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Skeletal system


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Skeletal system

  1. 1. Lecture 6 THE SKELETAL SYSTEM – Bones and Cartilage
  2. 2. Figure 6.1: The bones and cartilages of the human skeleton, p. 177. Epiglottis Larynx Trachea Lung Respiratory tube cartilages in neck and thorax = Hyaline cartilages Key: = Fibrocartilages = Elastic cartilages = Bones of axial skeleton = Bones of appendicular skeleton Cartilage in external ear Cartilages in nose Articular cartilage of a joint Costal cartilage Cartilage in intervertebral disc Pubic symphysis Articular cartilage of a joint Meniscus (padlike cartilage in knee joint)
  3. 3. Figure 6.2: Classification of bones on the basis of shape, p. 178. (a) (b) (d) (c) Long bone (humerus) Short bone (triquetral) Irregular bone (vertebra), left lateral view Flat bone (sternum)
  4. 4. 2 Types of Bone Tissue Spongy bone Compact bone
  5. 5. 2 types of bone tissue <ul><li>Compact bone tissue - composed of OSTEONS = structural units of compact bone. </li></ul><ul><li>Spongy bone tissue – like a honeycomb – composed of needle-like structures called TRABECULAE = structural units of spongy bone </li></ul>Compact bone Spongy bone
  6. 6. Structure of a long bone <ul><li>Epiphyses = expanded ends of long bones </li></ul><ul><li>spongy bone surrounded by a thin layer of compact bone </li></ul><ul><li>Diaphysis = shaft = long axis of a long bone </li></ul><ul><li>composed of a thick collar of compact bone which surrounds a Medullary Cavity – contains red bone marrow in childhood and yellow bone marrow in adulthood </li></ul><ul><li>Hematopoiesis, the process by which blood cells and platelets are formed, occurs only in red bone marrow </li></ul><ul><li>Membranes: Endosteum and Periosteum </li></ul>
  7. 7. Figure 6.3: The structure of a long bone (humerus of arm), p. 180. (b) (c) (a) Proximal epiphysis Articular cartilage Yellow bone marrow Endosteum Epiphyseal line Spongy bone Periosteum Compact bone Medullary cavity Spongy bone Compact bone Articular cartilage Compact bone Periosteum Perforating (Sharpey’s) fibers Nutrient arteries Diaphysis Distal epiphysis
  8. 8. Figure 6.3a: The structure of a long bone (humerus of arm), p. 180. (a) Proximal epiphysis Articular cartilage Epiphyseal line Spongy bone Periosteum Compact bone Medullary cavity Diaphysis Distal epiphysis Fat
  9. 9. Figure 6.6: Microscopic anatomy of compact bone, p. 183. (a) (b) (c) Perforating (Sharpe’s) fibers Compact bone Periosteal blood vessel Periosteum Lacuna Blood vessel Endosteum lining bony canals and covering trabeculae Central (Haversian) canal Spongy bone Blood vessel continues into medullary cavity containing marrow Central (Haversian) canal Canaliculus Lacuna Lamella Osteocyte Osteon (Haversian system) Circumferential lamellae Lamellae Osteon Interstitial lamellae Central canal Perforating (Volkmann’s) canal
  10. 10. Figure 6.5: A single osteon, p. 182. Lamellae Collagen fibers Twisting force Nerve fiber Vein Artery with capillaries Structures in the central canal
  11. 11. Figure 6.3c: The structure of a long bone (humerus of arm), p. 180. (c) Yellow bone marrow Endosteum Compact bone Periosteum Perforating (Sharpey’s) fibers Nutrient arteries
  12. 12. The 2 membranes <ul><li>Endosteum: covers the internal surfaces of bone such as the canals. It contains osteoblasts and osteoclasts </li></ul><ul><li>Periosteum: is double-layered – composed of the outer fibrous layer and the inner osteogenic layer. </li></ul><ul><li>The fibrous layer is composed of dense irregular connective tissue </li></ul><ul><li>The osteogenic layer contains of osteoblasts and osteoclasts . </li></ul><ul><li>The periosteum is attached to compact bone by tough collagenous fibers called the </li></ul><ul><li>SHARPEY”S(perforating) FIBERS </li></ul>
  13. 13. The bone cells <ul><li>Osteoblasts : bone-forming cells – secrete bone tissue </li></ul><ul><li>Osteogenic cells: give rise to osteoblasts </li></ul><ul><li>Osteocytes: matured osteoblasts </li></ul><ul><li>Osteoclasts: bone-resorbing cells – destroy bone tissue </li></ul>
  14. 14. Microscopic structure of compact bone <ul><li>Composed of osteons= structural units of compact bone </li></ul><ul><li>Each osteon is an elongated cylinder consisting of concentric tubes called LAMELLAE hence, compact bone is also known as Lamellar bone. The collagen fibers in adjacent lamellae run in opposite directions to resist twisting </li></ul><ul><li>HAVERSIAN CANAL = Central canal – runs in the core of each osteon contains blood vessels and nerves </li></ul><ul><li>Perforating or Volkmann’s canals – connect blood vessels and nerves between the periosteum and the Haversian canals </li></ul><ul><li>LACUNAE – shallow cavities in the solid bone matrix that house the osteocytes. </li></ul><ul><li>CANALICULI – tiny canals that connect lacunae to each other and to the Haversian canal to allow for transfer of substances from the blood vessel in the Haversian canal </li></ul>
  15. 15. Figure 6.6a: Microscopic anatomy of compact bone, p. 183. (a) Perforating (Sharpey’s) fibers Compact bone Periosteal blood vessel Periosteum Blood vessel Endosteum lining bony canals and covering trabeculae Central (Haversian) canal Spongy bone Blood vessel continues into medullary cavity containing marrow Osteon (Haversian system) Circumferential lamellae Lamellae Perforating (Volkmann’s) canal
  16. 16. Figure 6.15: Fetal primary ossification centers at 12 weeks, p. 198. Parietal bone Radius Ulna Humerus Femur Occipital bone Clavicle Scapula Ribs Vertebra Ilium Tibia Frontal bone of skull Mandible
  17. 17. OSSIFICATION ( Osteogenesis) - Development of the bony skeleton from the embryonic skeleton <ul><li>2 forms: PRENATAL AND POSTNATAL </li></ul><ul><li>Prenatal bone development – occurs before birth; 2 types </li></ul><ul><li>i) INTRAMEMBRANOUS OSSIFICATION ii)ENDOCHONDRAL OSSIFICATION </li></ul><ul><li>Intramembranous ossification : </li></ul><ul><li>develops from FIBROUS CONNECTIVE TISSUE MEMBRANE ( derived directly from mesenchyme) and results in the formation of MEMBRANE BONES = cranial bones and clavicles </li></ul><ul><li>Note : all membrane bones are flat bones. </li></ul>
  18. 18. Figure 6.7 : Intramembranous ossification, p. 184. Mesenchymal cell Collagen fiber Ossification center Osteoid Osteoblast Osteoid Osteocyte Newly calcified bone matrix Osteoblast An ossification center appears in the fibrous connective tissue membrane . • Selected centrally located mesenchymal cells cluster and differentiate into osteoblasts, forming an ossification center. Bone matrix (osteoid) is secreted within the fibrous membrane. • Osteoblasts begin to secrete osteoid, which is mineralized within a few days. • Trapped osteoblasts become osteocytes. 1 2
  19. 19. Figure 6.7 : Intramembranous ossification (continued), p. 184. Mesenchyme Condensing to form the periosteum Blood vessel Trabecula of woven bone Fibrous periosteum Osteoblast Plate of compact bone Diploë (spongy bone) cavities contain red marrow Woven bone and periosteum form. • Accumulating osteoid is laid down between embryonic blood vessels, which form a random network. The result is a network (instead of lamellae) of trabeculae. • Vascularized mesenchyme condenses on the external face of the woven bone and becomes the periosteum. Bone collar of compact bone forms and red marrow appears. • Trabeculae just deep to the periosteum thicken, forming a woven bone collar that is later replaced with mature lamellar bone. • Spongy bone (diploë), consisting of distinct trabeculae, persists internally and its vascular tissue becomes red marrow. 3 4 Intramembranous ossification : develops from FIBROUS CT MEMBRANE ( derived directly from mesenchyme) and results in the formation of MEMBRANE BONES = cranial bones and clavicles Note : all membrane bones are flat bones.
  20. 20. The Embryonic Skeleton
  21. 21. Figure 6.8: Endochondral ossification in a long bone, p. 185. Formation of bone collar around hyaline cartilage model. Hyaline cartilage Cavitation of the hyaline carti- lage within the cartilage model. Invasion of internal cavities by the periosteal bud and spongy bone formation. Formation of the medullary cavity as ossification continues; appearance of sec- ondary ossification centers in the epiphy- ses in preparation for stage 5. Ossification of the epiphyses; when completed, hyaline cartilage remains only in the epiphyseal plates and articular cartilages. Deteriorating cartilage matrix Epiphyseal blood vessel Spongy bone formation Epiphyseal plate cartilage Secondary ossificaton center Blood vessel of periosteal bud Medullary cavity Articular cartilage Spongy bone Primary ossification center Bone collar 1 2 3 4 5
  22. 22. <ul><li>Derived from HYALINE CARTILAGE produced by chondroblasts; (chondroblasts are derived from mesenchyme ) </li></ul><ul><li>The bones formed from endochondral ossification are called endochondral or cartilage bones = all bones in the body except the cranial bones and the clavicles </li></ul>Endochondral Ossification
  23. 23. Endochondral Ossification <ul><li>Hyaline cartilage is ossified into bone. </li></ul><ul><li>After endochondral ossification, hyaline cartilage still persists in two areas in the long bones as the: </li></ul><ul><li> 1. Articular cartilage – capping the ends of the epiphyses of long bones </li></ul><ul><li>2. Epiphyseal plates = at the junctions of the epiphyses and the diaphysis of a long bone </li></ul>
  24. 24. Postnatal Bone Growth <ul><li>Ossification that occurs after birth </li></ul><ul><li>2 types : Longitudinal bone growth and Appositional bone growth </li></ul><ul><li>Longitudinal bone growth = Linear bone growth increases the length of long bones = height </li></ul><ul><li>Appositional bone growth = increases the width/diameter of all bones </li></ul>
  25. 25. LONGITUDINAL Bone Growth <ul><li>Involves the EPIPHYSEAL PLATES </li></ul><ul><li>New hyaline cartilage is added on at the epiphyseal faces of the epihyseal plates </li></ul><ul><li>New bone tissue is added on at the diaphyseal faces of the epiphyseal plates </li></ul><ul><li>Results in lengthening of the diaphysis of the long bone = lengthening of the long bone </li></ul><ul><li>The amount of new hyaline cartilage added on the epiphyseal face = the amount of bone tissue formed on the diaphyseal face hence, the width ( thickness) of the epiphyseal plates does NOT change </li></ul>
  26. 26. Figure 6.9: Growth in length of a long bone – zones of the Epiphyseal plate Calcified cartilage spicule Osseous tissue (bone) covering cartilage spicules Growth ( proliferation) zone Cartilage cells undergo mitosis Resting (quiescent) zone Hypertrophic zone Older cartilage cells enlarge Ossification (osteogenic) zone New bone formation is occurring Resorption zone Calcification zone Matrix becomes calcified; cartilage cells die; matrix begins deteriorating Osteoblast depositing bone matrix Diaphyseal face Epiphyseal fac e
  27. 27. Figure 6.10: Long bone growth and remodeling during youth, p. 187. Growth Bone grows in length because: Cartilage grows here Cartilage grows here Cartilage replaced by bone here Cartilage replaced by bone here Remodeling Growing shaft is remodeled by: Articular cartilage Bone resorbed here Bone added by appositional growth here Bone resorbed here Epiphyseal plate 1 2 3 4 1 2 3
  28. 28. APPOSITIONAL BONE GROWTH <ul><li>All bones widen and increase in diameter/thickness via appositional bone growth </li></ul><ul><li>Sequence of events: </li></ul><ul><li> Osteoblasts in the osteogenic layer of the periosteum secrete new bone tissue onto the external surface of the bone </li></ul><ul><li>Osteoclasts in the endosteum slightly resorb bone tissue in the internal surface of the bone </li></ul><ul><li>Overall, more new bone tissue is added onto the external surface and old bone tissue is slightly resorbed from the internal surface resulting in a thicker but lighter bone. </li></ul>
  29. 29. Hormonal Control of Postnatal Bone Growth <ul><li>Growth hormone – stimulates hepatocytes to produce Insulin-like growth factors ( IGFs) </li></ul><ul><li>IGFs stimulate chondroblasts to produce hyaline cartilage on the epiphyseal faces of the epiphyseal plates and bone formation on the diaphyseal faces </li></ul><ul><li>Sex steroid hormones ( testosterone in the male and the estrogens in the female) synergize with growth hormone to cause “ growth spurt” </li></ul><ul><li>Towards the end of adolescence, the sex steroid hormones antagonize the actions of growth hormone and the epiphyseal plates become ossified = EPIPHYSEAL PLATE CLOSURE – height determined </li></ul>
  30. 30. Bone Remodeling <ul><li>Adult bones constantly undergo bone formation on the periosteal surface and bone resorption on the endosteal surface = Bone Remodeling </li></ul><ul><li>In healthy adults, the bone density remains constant because </li></ul><ul><li>Rate of Bone formation = rate of bone resorption </li></ul><ul><li>If the rate of resorption outpaces the rate of formation = OSTEOPOROSIS </li></ul><ul><li>Functions of Bone Remodeling: </li></ul><ul><li>i) To maintain calcium homeostasis </li></ul><ul><li>ii) To allow for bone repair after fractures </li></ul>
  31. 31. Normal and osteoporotic bone Normal bone Osteoporotic bone osteoporosis /DS00128
  32. 32. Figure 6.11 : Hormonal control of blood calcium levels , p. 189. PTH; calcitonin secreted Calcitonin stimulates calcium salt deposit in bone Parathyroid glands release parathyroid hormone (PTH) Thyroid gland Thyroid gland Parathyroid glands Osteoclasts degrade bone matrix and release Ca 2+ into blood Falling blood Ca 2+ levels Rising blood Ca 2+ levels Calcium homeostasis of blood: 9–11 mg/100 ml PTH Imbalance Imbalance
  33. 33. Factors that Control Bone Remodeling <ul><li>2 factors: Hormonal control and mechanical stress </li></ul><ul><li>Hormonal Control : </li></ul><ul><li>Under hypercalcemic conditions, CALCITONIN is released to stimulate osteoblasts to produce bone tissue and stimulate mineralization – uses calcium from blood </li></ul><ul><li>Under hypocalcemic conditions, PARATHYROID HORMONE (PTH) is released to stimulate osteoclasts to cause bone resorption to release calcium from bones into blood </li></ul><ul><li>I,25 dihydroxyvitamin D stimulates calcium absorption from the small intestine </li></ul><ul><li>Mechanical Stress : </li></ul><ul><li>Bones remodel/grow in response to mechanical stresses placed on the bones = WOLFF’S LAW </li></ul>
  34. 34. Forms of evidence in support of Wolff’s Law <ul><li>Bone attachment sites for active skeletal muscles appear thicker – projections such as trochanters, spines, </li></ul><ul><li>Bones of the upper limb often used are thicker than the less used limb – bones in the right arm of a right-handed individual are thicker than bones in the left arm and vice versa </li></ul><ul><li>Long bones are thickest in the middle region of the diaphysis where bending stresses are greatest </li></ul><ul><li>Bedridden individual not subjected to the stresses of walking or exercises lose bone density </li></ul><ul><li>Astronauts who spend appreciably amount of time in space (where there’s no gravity and they cannot walk), lose bone density </li></ul>
  35. 35. Figure 6.12: Bone anatomy and stress, p. 190 Load here (body weight) Head of femur Compression here Point of no stress Tension here