forms

Dissolved matrix is transcytosed across the osteoclast cell where it is secreted into the interstitial fluid and then into the blood

Bone Degeneration

Bone degenerates quickly Up to 1/3 of bone mass can be lost in a few weeks of inactivity

Minerals, vitamins, and nutrients

Rewired for bone growthA dietary source of calcium and phosphatesalts:

plus small amounts of magnesium, fluoride, iron, and manganese

Protein, vitamins C, D, and A

Hormones for Bone Growth and Maintenance

Table 6–2

Calcitriol

The hormone calcitriol:synthesis requires vitamin D3 (cholecalciferol)made in the kidneys (with help from the liver)helps absorb calcium and phosphorus from digestive tract

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The Skeleton as Calcium Reserve

Bones store calcium and other mineralsCalcium is the most abundant mineral in the bodyCalcium ions in body fluids must be closely regulated because:Calcium ions are vital to:

membranesneuronsmuscle cells, especially heart cellsblood clotting

Calcium Regulation: Hormonal Control

Homeostasis is maintained by calcitonin and parathyroid hormone which control storage, absorption, and excretionRising blood Ca2+ levels trigger the thyroid to release calcitoninCalcitonin stimulates calcium salt deposit in boneFalling blood Ca2+ levels signal the parathyroid glands to release PTHPTH signals osteoclasts to degrade bone matrix and release Ca2+ into the blood

Hormonal Control of Blood Ca

Figure 6.11

PTH;calcitoninsecreted

Calcitoninstimulatescalcium saltdepositin bone

Parathyroidglands releaseparathyroidhormone (PTH)

Thyroidgland

Thyroidgland

Parathyroidglands

Osteoclastsdegrade bonematrix and releaseCa2+ into blood

Falling bloodCa2+ levels

Rising bloodCa2+ levels

Calcium homeostasis of blood: 9–11 mg/100 ml

PTH

Imbalance

Imbalance

Calcitonin and Parathyroid Hormone Control

Bones:where calcium is stored

Digestive tract:where calcium is absorbed

Kidneys:where calcium is excreted

Parathyroid Hormone (PTH)

Produced by parathyroid glands in neckIncreases calcium ion levels by:

stimulating osteoclastsincreasing intestinal absorption of calcium decreases calcium excretion at kidneys

Secreted by cells in the thyroid glandDecreases calcium ion levels by:

inhibiting osteoclastactivityincreasing calcium excretion at kidneys

Actually plays very small role in adults

Calcitonin

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Fractures

Fractures:cracks or breaks in bonescaused by physical stress

Fractures are repaired in 4 steps

Fracture Repair Step 1: Hematoma

Hematoma formationTorn blood vessels hemorrhageA mass of clotted blood (hematoma) forms at the fracture siteSite becomes swollen, painful, and inflamed

Bone cells in the area die

Figure 6.13.1

Fracture Repair Step 2: Soft Callus

Cells of the endosteum and periosteum divide and migrate into fracture zoneGranulation tissue (soft callus) forms a few days after the fracture from fibroblasts and endotheliumFibrocartilaginous callus forms to stabilize fracture

external callus of hyaline cartilage surrounds breakinternal callus of cartilage and collagen develops in marrow cavity

Capillaries grow into the tissue and phagocytic cells begin cleaning debris

Figure 6.13.2

Stages in the Healing of a Bone Fracture

The fibrocartilaginous callus forms when:Osteoblasts and fibroblasts migrate to the fracture and begin reconstructing the boneFibroblasts secrete collagen fibers that connect broken bone endsOsteoblasts begin forming spongy boneOsteoblasts furthest from capillaries secrete an externally bulging cartilaginous matrix that later calcifies

Fracture Repair Step 3: Bony Callus

Bony callus formationNew spongy bone trabeculae appear in the fibrocartilaginous callusFibrocartilaginous callus converts into a bony (hard) callusBone callus begins 3-4 weeks after injury, and continues until firm union is formed 2-3 months later

Figure 6.13.3

Fracture Repair Step 4: Remodeling

Bone remodelingExcess material on the bone shaft exterior and in the medullary canal is removedCompact bone is laid down to reconstruct shaft wallsRemodeling for up to a year

reduces bone callusmay never go away completely

Usually heals stronger than surrounding bone

Figure 6.13.4

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Clinical advances in bone repairElectrical stimulation of fracture site.

results in increased rapidity and completeness of bone healingelectrical field may prevent parathyroid hormone from activating osteoclasts at the fracture site thereby increasing formation of bone and minimizing breakdown of bone,

Ultrasound.Daily treatment results in decreased healing time of fracture by about 25% to 35% in broken arms and shinbones. Stimulates cartilage cells to make bony callus.

Free vascular fibular graft technique. Uses pieces of fibula to replace bone or splint two broken ends of a bone. Fibula is a non-essential bone, meaning it does not play a role in bearing weight; however, it does help stabilize the ankle.

Bone substitutes.synthetic material or crushed bones from cadavers serve as

bone fillers (Can also use sea coral).

Aging and Bones

Bones become thinner and weaker with ageOsteopenia begins between ages 30 and 40 Women lose 8% of bone mass per decade, men 3%

Osteoporosis

Severe bone loss which affects normal functionGroup of diseases in which bone reabsorption outpaces bone depositThe epiphyses, vertebrae, and jaws are most affected, resulting in fragile limbs, reduction in height, tooth lossOccurs most often in postmenopausal womenBones become so fragile that sneezing or stepping off a curb can cause fracturesOver age 45, occurs in:

29% of women18% of men

Notice what happens in osteoporosis

Osteoporosis: Treatment

Calcium and vitamin D supplementsIncreased weight-bearing exerciseHormone (estrogen) replacement therapy (HRT) slows bone lossNatural progesterone cream prompts new bone growthStatins increase bone mineral densityPPIs may decrease density

Hormones and Bone Loss

Estrogens and androgens help maintain bone massBone loss in women accelerates after menopause

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Cancer and Bone Loss

Cancerous tissues release osteoclast-activating factor:

stimulates osteoclastsproduces severe osteoporosis

Paget’s Disease

Characterized by excessive bone formation and breakdownAn excessively high ratio of spongy to compact bone is formedReduced mineralization causes spotty weakening of boneOsteoclast activity wanes, but osteoblastactivity continues to work

Developmental Aspects of Bones

Mesoderm gives rise to embryonic mesenchymal cells, which produce membranes and cartilages that form the embryonic skeletonThe embryonic skeleton ossifies in a predictable timetable that allows fetal age to be easily determined from sonogramsAt birth, most long bones are well ossified (except for their epiphyses)

Developmental Aspects of Bones

By age 25, nearly all bones are completely ossifiedIn old age, bone resorption predominatesA single gene that codes for vitamin D docking determines both the tendency to accumulate bone mass early in life, and the risk for osteoporosis later in life

SUMMARY

Skeletal cartilageStructure and function of bone tissuesTypes of bone cellsStructures of compact bone and spongy boneBone membranes, peri- and endosteumOssification: intramembranous and endochondralBone minerals, recycling, and remodelingHormones and nutritionFracture repairThe effects of aging

Figure 6–16 (1 of 9)

The Major Types of Fractures

Simple (closed): bone end does not break the skinCompound (open): bone end breaks through the skinNondisplaced – bone ends retain their normal positionDisplaced – bone ends are out of normal alignmentComplete – bone is broken all the way throughIncomplete – bone is not broken all the way throughLinear – the fracture is parallel to the long axis of the boneTransverse – the fracture is perpendicular to the long axis of the boneComminuted – bone fragments into three or more pieces; common in the elderly

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Types of fractures (just FYI) More fractures