skeleton of the human

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    Skeleton

    Skeletal System Composit ion

    1. Bones Strong connective tissue with mineralized matrix of

    calcium phosphate and calcium carbonate.

    2. Carti lage Flexible connective tissue made of collagen and

    elastin fibers that cover joint surfaces of bones.

    3. Ligaments Regular fibrous tissue that holds bones together

    at the joints.

    4.

    Tendons Dense fibrous tissue that attach bone to muscle .

    Skeletal System Division

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    Functions of the Skeleton

    1. Support Framework that supports the body.

    2.

    Protection Skull protects the brain, vertebrae surround thespinal cord and rib cage protects vital organs of the thorax.

    3. Movement Act as levers with skeletal muscles to move

    body.

    4.

    Mineral Storage Calcium and phosphate ions are released

    into the bloodstream as needed for distribution to body.

    5.

    Blood pH Regulator Absorbs bicarbonate ions.

    6.

    Blood Formation Red bone marrow produces blood cells.

    Shapes and Bone Features

    1. Long Bones Greater in length and width, and includes

    bones of the arms and legs. E.g. Humerus and Femur.

    2.

    Short Bones More equal in length and width, and includecarpal (wrist) bones.

    3. Flat Bones Flattened and include bones of the skull and

    sternum. E.g. Parietal bone of roof skull.

    4. I rregular Bones Complex shape and include pelvis and

    spinal vertebrae.

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    Bone Cells

    1. Osteoprogenitor Cells

    !Mitotic stem cells found in the membranousperiosteum and endosteum .

    ! Differentiate into osteoblasts (bone-forming cells).

    2.

    Osteoblasts

    ! Form and help mineralize the organic matter of the bony

    matrix .

    !

    Non-mitotic.

    3. Osteocytes

    ! Matured version of osteoblasts when they have become

    trapped in the matrix they formed.

    ! Reside in lacunae (pocket) and are connected to

    osteocytes by gap junctions.

    ! They do not continually form bone.

    ! Important: Signal osteoclasts (bone-destroying cells)

    and osteoblasts about mechanical stresses on the bone.

    4. Osteoclasts

    ! Multinucleated cells that break down the calcified matrix.

    ! Secrete acids that dissolve the inorganic component of

    the calcified matrix.

    ! Release proteolyt ic enzymes that digest the organic

    components.

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    Bone Matrix

    1. Organic Matter ~1/3)

    !Collagen providing flexibility.

    ! Glycosaminoglycans, proteoglycans and glycoproteins.

    2. Inorganic Matter 2/3)

    ! Mostly hydroxyapatite crystallized calcium

    phosphate.

    ! The remainder is carbonate and other minerals (F-,

    Mg2+and K+).

    Compact Bone

    ! Very dense and has repeating shape.

    ! The osteon is the basic structural unit of compact bone.

    !

    Osteon comprised of cylinders of tissue formed from lamellae(layers) of matrix arranged around central canal holding a

    blood vessel.

    ! Lacunae containing osteocytes are located in between

    lamellae.

    Spongy Bone

    !Provides strength with little weight and trabeculae developalong stress lines of bones.

    ! Bone cells lie within the trabeculae and get nutrients from

    substance diffusing into the canaliculi .

    ! Trabeculae contain irregularly arranged lamellae and osteocytes

    interconnected by canaliculi.

    ! High vascular and contains red bone marrow for the production

    of blood cells.

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    Periosteum

    ! Fibrous white membrane layer covering bones.

    !Contains osteoblasts and osteoclasts that participate in bonegrowth and repair.

    ! Has two layers:

    1.

    Outer Fibrous Layer contains fibroblasts.

    2.

    Inner Osteogenic Layer contains osteoprogenitor

    cells that developed into osteoblasts.

    Endosteum

    ! Delicate connective tissue membrane that covers internal bone

    surfaces.

    ! Covers the trabeculae of spongy bone.

    ! Lines the central canals that pass through the compact bone.

    ! Have osteoblasts and osteoclasts.

    Bone General Features

    ! Compact dense) bone outer shell of long bone.

    ! Diaphysis elongated shaft of long bone providing leverage.

    ! Medullary cavity space in diaphysis that contains yellow

    bone marrow.

    ! Epiphysis enlarged ends of a long bone to strengthen joints.

    ! Epiphyseal plate cartilage that separates the marrow

    spaces of epiphysis and diaphysis.

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    Apposit ional Bone Growth

    ! Increase bone diameter of existing bones.

    !Osteoprogenitor cells in periosteum differentiate intoosteoblasts to add more bone matrix to outer surface of the

    bone.

    ! Osteoclasts remove matrix at inner surface to enlarge medullary

    cavity.

    Bone Marrow

    !

    Soft tissue that occupies the marrow cavity of a long bone and

    small spaces betweentrabeculae of spongy bone.

    ! Types:

    1. Red Marrow

    ! Mesh of network fibers and stem cells.

    !

    Hemopoietic t issue that gives rise to RBCs,WBCs and platelets.

    ! Found in the vertebrae, ribs, sternum (breastbone)

    and proximal heads of the femur and humerus.

    2. Yellow Marrow

    ! Fatty substance that fills the shafts of most long

    bones in adults.

    ! Non-hemopoietic and is an energy reservoir.

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    Calcium Homeostasis

    1. Parathyroid Low Ca2+Levels)

    !PTH secreted when calcium blood levels are low .

    ! Stimulates osteoclast production and activity.

    ! Promotes calcium reabsorption by the kidneys.

    ! Promotes calcitriol synthesis in the kidneys.

    2.

    Calcitonin High Ca2+Levels)

    !

    Secreted by C cells of the thyroid gland when calcium

    blood levels are high.

    ! Inhibits osteoclast activity, hence bone resorption.

    ! Stimulates Ca2+uptake and incorporation into bone

    matrix.

    3. Calcitr iol Low Ca2+Levels)

    ! Processes by the liver and kidney.

    ! Enters the blood to stimulate the intestine to absorb

    calcium and phosphate.

    ! Promotes osteoclast activity, raising blood Ca2+levels.

    Ossif ication formation of new bones.

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    Intramembranous Ossif ication

    ! Produces f lat bones of the skull and clavicle (collarbone).

    !Fibrous connective tissue membranes formed bymesenchymal cells .

    ! Mesenchymal (stems) cells differentiate into osteoblasts.

    4 Major Steps of Intramembranous Ossif ication

    1.

    Ossif ication center appears in f ibrous connective

    tissue membrane

    -

    Centrally located mesenchymal cells cluster and

    differentiate into osteoblasts, forming the center.

    2. Bone matrix osteoid) is secreted with the f ibrous

    membrane

    - Osteoblasts begin to secrete osteoid, being mineralized.

    -

    Trapped osteoblasts become osteocytes.

    3. Periosteum Form

    - Remaining mesenchyme at the surface gives rise to

    periosteum.

    4. Center remodeling

    -

    Osteoclasts remodel the center of the flat bone to containmarrow spaces.

    -

    Osteoblasts around blood vessels remodel the surface to

    form compact bone.

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    Endochondral Ossif ication

    ! Produces most bones of the body, especially below the skull.

    !Chondrocytes produces cartilage composed of collagen,elastin and proteoglycans.

    ! Uses hyaline cartilage as a model for bone construction.

    ! Bones grow in diameter and length, involving appositional bone

    deposition.

    Steps of Intramembranous Ossif ication

    1.

    In shaft, chondrocytes enlarge and matrix ossifies (turn into

    bone). Chondrocytes then die leaving cavities within cartilage.

    2. Blood vessels grow around cartilage edge. Osteoblasts then

    form a superficial layer of bone.

    3. Perichondrium around the cartilage stops producing

    chondrocytes and beings producing osteoblasts.

    4.

    Blood vessels invade the central region, enabling fibroblasts todifferentiate into osteoblasts.

    5. Osteoblasts begin to form spongy bone in spaces previously

    housing the cartilage.

    6. Medullary cavity created by cartilage replaced by osseous

    tissue.

    7.

    Secondary ossification centers form as capillaries andosteoblasts migrate into epiphyses.

    8.

    Epiphyses fill with spongy bone.

    9.

    As spongy bone is deposited in the diaphysis and epiphysis, a

    band of cartilage epiphyseal plate remains between the two

    ossification centers.

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    Healing Of Fractures

    1. Hematoma is formed and is converted to granulation tissue,

    formed by fibroblasts with high concentration of capillaries.

    2. A callus forms and begins as a soft callus of fibrocartilage that

    is eventually replaced by hard callus of bone.

    3. Bone remodeling occurs where small bone fragments are

    removed by osteoclasts.

    4.

    Osteoblasts then deposit spongy bone and then convert it to

    compact bone.