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Skeletal Skeletal SystemSystem
Structure & Function Development & Growth Bone Homeostasis Osteoporosis & other bone disorders
The evil that men do lives after them, the good is oft interred with their bones.
—William Shakespeare (1564-1616) Julius Caesar, Act III, Scene 2
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A Bone’s Shape Makes Possible A Bone’s Shape Makes Possible its Functionsits Functions
Sites of muscle attachments
• Shape & form
• Support
• Protection
• Movement
• Storage
• Hematopoiesis
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Bone StructureBone Structure
Structures of a long bone:– periosteum: tough fibrous membrane covering
– diaphysis: shaft - b/t epiphyses, long axis of bone
– articular cartilage: portion of epiphysis covered w/ cartilage
– epiphysis: expanded ends of bone -- proximal & distal
– medullary cavity: hollow chamber w/in diaphysis, endosteum membrane lines cavity & marrow resides w/in
– compact bone (cortical): near surface, continuous ECM w/ no spaces -- dense & hard
– spongy bone (cancellous): w/in compact bone, consists of network of thin strands of trabeculae
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Gross Anatomy of a Long Gross Anatomy of a Long BoneBone
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Cellular Structure Cellular Structure of Boneof Bone
• Compact bone: cells, ECM & mineral salts form an osteon– contain blood vessels & nerves surrounded by CT
– perforating canals transverse central canals
– osteocytes lie in small concavities (lacunae) b/t lamellae
– canaliculi - provide communication b/t osteocytes
• Spongy bone: cells & ECM lie w/in trabeculae– no osteon or central canals – irregular lamellae & osteocytes
– trabeculae are formed where stress is exerted on the bone
– receive nutrients via diffusion from the canaliculi
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Structural Unit of Compact Bone: OsteonStructural Unit of Compact Bone: Osteon
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Lamella
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Microscopic Structure of Spongy BoneMicroscopic Structure of Spongy Bone
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Gross Anatomy of a Long Gross Anatomy of a Long BoneBone
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Principal Cells of Bone Principal Cells of Bone TissueTissue
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Chemical Chemical Composition of Composition of
BoneBone Organic components:
– Cells: osteogenic, osteoblasts, osteocytes & osteoclasts
– ECM: contribute to bone’s structure & tensile strength
Inorganic components: – mineral salts: calcium phosphate & carbonate which
account for bone’s hardness
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OsteogenesisOsteogenesis Intramembranous ossification (flat & irregular):
- originate as sheet-like layers of CT
- partially differentiated CT form into osteoblasts
- deposit bony matrix -- forming spongy bone
Endochondral ossification (long & short):- develop from hyaline cartilage -- model for bone formation
- CT covering cartilage becomes infiltrated w/ blood vessels forming periosteum
- CT differentiates into osteoblasts forming spongy bone w/ ossification continuing deposition of compact bone occurs
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Intramembranous Intramembranous OssificationOssification
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Stages in Endochondral Stages in Endochondral Ossification Occurring in a Ossification Occurring in a
Long BoneLong Bone
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EndochonEndochondral dral
OssificatioOssificationn
Forming skeleton of an embryonic chicken, stained with Alizarin Red and Alcian Blue to differentiate between hardened bone (in red) and the remaining cartilage model (in blue).
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Bone Growth : elongation & Bone Growth : elongation & appositionalappositional
Epiphyseal Plate
http://highered.mcgraw-hill.com/classware/infoCenter.do?isbn=0072829532
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X-ray Depicting Epiphyseal X-ray Depicting Epiphyseal PlatePlate
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Osteoclasts & Medullary Osteoclasts & Medullary CavityCavity
Multinucleated cells originate from WBC -- break down calcified matrix (bone resorption):– lysosomal enzymes digest organic components– acids secreted dissolve inorganic portion of matrix– phagocytes digest bony matrix – osteoblasts invade depositing new bone
Formation of medullary cavity:– primary ossification center enlarges -- osteoclasts break down
spongy bone – cavity forms in center of diaphysis– cavity fills w/ marrow, blood & lymph vessels & nerve fibers
• red & yellow marrow
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Bone–Resorbing Bone–Resorbing OsteoclastOsteoclast
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Development of osteoblasts Development of osteoblasts &osteoclasts from bone &osteoclasts from bone
marrow progenitorsmarrow progenitors
Valsamis et al. Nutrition & Metabolism 2006 3:36
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Homeostasis: Homeostasis: Bone Bone
RemodelingRemodeling Continuous bone resorption
& bone deposition – regulated via 2 control loops: 1. negative feedback
(hormonal)
2. mechanical/gravitational
(Wolff’s law of bone)
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Hormonal Control of Blood Hormonal Control of Blood CaCa++++
• When Ca++ intake is -- blood [Ca++] are also
• PTH stimulates osteoclasts -- releasing Ca++ salts from ECM into blood
• High intake blood [Ca++] inhibits osteoclasts activity
• Calcitonin -- stimulates osteoblasts activity, bone resorption & Ca++ excretion
(hypercalcemia)
(hypocalcemia)
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Hormonal Control of CaHormonal Control of Ca++++ BalanceBalance
• Parathyroid hormone (PTH) – responsible for plasma [Ca++]– mobilize Ca++ from bone ( bone resorption)– enhances renal reabsorption of Ca++ – intestinal absorption of Ca++ (indirectly)
• Vitamin D3 – 1,25-dihydroxycholecalciferol (calcitriol):– obtained from diet & sunlight– supports effect of PTH – enhancing Ca++ uptake in small
intestine– PTH & prolactin regulate production
• Calcitonin:– released in association w/ plasma [Ca++]– bone resorption & renal excretion (action opposite to PTH)
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Factors Affecting Bone Factors Affecting Bone Development, Growth & Development, Growth &
RepairRepair Nutrition: calcium, phosphorus, vitamins D, A, C & K
UV radiation: dehydrocholesterol
Hormones: hGH, T3 & T4, PTH and male & female sex hormones
Physical activity: weight bearing exercise & skeletal muscle contraction (Wolff’s Law)
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Fractures & RepairFractures & Repair
• Fracture: classified by cause & nature of break (e.g., traumatic, compound)
– Blood vessels & periosteum rupture -- hematoma, swelling & inflammation to surrounding tissue
– Angiogenesis: osteoblasts invade hematoma generating spongy bone nearby & fibroblast produce fibrocartilage (cartilaginous callus) and ECM
– Phagocytic cells remove blood clot & osteoclasts resorb bone fragments
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Key Steps in Repair of a Key Steps in Repair of a FractureFracture
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Types of FracturesTypes of Fractures
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Rickets & OsteomalaciaRickets & Osteomalacia
• Pathology: failure of osteoid to calcify in a growing person, most commonly assoc. w/ vitamin D deficiency in hypocalcemia
• Signs & Symptoms: muscular hypotonia, thickening of skull, softening of long bones (bowlegs), knobby deformity in long bones & ribs, kyphoscoliosis
• Risk factors: dark skin, inner-city dwellers, breastfeed infants w/o vitamin D supplementation
• Treatment: UV light, vitamin D, calcium & phosphorus supplements,
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OsteoporosisOsteoporosis
• Pathology: bone mass & mineral content - w/in affected bones trabeculae are lost -- spaces/canals enlarge filling w/ fibrous & fatty tissues
• Signs & symptoms: bones fracture easily (long bones), spontaneous breaks - unable to support body weight
• Risk factors: Ca++ & vitamin D intake, phys. act., estrogen levels, cigarette smoking, alcohol abuse, medications: gender, age, body
size, ethnicity & genetics
• Screening & Treatment: DEXA; bisphosphonates; estrogen therapy (ERT); PTH & exercise
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Bone Mineral Bone Mineral Acquisition During Acquisition During PubertyPuberty
Gap in CaGap in Ca++++ Intake Intake
• Females reach 95% of adult BMD by age 18 yrs & w/ only modest gains up to 3rd decade of life
• Bone mineral density (BMD) most rapidly b/t ages of 11-14 yrs in girls & 14-17 yrs in boys
• 86% of girls & 65% of boys aged 12-18 yrs fail to meet RDA of 1200 mg/d for Ca++
• Intake for Ca++ -- 1300 mg/d; gap b/t the recommended & actual intakes has widened
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• Ca++ content of common foods: http://www.nof.org/
• Meeting RDA of Ca++ is challenging when dairy products are not consumed. Ca++ -fortified products offer a means of boosting Ca++ consumption through nondairy foods
• Nondairy sources of Ca++ such as breads, cereals, vegetables, and fish, have a lower content or less bioavailable form. Ca++ -rich foods such as DGLV, tofu, nuts, legumes & sardines are not part of the standard diet
• Inadequate vitamin D intake, lack of exposure to sunlight & reduced vitamin D receptors in older adults all contribute
• Lack of phys. act., smoking, excessive alcohol consumption, diets Na & phosphorus
Barriers to Calcium Intake & Other Barriers to Calcium Intake & Other Factors Affecting Bone Mineral Factors Affecting Bone Mineral DensityDensity
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Spinal Deviations of the Vertebral ColumnSpinal Deviations of the Vertebral Column
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Intervertebral Disc & Intervertebral Disc & HerniationHerniation
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Carpal Tunnel SyndromeCarpal Tunnel Syndrome
• Pathology: swelling of tendons reduces tunnel space -- squeezing & injuring median nerve
• Symptoms: numbness, tingling, pain, inflammation & clumsiness of the hand
• Diagnosis & treatment: Tinel’s test, Phalen’s test, nerve conductionvelocity studies, patient history & occupational evaluation; anti-inflammatory drugs, splints, avoidance of activities causing condition,surgery and alternative therapies