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Stages of Intramembranous Stages of Intramembranous OssificationOssification
An ossification center appears in the fibrous An ossification center appears in the fibrous connective tissue membraneconnective tissue membrane
Bone matrix is secreted within the fibrous Bone matrix is secreted within the fibrous membranemembrane
Woven bone and periosteum form Woven bone and periosteum form Bone collar of compact bone forms, and red Bone collar of compact bone forms, and red
marrow appearsmarrow appears
Stages of Intramembranous Stages of Intramembranous OssificationOssification
Figure 6.7.1
Stages of Intramembranous Stages of Intramembranous OssificationOssification
Figure 6.7.2
Stages of Intramembranous Stages of Intramembranous OssificationOssification
Figure 6.7.3
Stages of Intramembranous Stages of Intramembranous OssificationOssification
Figure 6.7.4
Endochondral OssificationEndochondral Ossification
Begins in the second month of developmentBegins in the second month of development Uses hyaline cartilage “bones” as models for Uses hyaline cartilage “bones” as models for
bone constructionbone construction Requires breakdown of hyaline cartilage prior Requires breakdown of hyaline cartilage prior
to ossificationto ossification
Stages of Endochondral Stages of Endochondral OssificationOssification
Formation of bone collarFormation of bone collar Cavitation of the hyaline cartilageCavitation of the hyaline cartilage Invasion of internal cavities by the periosteal bud, Invasion of internal cavities by the periosteal bud,
and spongy bone formationand spongy bone formation Formation of the medullary cavity; appearance of Formation of the medullary cavity; appearance of
secondary ossification centers in the epiphysessecondary ossification centers in the epiphyses Ossification of the epiphyses, with hyaline Ossification of the epiphyses, with hyaline
cartilage remaining only in the epiphyseal platescartilage remaining only in the epiphyseal plates
Stages of Endochondral OssificationStages 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
1
2
34
5
Postnatal Bone GrowthPostnatal Bone Growth
Growth in length of long bonesGrowth in length of long bones Cartilage on the side of the epiphyseal plate closest Cartilage on the side of the epiphyseal plate closest
to the epiphysis is relatively inactiveto the epiphysis is relatively inactive Cartilage abutting the shaft of the bone organizes Cartilage abutting the shaft of the bone organizes
into a pattern that allows fast, efficient growth into a pattern that allows fast, efficient growth Cells of the epiphyseal plate proximal to the Cells of the epiphyseal plate proximal to the
resting cartilage form three functionally different resting cartilage form three functionally different zones: growth, transformation, and osteogenic zones: growth, transformation, and osteogenic
Functional Zones in Long Bone Functional Zones in Long Bone GrowthGrowth
Growth zone – cartilage cells undergo mitosis, Growth zone – cartilage cells undergo mitosis, pushing the epiphysis away from the diaphysispushing the epiphysis away from the diaphysis
Transformation zone – older cells enlarge, the Transformation zone – older cells enlarge, the matrix becomes calcified, cartilage cells die, matrix becomes calcified, cartilage cells die, and the matrix begins to deteriorateand the matrix begins to deteriorate
Osteogenic zone – new bone formation occursOsteogenic zone – new bone formation occurs
Growth in Length of Long BoneGrowth in Length of Long Bone
Figure 6.9
Long Bone Growth and Long Bone Growth and RemodelingRemodeling
Growth in length – cartilage continually grows Growth in length – cartilage continually grows and is replaced by bone as shown and is replaced by bone as shown
Remodeling – bone is resorbed and added by Remodeling – bone is resorbed and added by appositional growth as shown appositional growth as shown
Long Bone Growth and Long Bone Growth and RemodelingRemodeling
Figure 6.10
During infancy and childhood, epiphyseal During infancy and childhood, epiphyseal plate activity is stimulated by growth hormoneplate activity is stimulated by growth hormone
During puberty, testosterone and estrogens: During puberty, testosterone and estrogens: Initially promote adolescent growth spurtsInitially promote adolescent growth spurts Cause masculinization and feminization of specific Cause masculinization and feminization of specific
parts of the skeletonparts of the skeleton Later induce epiphyseal plate closure, ending Later induce epiphyseal plate closure, ending
longitudinal bone growth longitudinal bone growth
Hormonal Regulation of Bone Hormonal Regulation of Bone Growth During YouthGrowth During Youth
Bone RemodelingBone Remodeling
Remodeling units – adjacent osteoblasts and Remodeling units – adjacent osteoblasts and osteoclasts deposit and resorb bone at osteoclasts deposit and resorb bone at periosteal and endosteal surfacesperiosteal and endosteal surfaces
Bone DepositionBone Deposition
Occurs where bone is injured or added Occurs where bone is injured or added strength is neededstrength is needed
Requires a diet rich in protein, vitamins C, D, Requires a diet rich in protein, vitamins C, D, and A, calcium, phosphorus, magnesium, and and A, calcium, phosphorus, magnesium, and manganesemanganese
Alkaline phosphatase is essential for Alkaline phosphatase is essential for mineralization of bonemineralization of bone
Bone DepositionBone Deposition
Sites of new matrix deposition are revealed by Sites of new matrix deposition are revealed by the:the: Osteoid seam – unmineralized band of bone matrixOsteoid seam – unmineralized band of bone matrix Calcification front – abrupt transition zone Calcification front – abrupt transition zone
between the osteoid seam and the older between the osteoid seam and the older mineralized bonemineralized bone
Bone ResorptionBone Resorption
Accomplished by osteoclastsAccomplished by osteoclasts Resorption bays – grooves formed by Resorption bays – grooves formed by
osteoclasts as they break down bone matrixosteoclasts as they break down bone matrix Resorption involves osteoclast secretion of:Resorption involves osteoclast secretion of:
Lysosomal enzymes that digest organic matrixLysosomal enzymes that digest organic matrix Acids that convert calcium salts into soluble formsAcids that convert calcium salts into soluble forms
Bone ResorptionBone Resorption
Dissolved matrix is transcytosed across the Dissolved matrix is transcytosed across the osteoclast’s cell where it is secreted into the osteoclast’s cell where it is secreted into the interstitial fluid and then into the bloodinterstitial fluid and then into the blood
Importance of Ionic Calcium in Importance of Ionic Calcium in the Bodythe Body
Calcium is necessary for:Calcium is necessary for: Transmission of nerve impulsesTransmission of nerve impulses Muscle contractionMuscle contraction Blood coagulationBlood coagulation Secretion by glands and nerve cellsSecretion by glands and nerve cells Cell divisionCell division
Control of RemodelingControl of Remodeling
Two control loops regulate bone remodelingTwo control loops regulate bone remodeling Hormonal mechanism maintains calcium Hormonal mechanism maintains calcium
homeostasis in the bloodhomeostasis in the blood Mechanical and gravitational forces acting on the Mechanical and gravitational forces acting on the
skeletonskeleton
Hormonal MechanismHormonal Mechanism
Rising blood CaRising blood Ca2+2+ levels trigger the thyroid to levels trigger the thyroid to release calcitoninrelease calcitonin
Calcitonin stimulates calcium salt deposit in Calcitonin stimulates calcium salt deposit in bonebone
Falling blood CaFalling blood Ca2+2+ levels signal the parathyroid levels signal the parathyroid glands to release PTHglands to release PTH
PTH signals osteoclasts to degrade bone PTH signals osteoclasts to degrade bone matrix and release Camatrix and release Ca2+2+ into the blood into the blood
Hormonal Control of Blood CaHormonal 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
Response to Mechanical StressResponse to Mechanical Stress
Wolff’s law – a bone grows or remodels in Wolff’s law – a bone grows or remodels in response to the forces or demands placed upon response to the forces or demands placed upon itit
Observations supporting Wolff’s law includeObservations supporting Wolff’s law include Long bones are thickest midway along the shaft Long bones are thickest midway along the shaft
(where bending stress is greatest)(where bending stress is greatest) Curved bones are thickest where they are most Curved bones are thickest where they are most
likely to bucklelikely to buckle
Response to Mechanical StressResponse to Mechanical Stress
Trabeculae form along lines of stressTrabeculae form along lines of stress Large, bony projections occur where heavy, Large, bony projections occur where heavy,
active muscles attachactive muscles attach
Bone Fractures (Breaks)Bone Fractures (Breaks)
Bone fractures are classified by:Bone fractures are classified by: The position of the bone ends after fractureThe position of the bone ends after fracture The completeness of the breakThe completeness of the break The orientation of the bone to the long axisThe orientation of the bone to the long axis Whether or not the bones ends penetrate the skinWhether or not the bones ends penetrate the skin
Types of Bone FracturesTypes of Bone Fractures
Nondisplaced – bone ends retain their normal Nondisplaced – bone ends retain their normal positionposition
Displaced – bone ends are out of normal Displaced – bone ends are out of normal alignmentalignment
Types of Bone FracturesTypes of Bone Fractures
Complete – bone is broken all the way throughComplete – bone is broken all the way through Incomplete – bone is not broken all the way Incomplete – bone is not broken all the way
throughthrough Linear – the fracture is parallel to the long axis Linear – the fracture is parallel to the long axis
of the boneof the bone
Types of Bone FracturesTypes of Bone Fractures
Transverse – the fracture is perpendicular to Transverse – the fracture is perpendicular to the long axis of the bonethe long axis of the bone
Compound (open) – bone ends penetrate the Compound (open) – bone ends penetrate the skinskin
Simple (closed) – bone ends do not penetrate Simple (closed) – bone ends do not penetrate the skinthe skin
Common Types of FracturesCommon Types of Fractures
Comminuted – bone fragments into three or Comminuted – bone fragments into three or more pieces; common in the elderlymore pieces; common in the elderly
Spiral – ragged break when bone is Spiral – ragged break when bone is excessively twisted; common sports injuryexcessively twisted; common sports injury
Depressed – broken bone portion pressed Depressed – broken bone portion pressed inward; typical skull fractureinward; typical skull fracture
Common Types of FracturesCommon Types of Fractures
Compression – bone is crushed; common in Compression – bone is crushed; common in porous bonesporous bones
Epiphyseal – epiphysis separates from Epiphyseal – epiphysis separates from diaphysis along epiphyseal line; occurs where diaphysis along epiphyseal line; occurs where cartilage cells are dyingcartilage cells are dying
Greenstick – incomplete fracture where one Greenstick – incomplete fracture where one side of the bone breaks and the other side side of the bone breaks and the other side bends; common in childrenbends; common in children
Common Types of FracturesCommon Types of Fractures
Table 6.2.1
Common Types of FracturesCommon Types of Fractures
Table 6.2.2
Common Types of FracturesCommon Types of Fractures
Table 6.2.3
Stages in the Healing of a Bone Stages in the Healing of a Bone FractureFracture
Hematoma formationHematoma formation Torn blood vessels Torn blood vessels
hemorrhagehemorrhage A mass of clotted A mass of clotted
blood (hematoma) blood (hematoma) forms at the fracture forms at the fracture sitesite
Site becomes swollen, Site becomes swollen, painful, and inflamedpainful, and inflamed
Figure 6.13.1
Stages in the Healing of a Bone Stages in the Healing of a Bone FractureFracture
Fibrocartilaginous Fibrocartilaginous callus formscallus forms
Granulation tissue (soft Granulation tissue (soft callus) forms a few callus) forms a few days after the fracturedays after the fracture
Capillaries grow into Capillaries grow into the tissue and the tissue and phagocytic cells begin phagocytic cells begin cleaning debriscleaning debris
Figure 6.13.2
Stages in the Healing of a Bone Stages in the Healing of a Bone FractureFracture
The fibrocartilaginous callus forms when:The fibrocartilaginous callus forms when: Osteoblasts and fibroblasts migrate to the fracture Osteoblasts and fibroblasts migrate to the fracture
and begin reconstructing the boneand begin reconstructing the bone Fibroblasts secrete collagen fibers that connect Fibroblasts secrete collagen fibers that connect
broken bone endsbroken bone ends Osteoblasts begin forming spongy boneOsteoblasts begin forming spongy bone Osteoblasts furthest from capillaries secrete an Osteoblasts furthest from capillaries secrete an
externally bulging cartilaginous matrix that later externally bulging cartilaginous matrix that later calcifiescalcifies
Stages in the Healing of a Bone Stages in the Healing of a Bone FractureFracture
Bony callus formationBony callus formation New bone trabeculae New bone trabeculae
appear in the appear in the fibrocartilaginous callusfibrocartilaginous callus
Fibrocartilaginous callus Fibrocartilaginous callus converts into a bony (hard) converts into a bony (hard) calluscallus
Bone callus begins 3-4 Bone callus begins 3-4 weeks after injury, and weeks after injury, and continues until firm union continues until firm union is formed 2-3 months lateris formed 2-3 months later
Figure 6.13.3
Stages in the Healing of a Bone Stages in the Healing of a Bone FractureFracture
Bone remodelingBone remodeling Excess material on the Excess material on the
bone shaft exterior and bone shaft exterior and in the medullary canal in the medullary canal is removedis removed
Compact bone is laid Compact bone is laid down to reconstruct down to reconstruct shaft wallsshaft walls
Figure 6.13.4
Homeostatic ImbalancesHomeostatic Imbalances
OsteomalaciaOsteomalacia Bones are inadequately mineralized causing Bones are inadequately mineralized causing
softened, weakened bonessoftened, weakened bones Main symptom is pain when weight is put on the Main symptom is pain when weight is put on the
affected boneaffected bone Caused by insufficient calcium in the diet, or by Caused by insufficient calcium in the diet, or by
vitamin D deficiencyvitamin D deficiency
Homeostatic ImbalancesHomeostatic Imbalances
RicketsRickets Bones of children are inadequately mineralized Bones of children are inadequately mineralized
causing softened, weakened bonescausing softened, weakened bones Bowed legs and deformities of the pelvis, skull, Bowed legs and deformities of the pelvis, skull,
and rib cage are commonand rib cage are common Caused by insufficient calcium in the diet, or by Caused by insufficient calcium in the diet, or by
vitamin D deficiencyvitamin D deficiency
Isolated Cases of RicketsIsolated Cases of Rickets
Rickets has been essentially eliminated in the Rickets has been essentially eliminated in the USUS
Only isolated cases appearOnly isolated cases appear Example: Infants of breastfeeding mothers Example: Infants of breastfeeding mothers
deficient in Vitamin D will also be Vitamin D deficient in Vitamin D will also be Vitamin D deficient and develop ricketsdeficient and develop rickets
Homeostatic ImbalancesHomeostatic Imbalances
OsteoporosisOsteoporosis Group of diseases in which bone reabsorption Group of diseases in which bone reabsorption
outpaces bone depositoutpaces bone deposit Spongy bone of the spine is most vulnerableSpongy bone of the spine is most vulnerable Occurs most often in postmenopausal womenOccurs most often in postmenopausal women Bones become so fragile that sneezing or stepping Bones become so fragile that sneezing or stepping
off a curb can cause fracturesoff a curb can cause fractures
Osteoporosis: TreatmentOsteoporosis: Treatment
Calcium and vitamin D supplementsCalcium and vitamin D supplements Increased weight-bearing exerciseIncreased weight-bearing exercise Hormone (estrogen) replacement therapy Hormone (estrogen) replacement therapy
(HRT) slows bone loss(HRT) slows bone loss Natural progesterone cream prompts new bone Natural progesterone cream prompts new bone
growthgrowth Statins increase bone mineral densityStatins increase bone mineral density
Thought to kill off osteoclasts and/or promote Thought to kill off osteoclasts and/or promote growth and development of osteoblastsgrowth and development of osteoblasts
Paget’s DiseasePaget’s Disease
Characterized by excessive bone formation and Characterized by excessive bone formation and breakdownbreakdown
Pagetic bone with an excessively high ratio of Pagetic bone with an excessively high ratio of woven to compact bone is formedwoven to compact bone is formed
Pagetic bone, along with reduced Pagetic bone, along with reduced mineralization, causes spotty weakening of bonemineralization, causes spotty weakening of bone
Osteoclast activity wanes, but osteoblast activity Osteoclast activity wanes, but osteoblast activity continues to workcontinues to work
Paget’s DiseasePaget’s Disease
Usually localized in the spine, pelvis, femur, Usually localized in the spine, pelvis, femur, and skulland skull
Unknown cause (possibly viral)Unknown cause (possibly viral) Treatment includes the drugs Didronate and Treatment includes the drugs Didronate and
FosamaxFosamax
Fetal Primary Ossification CentersFetal Primary Ossification Centers
Figure 6.15
Developmental Aspects of BonesDevelopmental Aspects of Bones
Mesoderm gives rise to embryonic Mesoderm gives rise to embryonic mesenchymal cells, which produce membranes mesenchymal cells, which produce membranes and cartilages that form the embryonic and cartilages that form the embryonic skeletonskeleton
The embryonic skeleton ossifies in a The embryonic skeleton ossifies in a predictable timetable that allows fetal age to predictable timetable that allows fetal age to be easily determined from sonogramsbe easily determined from sonograms
At birth, most long bones are well ossified At birth, most long bones are well ossified (except for their epiphyses)(except for their epiphyses)
Developmental Aspects of BonesDevelopmental Aspects of Bones
By age 25, nearly all bones are completely By age 25, nearly all bones are completely ossifiedossified
In old age, bone resorption predominatesIn old age, bone resorption predominates A single gene that codes for vitamin D A single gene that codes for vitamin D
docking determines both the tendency to docking determines both the tendency to accumulate bone mass early in life, and the accumulate bone mass early in life, and the risk for osteoporosis later in liferisk for osteoporosis later in life