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ch. 6 pt 3

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  • endochondrral ossification

    -bones form by replacing hyaline cartilage

    form most of skeleton

    osteoblasts lay down bone tissue on the cartilage model where the cartilage from that model is replaced with bone tissue to form a solid bone

    -begins late in month 2 of development

  • central canal

    runs through the core of osteon

    -contains blood vessels and nerve fibers

  • perforating (volkmann's) canals

    canals lined with endosteum that occur at right angles to central canal

    -connects blood vessels and nerves of periosteum, medullary cavity, and central canal

  • lacunae

    small cavities that contain osteocytes

  • canaliculi

    hairlike canals that connect lacunae to eachother and to central canal

    -when matrix hardens and cells are trapped the canaliculi form

    (allow communication between all osteocytes of osteon and permit nutrients and wastes to be relayed from one cell to another)

  • interstitial lamellae

    -not part of the osteon

    -some fill gaps between forming osteons;others are remnants of osteon cut by bone remodeling

  • Circumferential lamellae

    -depp to periosteum, but superficial to endosteum, these layers of lamellae extend around surface of diaphysis

    -help long bone resist twisting

  • spongy bone

    -helps bone resist stress

    -trabeculae confer strength to bone

  • organic components

    osteogenic cells, osteoblasts, osteocytes, bone-lining cells, osteoclasts, and osteoid

  • inorganic components

    hydroxypapitites (mineral salts)

    -make up 65% of bone mass

    -consists mainly of calcium phosphate crystals in and around collagen fibers

    -lasts long after death because of mineral composition

  • ossification (osteogenesis)

    the process of bone tissue formation

    -formation of bony skeleton begins in month 2 of development

    -postnatal bone growth occurs until early adulthood

    -bone remodeling and repair are lifelong

    -up to about week 8, fibrous membranes and hyaline cartilage of fetal skeleton are replaced with bone tissue

  • intramembranous ossification

    -bone develops from fibrous membranes formed by mesenchymal cells

    -bones are called membrane bones

  • 5 main steps of ossification

    1. bone collar forms around diaphysis of cartilage model

    2. central cartilage in diaphysis calcifies, then develops cavities

    3. periosteal bud invades cavities, leading to formation of spongy bone

    4. diaphysis elongates and medullary cavity forms

    5. epiphysis ossify

  • mesenchymal cells

    forms frontal, parietal, occipital, temporal and clavicle bones

  • 4 major steps of intramembranous ossification

    1. ossification centers are formed when mesenchymal cells cluster and become osteoblasts

    2. osteoid is secreted, then calcified

    3. women bone is formed when osteoid is laid down around blood vessels, resulting in trabeculae

    4. lamellar bone replaces woven bone and red marrow appears

  • epiphyseal plate consists of five zones:

    1. resting (quiescent) zone

    2. proliferation(growth) zone

    3. hypertrophic zone

    4. calcification zone

    5. ossification (osteogenic) zone

  • resting zone

    area of cartilage on epiphyseal side of epiphyseal plate that is relatively inactive

  • proliferation zone

    -area of cartilage on diaphysis side of epiphyseal plate that is rapidly dividing

    -new cells formed move upward, pushing epiphysis away from diaphysis, causing lengthening

  • hypertonic zone

    -area with older chondrocytes closer to diaphysis

    -cartilage lacunae enlarge and erode, forming interconnecting spaces

  • calcification zone

    surrounding cartilage matrix calcifies; chondrocytes die and deteriorate

  • ossification zone

    -chondrocyte deterioration leaves long spicules of calcified cartilage at epiphysis-diaphysis junction

    -spicules are then roded by osteoclasts, then covered with new bone by osteoblasts

    -replaced with spngy bone

    -medullary cavity enlarges as spicules erode

  • epiphyseal plate closure occurs when

    epiphysis and diaphysis fuse

  • bone lengthening ceases in

    females around 18

    males around 21

  • growth hormone

    most important hormone in stimulating epiphyseal plate activity in infancy and childhood

  • thyroid hormone

    modulates activity of growth hormone, ensuring proper proportions

  • testosterone (males)

    estrogen (females)

    promote adolescent growth spurts

  • about ____ of bone mass is recycled each week

    5-7%

    -spongy bone replaced- every 3-4 years

    -compact bone replaced- every 10 years

  • bone remodeling

    consists of both bone deposit and bone resorption

    -occurs at surfaces of both periosteum and endosteum

  • remodeling units

    packets of adjacent osteoblasts and osteoclasts coordinate remodeling process

  • resorption

    function of osteoclasts

    -dig depressions or grooves as they break down matrix

    -secrete lysosomal enzymes and protons (H+) that digest matrix

    -acidity converts calcium salts to soluble forms

  • osteoclasts also phagocytize

    demineralized matrix and dead osteocytes

    -digested products are transcytosed across cell and released into interstitial fluid and then into blood

    -once resorption is complete, osteoclasts undergo apoptosis

  • osteoclast activation involves

    PTH (parathyroid hormone) and immune T cells

  • osteoid stem

    band of unmineralized bone matrix that marks area of new matrix

  • calcification front

    abrupt transition zone between osteoid seam and older mineralized bone

  • hormonal controls

    -negative feedback loop that control blood calcium levels

    -calcium functions: nerve transmission, muscle contraction, blood coagulation, gland and nerve secretions, cell division

    -99% of calcium found in bone

    intestinal absorption requires vitamin D

  • parathyroid hormone (PTH)

    produced by parathyroid glands in response to low blood calium levels

    -stimulates osteoclasts to resorb bone

    -calcium is released into blood

    -PTH secretion stops when homeostatic calcium levels are reached

  • Calcitonin

    produced by parafollicular cells of thyroid gland in response to high levels of blood calcium levels

    -effects are negligible; but at high pharmacological doses it can lower blood calcium levels temporarily

  • leptin

    -hormone released by adipose tissue

    -may play role in bone density regulation by inhibiting osteoblasts

  • serotonin

    -neurotransmitter regulates mood and sleep; also interferes with osteoblast activity

    -made in guy

    -secreted in blood after meal

    -may inhibit bone turnover after a meal, so bone calcium is locked in when new calcium is flooding into bloodstream