BIO 201

Skeletal System 1

Skeletal SystemSkeletal System

Functions of the skeletal Functions of the skeletal systemsystemsystemsystemTissues and organs of Tissues and organs of the skeletal systemthe skeletal systemHistology of osseous Histology of osseous tissuetissuePhysiology of bone Physiology of bone tissuetissuetissuetissueOssificationOssificationOsteopathologyOsteopathology

Functions of the Skeletal SystemFunctions of the Skeletal System

SupportSupport–– Structure of ears, noseStructure of ears, nose–– Scaffold for musclesScaffold for muscles

ProtectionProtection–– Major organs enclosed by bony cagesMajor organs enclosed by bony cages

MovementMovement–– Leverage for muscular movementLeverage for muscular movement

Blood formationBlood formationMineral reservoirMineral reservoir–– Calcium (CaCalcium (Ca2+2+) and phosphorus (phosphate PO) and phosphorus (phosphate PO 33--))–– Calcium (CaCalcium (Ca ) and phosphorus (phosphate PO) and phosphorus (phosphate PO44 ))

pH balance pH balance –– Storage of bicarbonate from bloodStorage of bicarbonate from blood

BIO 201

Skeletal System 2

Shapes of BonesShapes of Bones

Features of Long BonesFeatures of Long Bones

Enlarged ends (Enlarged ends (epiphysesepiphyses) ) –– spongy bone (cancellous) spongy bone (cancellous)

covered with a layer of compact covered with a layer of compact bonebonebonebone

Shaft (Shaft (diaphysisdiaphysis) ) –– contains marrow cavity contains marrow cavity

(medullary cavity) that is lined (medullary cavity) that is lined with with endosteumendosteum (layer of (layer of osteogenicosteogenic cells)cells)

Joint surface covered with Joint surface covered with articulararticular cartilagecartilageRemainder of bone coveredRemainder of bone coveredRemainder of bone covered Remainder of bone covered with with periosteumperiosteum–– inner osteogenic layer important inner osteogenic layer important

for growth and healingfor growth and healing

Epiphyseal plateEpiphyseal plate or or linelinedepends on agedepends on age

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Skeletal System 3

Features of a Flat BoneFeatures of a Flat Bone

External and internalExternal and internalExternal and internal External and internal surfaces of flat bone are surfaces of flat bone are composed of compact composed of compact bonebone–– Increased protectionIncreased protection

Middle layer is spongy Middle layer is spongy bonebone–– No marrow cavityNo marrow cavity

Bone as a TissueBone as a Tissue

Dynamic tissue that continually remodels itselfDynamic tissue that continually remodels itself

Bones and bone tissueBones and bone tissue–– bone or bone or osseous tissueosseous tissue is a connective tissue with a matrix hardened is a connective tissue with a matrix hardened

by mineralsby mineralsby mineralsby minerals(calcium phosphate (calcium phosphate ---- hydroxyapatitehydroxyapatite))

–– bones make up the skeletal system bones make up the skeletal system individual bones are made up of boneindividual bones are made up of bonetissue, marrow, cartilage and periosteumtissue, marrow, cartilage and periosteum

BIO 201

Skeletal System 4

Cells of Osseous TissueCells of Osseous Tissue

OsteogenicOsteogenic cells reside in endosteum, periosteum or central cells reside in endosteum, periosteum or central canalscanals–– multiply continuously and multiply continuously and differentiatedifferentiate into noninto non--dividing dividing osteoblastsosteoblasts in in

response to stress or fracturesresponse to stress or fractures

OsteoblastsOsteoblasts form and help mineralize organic matter of matrix form and help mineralize organic matter of matrix

OsteocytesOsteocytes are osteoblasts that have become trapped in the are osteoblasts that have become trapped in the matrix they formedmatrix they formed–– cells in cells in lacunaelacunae connected by connected by gap junctionsgap junctions inside inside canaliculicanaliculi

–– signal signal osteoclastsosteoclasts and osteoblasts about mechanical stressesand osteoblasts about mechanical stresses

Osteoblast vs. OsteocyteOsteoblast vs. Osteocyte

BIO 201

Skeletal System 5

Bone Making Cells in a Nutshell…Bone Making Cells in a Nutshell…

More Cells of Osseous TissueMore Cells of Osseous Tissue

OsteoclastsOsteoclasts develop in bone marrow by the fusion of 3develop in bone marrow by the fusion of 3--50 of 50 of stem cells stem cells

Reside in pits called Reside in pits called resorption baysresorption bays that they have eaten that they have eaten into the surface of the boneinto the surface of the bone

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Skeletal System 6

A Bone Eating Cell in a Nutshell…A Bone Eating Cell in a Nutshell…

Compact BoneCompact Bone

OsteonOsteon (haversian system) (haversian system) = basic structural unit= basic structural unit–– cylinders of tissue formed cylinders of tissue formed

from layers (from layers (lamellaelamellae) of) offrom layers (from layers (lamellaelamellae) of ) of matrix arranged around matrix arranged around central canalcentral canal holding a blood holding a blood vesselvessel

–– osteocytesosteocytes connected to connected to each other and their blood each other and their blood supply by tiny cell processes supply by tiny cell processes in in canaliculicanaliculi

Perforating canals orPerforating canals orPerforating canals or Perforating canals or Volkmann canalsVolkmann canals–– vascular canals vascular canals

perpendicularly joining perpendicularly joining central canalscentral canals

BIO 201

Skeletal System 7

Histology of Histology of Compact Compact

BoneBone

OsteonsOsteons

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Skeletal System 8

Compact Bone in a Nutshell… Compact Bone in a Nutshell…

Spongy BoneSpongy BoneSpongelike appearance Spongelike appearance formed by rods and formed by rods and plates of bone called plates of bone called trabeculaetrabeculae–– spaces filled with red spaces filled with red

bone marrowbone marrow

Trabeculae have few Trabeculae have few osteons or central osteons or central canalscanals–– no osteocyte is far from no osteocyte is far from

blood of bone marrowblood of bone marrow

Provides strength with Provides strength with little weightlittle weight

BIO 201

Skeletal System 9

Bone MarrowBone Marrow

Soft tissue that occupies the Soft tissue that occupies the medullary medullary cavitycavity of a long bone or the of a long bone or the spaces amid spaces amid the trabeculae of spongy bonethe trabeculae of spongy bonethe trabeculae of spongy bonethe trabeculae of spongy boneRed marrow looks like thick bloodRed marrow looks like thick blood–– mesh of reticular fibers and immature cellsmesh of reticular fibers and immature cells–– hemopoietichemopoietic produces blood cellsproduces blood cells–– found in vertebrae, ribs, sternum, pelvic girdle found in vertebrae, ribs, sternum, pelvic girdle

and proximal heads of femur and humerus in and proximal heads of femur and humerus in adults adults

Y llY llYellow marrowYellow marrow–– fatty marrow of long bones in adultsfatty marrow of long bones in adults

Gelatinous marrow of old ageGelatinous marrow of old age–– yellow marrow replaced with reddish jellyyellow marrow replaced with reddish jelly

Extracellular Matrix of Osseous TissueExtracellular Matrix of Osseous Tissue

Composite material composed of approximately 1/3 organic & Composite material composed of approximately 1/3 organic & 2/3 inorganic matter2/3 inorganic matter

Organic matterOrganic mattergg–– collagen, glycosaminoglycans, proteoglycans and glycoproteinscollagen, glycosaminoglycans, proteoglycans and glycoproteins

Inorganic matterInorganic matter–– 85% 85% hydroxyapatitehydroxyapatite (crystallized calcium phosphate salt)(crystallized calcium phosphate salt)

–– 10% 10% calcium carbonatecalcium carbonate

–– other minerals (fluoride, sulfate, potassium, magnesium)other minerals (fluoride, sulfate, potassium, magnesium)

Combination provides forCombination provides for strengthstrength and and resilienceresiliencepp gg–– minerals resist compression; collagen resists tensionminerals resist compression; collagen resists tension

–– bone adapts to tension and compression by varying proportions bone adapts to tension and compression by varying proportions of minerals and collagen fibersof minerals and collagen fibers

BIO 201

Skeletal System 10

Bony MatrixBony Matrix

Mineral DepositionMineral Deposition

MineralizationMineralization is crystallization process in which ions is crystallization process in which ions (calcium, phosphate, bicarbonate and others) are removed (calcium, phosphate, bicarbonate and others) are removed from blood plasma and deposited in bone tissuefrom blood plasma and deposited in bone tissue

Steps of the mineralization processSteps of the mineralization process–– collagen fibers spiral along the length of the osteoncollagen fibers spiral along the length of the osteon

–– fibers become encrusted with minerals by work of osteoblastsfibers become encrusted with minerals by work of osteoblasts

Ectopic ossification is abnormal calcificationEctopic ossification is abnormal calcification

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Skeletal System 11

Mineral Mineral ResorptionResorptionProcess of dissolving bone and releasing minerals into the Process of dissolving bone and releasing minerals into the bloodblood–– performed by osteoclast at “ruffled border”performed by osteoclast at “ruffled border”

hydrogen pumps in the cell membrane secrete hydrogen ions into the hydrogen pumps in the cell membrane secrete hydrogen ions into the space between the osteoclast & the bonespace between the osteoclast & the bone

id i h H f 4 di l b i l hi h l d b k iid i h H f 4 di l b i l hi h l d b k iacid with a pH of 4 dissolves bone minerals which are released back into acid with a pH of 4 dissolves bone minerals which are released back into the bloodthe blood

Functions of Calcium and PhosphateFunctions of Calcium and Phosphate

Phosphate is a component of DNA, RNA, ATP, Phosphate is a component of DNA, RNA, ATP, phospholipidsphospholipids

Calcium is needed for communication between neurons,Calcium is needed for communication between neurons,Calcium is needed for communication between neurons, Calcium is needed for communication between neurons, muscle contraction, blood clotting and exocytosismuscle contraction, blood clotting and exocytosis

Ion imbalancesIon imbalances–– hypocalcemia is deficiency of blood calciumhypocalcemia is deficiency of blood calcium

excitability of nervous system excitability of nervous system muscle spasms or seizuresmuscle spasms or seizures

–– hypercalcemia is too much calcium in bloodhypercalcemia is too much calcium in blooddepression of nervous system depression of nervous system coma or deathcoma or death

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Skeletal System 12

Hormonal Control of Calcium BalanceHormonal Control of Calcium Balance

Calcitriol, PTH and calcitoninCalcitriol, PTH and calcitonin maintain normal blood calcium maintain normal blood calcium concentrationconcentration

Calcitriol (Activated Vitamin D)Calcitriol (Activated Vitamin D)Precursor produced by the UV Precursor produced by the UV radiation penetrating the epidermal radiation penetrating the epidermal keratinocytes or ingestion; processed keratinocytes or ingestion; processed by liver and kidneyby liver and kidneyCalcitriolCalcitriol behaves as a hormone behaves as a hormone (blood(blood--borne messenger)borne messenger)–– stimulates intestine to absorb calcium, stimulates intestine to absorb calcium,

phosphate and magnesiumphosphate and magnesium–– weakly promotes urinary reabsorption of weakly promotes urinary reabsorption of

calcium ionscalcium ions–– promotes osteoclast activity to raise blood promotes osteoclast activity to raise blood

calci m concentration to the le el neededcalci m concentration to the le el needed

calcidiol

calcium concentration to the level needed calcium concentration to the level needed for bone depositionfor bone deposition

Abnormal softness of the bones is Abnormal softness of the bones is called rickets in children and called rickets in children and osteomalacia in adultsosteomalacia in adults

calcitriol

BIO 201

Skeletal System 13

Calcitonin and ParathyroidCalcitonin and Parathyroid

CalcitoninCalcitoninS t d b C ll f th th idS t d b C ll f th th id

ParathyroidParathyroidSecreted by the parathyroidSecreted by the parathyroidSecreted by C cells of the thyroid Secreted by C cells of the thyroid

glandgland

Released when calcium Released when calcium concentration too highconcentration too high

FunctionsFunctions–– reduces osteoreduces osteoclastclast activity by as activity by as

much as 70% in 15 minutesmuch as 70% in 15 minutes

–– increases the number and activity increases the number and activity f tf t bl tbl t

Secreted by the parathyroid Secreted by the parathyroid glands glands Released when calcium blood Released when calcium blood level is too lowlevel is too lowFunctionsFunctions

–– stimulates osteostimulates osteoclasts clasts multiplication and activitymultiplication and activity

–– promotes calcium resorption by promotes calcium resorption by the kidneysthe kidneys

of osteoof osteoblastsblasts –– promotes calcitriol synthesis in the promotes calcitriol synthesis in the kidneyskidneys

–– inhibits collagen synthesis and inhibits collagen synthesis and bone deposition by osteobone deposition by osteoblastsblasts

Intramembranous OssificationIntramembranous OssificationProduces flat bones of skull and clavicleProduces flat bones of skull and clavicle

StepsSteps1.1. embryonic mesenchyme condenses into a sheet of soft tissueembryonic mesenchyme condenses into a sheet of soft tissue

transforms into a network of soft trabeculaetransforms into a network of soft trabeculae

2.2. osteoblasts gather on the trabeculae to form osteoid tissue osteoblasts gather on the trabeculae to form osteoid tissue (uncalcified bone)(uncalcified bone)

3.3. calcium phosphate is deposited in the matrix transforming the calcium phosphate is deposited in the matrix transforming the osteoblasts into osteocytesosteoblasts into osteocytes

4.4. osteoclasts remodel the center to contain marrow spaces & osteoclasts remodel the center to contain marrow spaces & osteoblasts remodel the surface to form compact boneosteoblasts remodel the surface to form compact bone

5.5. mesenchyme left at the surface gives rise to periosteummesenchyme left at the surface gives rise to periosteumy g py g p

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Skeletal System 14

Endochondral OssificationEndochondral OssificationPrimary ossification centerPrimary ossification center forms forms in in cartilage modelcartilage model–– chondrocyteschondrocytes near the center swell to near the center swell to

form primary ossification centerform primary ossification centerp yp y

–– matrix is reduced and model becomes matrix is reduced and model becomes weak at that pointweak at that point

–– cells of the perichondrium produce a cells of the perichondrium produce a bony collarbony collar

–– cuts off diffusion of nutrients and cuts off diffusion of nutrients and hastens chondrocyte deathhastens chondrocyte death

Primary marrow spacePrimary marrow space formedformedPrimary marrow spacePrimary marrow space formed formed –– osteogenic cellsosteogenic cells invade and transform invade and transform

into osteoblastsinto osteoblasts

–– osteoid tissue deposited and calcified osteoid tissue deposited and calcified into trabeculae; at same time into trabeculae; at same time osteoclasts work to enlarge the osteoclasts work to enlarge the primary marrow cavityprimary marrow cavity

Endochondral Ossification in a Endochondral Ossification in a Nutshell…Nutshell…

BIO 201

Skeletal System 15

Cartilage Cartilage BoneBone

Metaphysis

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Skeletal System 16

Growth and RepairGrowth and Repair

Bones grow, remodel and repair themselves throughout lifeBones grow, remodel and repair themselves throughout life–– “use ‘em or lose ‘em”“use ‘em or lose ‘em”Stress fracture is a break caused by abnormal trauma to a Stress fracture is a break caused by abnormal trauma to a bonebonePathological fracture is a break in a bone weakened by Pathological fracture is a break in a bone weakened by some other diseasesome other diseaseNormal healing takes 8 Normal healing takes 8 -- 12 weeks 12 weeks Stages of healingStages of healing

1.1. fracture hematomafracture hematomabroken vessels form a blood clotbroken vessels form a blood clot

2.2. granulation tissuegranulation tissueggfibrous tissue formed by fibroblasts & infiltrated by capillariesfibrous tissue formed by fibroblasts & infiltrated by capillaries

3.3. callus formationcallus formationsoft callus of fibrocartilage soft callus of fibrocartilage replaced by hard callus of bone in ~6 weeksreplaced by hard callus of bone in ~6 weeks

4.4. remodelingremodelingoccurs over next 6 months as spongy bone is replaced with compact occurs over next 6 months as spongy bone is replaced with compact bonebone

Fracture Fracture Repair Initial Repair Initial

StepsSteps

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Skeletal System 17

Finishing UpFinishing Up

In a Nutshell…In a Nutshell…

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Skeletal System 18

OsteoporosisOsteoporosisMost common bone Most common bone diseasedisease

Bones lose mass and Bones lose mass and become brittle due to become brittle due to loss of both organic and loss of both organic and minerals componentsminerals components

Postmenopausal white Postmenopausal white women at greatest riskwomen at greatest risk

ERT slows bone ERT slows bone resorption, but best resorption, but best treatment is prevention treatment is prevention ---- exercise and calcium exercise and calcium intake intake

Therapies to stimulate Therapies to stimulate bone deposition still bone deposition still under investigationunder investigation