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Chapter 6

Chapter 6. The Skeletal System: Bone Tissue. INTRODUCTION. Bone composed of numerous tissue types: cartilage dense connective tissue epithelium blood-forming, adipose, & nervous tissues Each individual bone is an organ Bone, along with cartilage, makes up skeletal system

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Chapter 6

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  1. Chapter 6 The Skeletal System: Bone Tissue

  2. INTRODUCTION • Bone composed of numerous tissue types: • cartilage • dense connective tissue • epithelium • blood-forming, adipose, & nervous tissues • Each individual bone is an organ • Bone, along with cartilage, makes up skeletal system • Dynamic tissue: ever-changing throughout life

  3. Functions of Bone • Support • Protection of soft tissues • Movement • Mineral homeostasis • Blood cell production in red bone marrow • Energy (TG) storage in yellow bone marrow

  4. Anatomy of Long Bone • Diaphysis = shaft • Epiphysis = distal & proximal ends • Metaphyses = where epiphysis & diaphysis meet • epiphyseal plate in growing bone • layer of hyaline cartilage • Articular cartilage over joint surfaces ↓ friction & shock • Medullary cavity = marrow cavity w/in diaphysis • Periosteum = tough membrane covering bone • enables widthwise growth • assists in fracture repair • nourishes bone • attachmt point for tendons/ligaments • Endosteum = lining of marrow cavity

  5. HISTOLOGY OF BONE • Inorganic salts & minerals within matrix • Hydroxyapatite & calcium carbonate • Mg+2, K+, SO4-2, F- • Salts deposited in a framework of collagen fibers during calcification (mineralization) • Occurs only in the presence of collagen fibers • Mineral salts give hardness to bone • Collagen fibers give tensile strength

  6. TYPES OF BONE CELLS • Osteogenic cells • undergo cell division • develop into osteoblasts • Osteoblasts = bone-building cells • Osteocytes • mature bone cells • principal cells of bone tissue • Osteoclasts • derived from monocytes • break down bone tissue (resorption)

  7. Bone Matrix • Inorganic mineral salts provide hardness • hydroxyapatite (CaPO4 + Ca(OH)2) & CaCO3 • Collagen fibers provide flexibility • their tensile strength resists being stretched or torn • Bone is not completely solid  has small spaces for vessels & red bone marrow • spongy bone has many such spaces • compact bone has very few such spaces

  8. Compact Bone • Arranged in units called osteons or Haversian systems • Osteons contain: • blood & lymphatic vessels • nerves • osteocytes • calcified matrix • Osteons aligned in same direction along stress lines • lines can slowly change as the stresses on bone change

  9. Compact Bone • Looks like solid, hard layer of bone • Shaft of long bones & external layer of all bones • Resists stresses produced by weight & movement

  10. Histology of Compact Bone • Osteon = concentric rings (lamellae) of calcified matrix surrounding vertically oriented blood vessel • Osteocytes found in lacunae • Osteocytes communicate through canaliculi filled with ECF • Canaliculi connect cells • Interstitial lamellae represent older osteons that were partially removed during bone remodeling

  11. Spongy Bone • Does not contain osteons • Consists of trabeculae surrounding many red marrow-filled spaces (Figure 6.3b) • Most of structure of short, flat, & irregular bones • Epiphyses of long bones • Lightweight • Supports & protects red bone marrow

  12. Trabeculae of Spongy Bone • Oriented along lines of stress & where stress applied from many directions, but not in areas of high stress • Spaces between trabeculae are filled with red marrow where blood cells develop • Lacunae contain osteocytes • Directly nourished by blood in medullary cavities

  13. Blood & Nerve Supply of Bone • Highly vascularized & innervated • Periosteal arteries enter diaphysis thru Volkmans canal • supply periosteum • Nutrient arteries • enter thru nutrient foramen in center of diaphysis • supply compact bone of diaph., spongy bone, & red marrow up to epiphys. plates • Metaphys & epiphyseal arteries • enter thru meta-/epiphysis • supply red marrow & bone tissue of meta-/epiphyses

  14. Blood Supply c’td • Veins carry blood away from long bones • Nutrient veins accompany nutr. artery in diaphy. • Numerous epiphyseal/metaphyseal veins • exit thru epiphyses • Periosteal arteries exit thru perisoteum • Nerves accompany blood vessels • Sensory nerves in periosteum • REMEMBER:Arteries carry oxygenated blood FROM the heart to the tissues (bone) & veins carry deoxygenated blood from the tissues TO the heart.

  15. BONE FORMATION • Osteogenesis or ossification • Begins when mesenchymal cells provide template for subsequent ossification • Two types of ossification: • Intramembranous ossification = formation of bone directly within mesenchyme • arranged in sheetlike layers • resemble membranes • Endochondral ossification = formation of bone from hyaline cartilage that develops from mesenchyme

  16. Intramembranous Ossification • Forms flat bones of skull and mandible • Formation of ossification center • mesenchymal cells differentiate into osteogenic cells & then into osteoblasts • osteoblasts secrete XC matrix until surrounded by it • Calcification • matrix secretion stops • osteoblast becomes osteocyte • Ca & other minerals deposited  calcification

  17. Intramembranous Ossification (c’td) • Trabeculae formation • XC matrix centers join to form bridges of trabeculae that constitute spongy bone • blood vessels fill spaces btwn trabeculae • CT develops into red bone marrow • Formation of periosteum • mesenchyme condenses at outer edge & becomes periosteum • thin layer of compact bone covers spongy bone center

  18. Endochondral Ossification • Replacement of cartilage by bone • Forms most bones of the body • Formation of cartilage model • Mesenchymal cells cluster in shape of future bone & develop into chondroblasts • Chondroblasts secrete cartilage ECM  form cartilage model • Perichondrium formation • Growth of cartilage model • Chondrocytes continually divide  growth in length (interstitial) • Addition of ECM from chondroblasts in perichond. growth in thickness (appositional) • Chondrocytes in mid-region hyptertrophy & calcify • Other chondrocytes die  form lacunae

  19. Endochondral Ossification • Development of Primary Ossification Center • proceeds inward from external surface of bone • nutrient artery stimulates differentiation of osteogenic cells in perichondrium into osteoblasts • penetrates perichondrium thru ctr of cartilage model • perichondrium becomes periosteum • @ ctr of model, periosteal capillaries grow & stimulate growth of prim. ossific. ctr • where bone will replace most of cartilage • osteoblasts deposit bone matrix over calcified cartilage • form spongy bone trabeculae

  20. Endochondral Ossification • Development of medullary cavity • Osteoclasts break down some of newly formed spongy bone trabeculae • Resultant cavity in diaphysis becomes medullary (marrow) cavity • Wall of diaphysis eventually replaced by compact bone

  21. Endochondral Ossification • Development of secondary ossification center • Formed @ birth when epiphyseal artery enters epiphyses • Spongy bone remains @ center of epiphyses • No formation of medullary cavity (as in 1° ossification) • Proceeds outward from center of epiphysis to outer surface • Formation of articular cartilage & growth plates • Hyaline cartilage covering epiphyses becomes articular cartilage • Epiphyseal plates responsible for lengthwise growth of long bones • When growth plates “close,” growth stops

  22. Bone Scan • Radioactive tracer is given intravenously • Amount of uptake is related to amount of blood flow to the bone • “Hot spots” are areas of increased metabolic activity that may indicate cancer, abnormal healing or growth • “Cold spots” indicate decreased metabolism of decalcified bone, fracture or bone infection

  23. BONE GROWTH • Two types • Appositional (↑ in thickness) • All bones • Lengthwise • Long bones

  24. Growth in Length • Epiphyseal plate consists of four zones: • Zone of resting cartilage • Nearest epiphysis • Scattered chondrocytes • Do not function in bone growth • Anchor epiphyseal plate to epiphysis • Zone of proliferation cartilage • Chondrocytes divide  replace dying chondrocytes • Zone of hypertrophic cartilage  large chondrocytes

  25. Growth in Length • Zone of calcified cartilage • Dead chondrocytes  ECM has calcified • Osteoclasts dissolve calcified cartilage • Osteoblasts lay down bone ECM • Replace calcified cartilage  become “new diaphysis” • Activity of epiphyseal plate = only means by which diaphysis can increase in length • New chondrocytes formed on epiphys side of plate • Old chondrocytes on diaphys side replaced by bone • Thickness remains constant, growth in length occurs

  26. Bone Growth in Length • Closure of epiphyseal plate = completion of growth in length • Epiphyseal line forms • Damage to epiphyseal plate accelerates closure • Between ages 18 & 25, epiphyseal plates close • Females: about age 18 • Males: about age 21

  27. Growth in Thickness • Appositional growth • Four steps in this process: • Periosteal cells differentiate into osteoblasts • secrete collagen fibers & other substances • osteoblasts become surrounded by ECM  osteocytes • Ridges fuse to form canal for blood vessels • periosteum becomes endosteum • New concentric lamellae formed when osteoblasts in endosteum deposit bone ECM  inward • As new osteon forms, osteoblasts under periosteum form new outer lamellae • Repetition of step 2  growth continues

  28. Bone Remodeling • Ongoing replacement of old bone tissue w/ new bone tissue • Resorption: osteoclasts destroy old bone • Deposition: osteoblasts construct new bone • @ any given time, 5% of total bone mass remodeled • Renewal varies w/ bone type & location within body • Removes injured bone • Triggered by a number of factors • Bone strength related to degree by which it is stressed • Lifting weights strengthens bone  thickens it • Controlled by hormones, vitamins & minerals

  29. Bone Remodeling • Resorption • Osteoclasts attach to endosteum/periosteum & form leak-proof seal around cell edges • Secrete lysosomal enzymes & acids • enzymes digest collagen • acids dissolve bone minerals • Degraded proteins, Ca & P released into interstitial fluid  diffuse into capillaries • Deposition • Osteoclasts depart site of remodeling • Osteoblasts begin new bone deposition

  30. Factors Affecting Bone Growth • Nutrition • Minerals • primarily Ca and P for bone growth • also F, Mg, Fe & Mn in lesser quantities • Vitamins • vitamin C for collagen formation • vitamins K and B12 for protein synthesis • vitamin A activates osteoblasts

  31. Factors Affecting Bone Growth • Sufficient hormones • During childhood • insulinlike growth factor (IGF) most important • promotes cell division at epiphyseal plate • enhance protein synthesis • stimulated by human growth hormone (hGH) • Thyroid hormones (T3 &T4) • Estrogens & androgens • @ puberty: stimulate sudden growth • ↑ osteoblast activity & synthesis of bone ECM  “growth spurt” • modifications of skeleton characteristic of male/female forms • adulthood: contribute to remodeling • Vitamin D, parathyroid hormone (PTH) & calcitonin

  32. Fracture and Repair of Bone • A fracture = any break in bone • 4 steps to fracture repair (Figure 6.10) • Formation of fracture hematoma • Blood leaks from vessels crossing fracture line  clot forms • Nearby bone cells die  swelling & inflammation occur • Phagocytes & osteoclasts remove damaged bone tissue • May last up to several weeks • Fibrocartilaginous callus formation • Mass of repair tissue consisting of collagen fibers & cartilage • Bridges broken ends of bone • Secreted by blast cells in periosteum • Takes about 3 weeks

  33. Fracture and Repair of Bone • Bony callus formation • Osteogenic cells differentiate into osteoblasts • Well-vascularized tissue • Form spongy bone trabeculae • Trabeculae join living & dead parts of bone • Fibrocartilage converted into spongy bone  bony callus • 3-4 months • Bone remodeling • Osteoclasts gradually resorb dead portions of fragments • Compact bone replaces spongy bone @ periphery of fracture • Usually a thickened area remains on bone surface  evidence of break

  34. Fractures • Named for shape or position of fracture line • Common types of fracture • greenstick -- partial fracture • impacted -- one side of fracture driven into the interior of other side

  35. Fractures • Named for shape or position of fracture line • Common types of fracture • closed -- no break in skin • open fracture --skin broken • comminuted -- broken ends of bones are fragmented

  36. Fractures • Named for shape or position of fracture line • Common types of fracture • Pott’s -- distal fibular fracture • Colles’s -- distal radial fracture • stress fracture -- microscopic fissures from repeated strenuous activities

  37. Calcium Homeostasis & Bone Tissue • 99% of total body calcium found in bone • Ca+2 ions required by several body systems • nerve & muscle cell function • blood clotting • enzyme function in many biochemical reactions • Plasma level maintained @ 9-11mg/100mL • Bone helps maintain plasma [Ca+2] • balance rates of deposition & resorption • Small changes in blood levels of Ca+2 can be deadly • cardiac arrest if too high • respiratory arrest if too low

  38. Hormonal Influences • Parathyroid hormone (PTH) is secreted if Ca+2 levels falls • PTH gene turned on  more PTH secreted from gland • osteoclast activity increased • kidney retains Ca+2 • stimulates calcitriol production • Calcitonin secreted from parafollicular cells in thyroid if Ca+2 blood levels get too high • inhibits osteoclast activity • increases bone formation by osteoblasts

  39. Parathyroid hormone acts to RAISE blood Ca+2 levels • Calcitonin works to DECREASE blood Ca+2 levels

  40. EXERCISE AND BONE TISSUE • Ability to alter its strength in response to mechanical stress • increased deposition of mineral salts & production of collagen fibers • removal of mechanical stress leads to weakening of bone through demineralization (loss of bone minerals) and collagen reduction. • reduced activity while in a cast • astronauts in weightless environment • bedridden person • Weight-bearing activities help build & retain bone mass

  41. AGING AND BONE TISSUE • loss of calcium and other minerals from bone matrix (demineralization) • may result in osteoporosis • very rapid in women 40-45 as estrogens levels decrease • in males, begins after age 60 • decreased rate of protein synthesis • decrease in collagen production which gives bone its tensile strength • decrease in growth hormone • bone becomes brittle & susceptible to fracture

  42. Osteoporosis • Decreased bone mass resulting in porous bones • Those at risk • white, thin menopausal, smoking, drinking female with family history • athletes who are not menstruating due to decreased body fat & decreased estrogen levels • people allergic to milk or with eating disorders whose intake of calcium is too low • Prevention or decrease in severity • adequate diet, weight-bearing exercise, & estrogen replacement therapy (for menopausal women) • behavior when young may be most important factor

  43. Disorders of Bone Ossification • Rickets • calcium salts are not deposited properly • bones of growing children are soft • bowed legs, skull, rib cage, and pelvic deformities result • Osteomalacia • new adult bone produced during remodeling fails to ossify • hip fractures are common

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