BONES%20AND%20SKELETAL%20TISSUES

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BONESANDSKELETAL TISSUES
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SKELETAL CARTILAGES

• Skeletal cartilages
• Made of some variety of cartilage
• Consists primarily of water
• High water content accounts for its resilience
• Ability to spring back to its original shape
  after being compressed
• Contains no nerves or blood vessels
• Surrounded by a layer of dense irregular
  connective tissue called the perichondrium
• Acts like a girdle to resist outward expansion
  when the cartilage is compressed
• Contains the blood vessels from which nutrients
  diffuse through the matrix to reach the cartilage
  cells
• This mode of nutrient delivery limits cartilage
  thickness

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SKELETAL CARTILAGES

• Three types
• Hyaline
• Elastic
• Fibrocartilage
• All three types have the same basic components
• Cells called chondrocytes
• Encased in small cavities (lacunae) within an
  extracellular matrix containing a jellylike
  ground substance and fibers

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HYALINE CARTILAGE

• Looks like a frosted glass when freshly exposed
• Provides support with flexibility and resilience
• Most abundant skeletal cartilage
• Chondrocytes appear spherical
• Only fiber type in the matrix is fine collagen
  fibers

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HYALINE CARTILAGE
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HYALINE CARTILAGE

• Includes
• Articular cartilages
• Cover the ends of most bones at movable joints
• Costal cartilages
• Connect the ribs to the sternum (breastbone)
• Respiratory cartilages
• Form the skeleton of the larynx (voicebox)
• Reinforces other respiratory passageways
• Nasal cartilages
• Support the external nose

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BONE CARTILAGE
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ELASTIC CARTILAGE

• Looks very much like hyaline cartilages, but they
  contain more stretchy elastic fibers and so are
  better able to stand up to repeated bending

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ELASTIC CARTILAGE
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ELASTIC CARTILAGE

• More flexible than hyaline
• Located only in the
• External ear
• Epiglottis of the larynx
• Flap that bends to cover the opening of the
  larynx each time we swallow

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BONE CARTILAGE
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FIBROCARTILAGE

• Highly compressible and have great tensile
  strength
• Perfect intermediate between hyaline and elastic
  cartilages
• Consist of roughly parallel rows of chondrocytes
  alternating with thick collagen fibers

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FIBROCARTILAGE
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FIBROCARTILAGE

• Occur in sites that are subjected to both heavy
  pressure and stretch
• Padlike cartilages (menisci) of the knee
• Discs between vertebrae

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BONE CARTILAGE
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Growth of Cartilage

• Growth occurs in two ways
• Appositional growth (growth from the outside)
  results in outward expansion due to the
  production of cartilage matrix on the outside of
  the tissue
• Cartilage-forming cells in the surrounding
  perichondrium secrete new matrix against the
  external face of the existing cartilage tissue
• Interstitial growth (growth from inside) results
  in expansion from within the cartilage matrix due
  to division of lacunae-bound chondrocytes and
  secretion of matrix
• Lacunae-bound chondrocytes divide and secrete new
  matrix, expanding the cartilage from within

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Growth of Cartilage

• Typically, cartilage growth ends during
  adolescence when the skeleton stops growing
• Under certain conditionsduring normal bone
  growth in youth and during old agecalcium salts
  may be deposited in the matrix and cause it to
  harden, a process called calcification
• NOTE calcified cartilage is NOT bone

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CLASSIFICATION OF BONES

• 206 named bones of the human skeleton are divided
  into two groups
• Axial skeleton
• Forms the long axis of the body
• Protect, support, or carry other body parts
• Includes
• Skull
• Vertebral column
• Rib cage
• Appendicular skeleton
• Bones of the upper and lower limbs, and the
  girdles (pectoral/shoulder and pelvic/hip) that
  attach them to the axial skeleton
• Help us to get from place to place (location)

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BONE CARTILAGE
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CLASSIFICATION OF BONES

• Classified by shape
• Long
• Short
• Flat
• Irregular

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BONE SHAPE
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Long Bones

• Longer than they are wide
• Have a definite shaft and two ends
• Consist of all limb bones except patellas, the
  carpels (wrist), and tarsals (ankles)
• Named for their elongated shape, NOT their
  overall size
• Example the three bones in your fingers (digits)
  are long bones, even though they are very small

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BONE SHAPE
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Short Bones

• Somewhat cube-shaped
• Include
• Carpals wrist
• Tarsals ankle
• Sesamoid bone
• Shaped like a sesame seed
• Special type of bone that forms in a tendon
• Example patella
• Vary in size
• Some clearly act to alter direction of pull of a
  tendon
• Functions of others is not known

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BONE SHAPE
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Flat Bone

• Thin, flattened and often curved
• Include
• Most skull bones
• Sternum (breastbone)
• Scapulae (shoulder blades)
• Ribs

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BONE SHAPE
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Irregular Bones

• Complicated shapes that do not fit in any of the
  previous classes
• Example
• Vertebrae
• Coxae (hip bone)

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BONE SHAPE
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FUNCTIONS OF BONES

• Besides contributing to body shape and form, our
  bones perform several important functions
• 1. Support
• 2. Protection
• 3. Movement
• 4. Mineral storage
• 5. Blood cell formation

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Support

• Bones provide a framework that supports the body
  and cradles its soft organs
• Examples
• Bones of the lower limbs act as pillars to
  support the body trunk
• Rib cage supports the thoracic wall

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Protection

• Fused bones at the skull protect the brain
• Vertebrae surround the spinal cord
• Rib cage helps protect the vital organs of the
  thorax

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Movement

• Skeletal muscles, which attach to bones by
  tendons, use bones as levers to move the body and
  its parts
• As a result, we can walk, grasp objects, and
  breathe

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Mineral Storage

• Bone is a reservoir for minerals
• Most important are calcium and phosphates
• Stored minerals are released into the bloodstream
  as needed for distribution to all parts of the
  body
• Deposits and withdrawals of minerals to and from
  the bones go on almost continuously

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Blood Cell Formation

• Most blood cell formation (hematopoiesis) occurs
  in the marrow cavities of certain bones

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Bone Structure

• Because bones contain various types of tissue,
  bones are organs
• Bone (osseous) tissue
• Nervous tissue in their nerves
• Cartilage tissue in their articular cartilages
• Fibrous connective tissue lining their cavities
• Muscle and epithelial tissues in their blood
  vessels

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Gross Anatomy

• Bone markings are projections, depressions, and
  openings found on the surface of bones that
  function as sites of muscle, ligament, and tendon
  attachment, as joint surfaces, and as openings
  for the passage of blood vessels and nerves

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Names of Bone Markings

• Projections (bulges) grow outward from the bone
  surface
• Projections that sites of muscle and ligament
  attachment
• Tuberosity elevated round swelling
• Large and rounded
• May be roughened
• Crest elongated prominence
• Narrow ridge
• Usually prominent
• Trochanter to run
• Very large
• Blunt
• Irregularly shaped
• ONLY in the femur
• Line
• Narrow ridge
• Less prominent than crest

• Tubercle little swelling
• Small and rounded
• Epicondyle above a knuckle (condyle)
• Raised area on or above a condyle
• Spine
• Sharp, slender, often pointed projection
• Process
• Any bone prominence

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Names of Bone Markings

• Projections That Help to Form Joints
  articulation
• Head
• Bony expansion carried on a narrow neck
• Facet small face
• Smooth, nearly flat articular surface
• Condyle knuckle
• Rounded articular projection
• Ramus branch
• Armlike bar of bone

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Names of Bone Markings

• Depressions and openings
• Allow blood vessels and nerves to pass
• Meatus passage/opening
• Canal-like passageway
• Sinus curve, hollow
• Cavity within a bone, filled with air and lined
  with mucous membrane
• Fossa furrow or shallow depression
• Shallow, basinlike depression in a bone
• Often serves as an articular surface
• Groove ditch
• Furrow
• Fissure slender deep furrow
• Narrow, slitlike opening
• Foramen passage/opening
• Round or oval opening through a bone

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Bone Textures

• Compact
• All bones have a dense outer layer consisting of
  compact bone that appears smooth and solid
• Spongy Bone
• Internal to compact bone is spongy bone, which
  consists of honeycomb, needle-like, or flat
  pieces, called trabeculae (little beam)
• In living bones the open spaces between
  trabeculae are filled with red or yellow bone
  marrow

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Compact/Spongy Bone
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Typical Long Bone Structure

• All long bones have the same general structure
• Diaphysis dia (through) / physis (growth)
• Tubular shaft
• Forms the long axis of the bone
• Constructed of a relatively thick collar of
  compact bone that surrounds a central medullary
  cavity or marrow cavity
• In adults, the medullary cavity contains fat
  (yellow marrow) and is called the yellow bone
  marrow
• Epiphyses epi (upon) / epiphyses (singular)
• The ends of the bone
• Consist of internal spongy bone covered by an
  outer layer of compact bone
• Joint surfaces of each epiphysis is covered with
  a thin layer of articular (hyaline) cartilage,
  which cushions the opposing bone ends during
  joint movement and absorbs stress

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LONGBONE
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Typical Long Bone Structure

• Epiphyseal line sometimes called the metaphysis
• Located between the epiphyses and diaphysis in an
  adult
• Is the remnant of the epiphyseal plate, a disc of
  hyaline cartilage that grows during childhood to
  lengthen the bone

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LONGBONE
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Typical Long Bone Structure

• Membranes Periosteum
• The external surface of the entire bone except
  the joint surfaces is covered by a glistening
  white, double-layered membrane called the
  periosteum (periaround / osteobone)
• Outer fibrous layer is dense irregular connective
  tissue
• Inner osteogenic layer abutting the bone surface
  consists primarily of
• Bone-forming cells osteoblasts
• Bone-destroying cells osteoclasts

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LONGBONE
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Typical Long Bone Structure

• Membranes Periosteum
• Richly supplied with nerve fibers, lymphatic
  vessels, and blood vessels, which enter the
  diaphysis via a nutrient foramen
• Secured to the underlying bone by perforating
  (Sharpeys) fibers
• Tufts of collagen fibers that extend from its
  fibrous layer into the bone matrix
• Provides anchoring points for tendons and
  ligaments
• At these points the perforating fibers are
  exceptionally dense

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LONGBONE
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Typical Long Bone Structure

• Membranes Endosteum (within bone)
• The internal surface of the bone is lined by a
  connective tissue membrane called the endosteum
• Covers the trabeculae of spongy bone and lines
  the canals that pass through the compact bone
• Like the periosteum, the endosteum contains both
  osteoblasts and osteoclasts

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LONGBONE
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Structure of Short, Flat, and Irregular Bones

• Short, flat, and irregular bones consist of thin
  plates of periosteum-covered compact bone on the
  outside, and endosteum-covered spongy bone
  inside, which houses bone marrow between the
  trabeculae (no marrow cavity is present)
• Not cylindrical
• No shaft or epiphyses
• Called the diploe (folded)
• Arrangement resembles a sandwich

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FLATBONE
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Location of Hematopoietic Tissue in Bones

• Hematopoietic tissue of bones, red bone marrow,
  is located within
• The trabecular cavities of the spongy bone in
  flat bones
• The trabecular cavities of the spongy bone of the
  epiphyses in the long bones
• Red bone marrow is found in
• All flat bones
• Epiphyses, and medullary cavities of infants
• In adults, distribution is restricted to flat
  bones and the proximal epiphyses of the humerus
  and femur
• Hence, blood cell production in adult long bones
  routinely occurs only in the head of the femur
  and humerus
• Red marrow found in the diploe of flat bones
  (such as the sternum) and in some irregular bones
  (such as the hip bones) is much more active in
  hematopoiesis
• These are the sites used for obtaining red marrow
  samples
• Yellow marrow in the medullary cavity can revert
  to red marrow if a person becomes very anemic and
  needs enhanced red blood cell production

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Microscope Anatomy of Bone

• Although compact bone looks dense and solid, a
  microscope reveals that it is riddled with
  passageways that serve as conduits for nerves,
  blood vessels, and lymphatic vessels

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COMPACT BONE
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Microscope Anatomy of BoneCompact Bone

• The structural unit of compact bone is the
  osteon, or Haversian system
• Each osteon is an elongated cylinder oriented
  parallel to the long axis of the bone
• Tiny weight bearing pillars
• Group of hollow tubes of bone matrix, one placed
  outside the next like the growth rings of a tree
  trunk
• In diagram osteon are drawn as if pulled out
  like a telescope to illustrate the individual
  lamellae
• Each matrix tube is a lamella (little plate), and
  for this reason compact bone is often called
  lamellar bone
• Although all of the collagen fibers in a
  particular lamella run in a single direction, the
  collagen fibers in adjacent lamella always run in
  opposite directions
• This alternating pattern is beautifully designed
  to withstand torsion (twisting) stressesthe
  adjacent lamella reinforce one another to resist
  twisting

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OSTEON
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Microscope Anatomy of BoneCompact Bone

• Collagen fibers are not the only part of bone
  lamellae that are beautifully ordered
• Tiny crystals of bone salts align with the
  collagen fibers and thus also alternate their
  direction in adjacent lamellae
• Running through the core of each osteon is
• The Central (Haversian) Canal that containing
  small blood vessels and nerve fibers that serve
  the needs of the osteons cells
• Perforating (Volkmanns) Canals lie at right
  angles to the long axis of the bone, and connect
  the blood and nerve supply of the periosteum to
  that of the central canals and medullary cavity
• BOTH Haversian and Volkmann Canal are lined with
  endosteum

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COMPACT BONE
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Microscope Anatomy of BoneCompact Bone

• (b)Osteocytes (spider-shaped mature bone cells)
  occupy lacunae (small space, cavity, or
  depression occupied by cells) at the junctions of
  the lamellae
• Hair-like canals called canaliculi connect the
  lacunae to each other and to the central canal
• Tie all the osteocytes in an osteon together,
  permitting nutrients and wastes to be relayed
  from one osteocyte to the next throughout the
  osteon
• Although bone matrix is hard and impermeable to
  nutrients, its canaliculi and cell-to-cell relays
  (via gap junctions) allow bone cells to be well
  nourished
• Function is to maintain the bone matrix
• If they die, the surrounding matrix is resorbed
  (remove-assimilated)

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COMPACT BONE
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Microscope Anatomy of BoneCompact Bone

• Not all the lamellae in compact bone are part of
  osteons
• (c) Lying between intact osteons are incomplete
  lamella called interstitial lamella
• These either fill the gaps between forming
  osteons or are remnants of osteons that have been
  cut through by bone remodeling
• (a) Circumferential lamellae are located just
  beneath the periosteum, extending around the
  entire circumference of the bone
• Effectively resist twisting of the long bone

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COMPACT BONE
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Spongy Bone

• Lacks osteons
• Trabeculae (honeycomb network) align along lines
  of stress and help the bone resist stress as much
  as possible
• These tiny bone struts are as carefully
  positioned as the flying buttresses of a Gothic
  cathedral
• Irregularly arranged lamella and osteocytes
  interconnected by canaliculi
• Nutrients reach the osteocytes by diffusing
  through the canaliculi from capillaries in the
  endosteum surrounding the trabeculae

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LONGBONE
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FLATBONE
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COMPACT BONE
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Chemical Composition of Bone

• Organic components
• Cells (osteoblasts, osteocytes, and osteoclasts)
• Osteoid nonliving
• Composed of secretions from the osteoblasts which
  contribute to the flexibility and tensile
  strength of bone that allows the bone to resist
  stretch and twisting
• Ground substance proteoglycans and
  glycoproteins
• Collagen fibers
• Bonds in or between collagen molecules break
  easily on impact dissipating energy to prevent
  the force from rising to a fracture value
• In the absence of continued or additional trauma,
  most of the bonds reform

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Chemical Composition of Bone

• Inorganic components
• Make up 65 of bone by mass
• Consist of hydroxyapatite (mineral salts) that is
  largely calcium phosphate, which accounts for the
  hardness and compression resistance of bone
• Present in the form of tightly packed tiny
  crystals surrounding the collagen fibers in the
  extracellular matrix
• Because of the salts they contain, bones last
  long after death and provide an enduring
  monument
• Healthy bone is half as strong as steel in
  resisting compression and fully as strong as
  steel in resisting tension (stretching)

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BONE DEVELOPMENT

• Ossification and osteogenesis are synonyms
  meaning the process of bone formation (osbone /
  genesisbeginning)
• In embryos leads to the formation of the
  skeleton
• Early adulthood bones increase in length
• Throughout life bones are capable of growing in
  thickness
• Adults ossification serves mainly for bone
  remodeling and repair

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Formation of the Bony Skeleton

• Before week 8, the skeleton of a human embryo is
  constructed entirely from fibrous membranes and
  hyaline cartilage
• Bone tissue begins to develop at about this time
  and eventually replaces most of the existing
  fibrous or cartilage structures
• When a bone develops from a fibrous membrane, the
  process is intramembranous ossification, and the
  bone is called a membrane bone
• Bone development by replacing hyaline cartilage
  is called endochondral ossification (endowithin
  / chondocartilage), and the resulting bone is
  called a cartilage (endochondral) bone

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Intramembranous Ossification

• Results in the formation of cranial bones of the
  skull (frontal, parietal, occipital, and temporal
  bones) and the clavicles
• All bones formed by this process are flat bones
• Four Major Steps 1, 2, 3, 4

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Intramembranous Ossification
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Intramembranous Ossification
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Endochondral Ossification

• Replaces hyaline cartilage, forming all bones
  below the skull except for the clavicles
• Begins in the second month of development
• Five Steps 1,2,3,4,5

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Endochondral Ossification

• 1. Initially, osteoblasts secrete osteoid,
  creating a bone collar around the diaphysis of
  the hyaline cartilage model

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Endochondral Ossification
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Endochondral Ossification

• 2. Cartilage in the center of the diaphysis
  calcifies
• Because calcified cartilage matrix is impermeable
  to diffusing nutrients, the chondrocytes die and
  deteriorate forming cavities

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Endochondral Ossification
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Endochondral Ossification

• 3. The periosteal bud (nutrient artery and vein,
  lymphatics, nerve fibers, red marrow elements,
  osteoblast, and osteoclasts) invades the internal
  cavities and spongy bone forms around the
  remaining fragments of hyaline cartilage

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Endochondral Ossification
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Endochondral Ossification

• 4. The diaphysis elongates as the cartilage in
  the epiphyses continue to lengthen and a
  medullary cavity forms through the action of
  osteoclasts within the center of the diaphysis

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Endochondral Ossification
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Endochondral Ossification

• 5. The epiphyses ossify shortly after birth
  through the development of secondary ossification
  centers
• When complete, hyaline cartilage remains only at
  two places
• On the epiphyseal surfaces (articular cartilages)
• Junction of the diaphysis and epiphysis, where it
  forms the epiphyseal plates

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Endochondral Ossification
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Postnatal Bone Growth

• During infancy and youth
• Long bones lengthen entirely by interstitial
  growth of the epiphyseal plates
• All bones grow in thickness by appositional growth

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Growth in Length of Long Bones

• Side of the epiphyseal plate cartilage facing the
  epiphysis, the cartilage is relatively quiescent
  and inactive
• Side of the epiphyseal plate cartilage abutting
  the diaphysis organizes into a pattern that
  allows fast, efficient growth (osteogenic zone)
• As the cells divide the epiphysis is pushed away
  from the diaphysis
• Long bone lengthens

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LENGTH GROWTH
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Bone Growth
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Growth in Length of Long Bones

• During growth, the epiphyseal plate maintains a
  constant thickness because the rate of cartilage
  growth on its epiphyseal-facing side is balanced
  by its replacement with bony tissue on its
  diaphysis-facing side

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Bone Growth
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Growth in Length of Long Bones

• As adolescence draws to an end, the chondroblasts
  of the epiphyseal plates divide less often and
  the plates become thinner and thinner until they
  are entirely replaced by bone tissue
• Longitudinal bone growth ends when the bone of
  the epiphysis and diaphysis fuses
• This process, called epiphyseal plate closure,
  happens at about 18 years of age in females and
  21 years of age in males
• However, an adult bone can still increase in
  diameter or thickness by appositional growth if
  stressed by excessive activity or body weight

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Growth in Width (Thickness)

• Growing bones widen as they lengthen
• Increases in thickness by appositional growth

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APPOSITIONAL GROWTH
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Growth in Width (Thickness)

• Osteoblast beneath the periosteum secrete bone
  matrix on the external bone surface
• Osteoclasts on the endosteal surface of the
  diaphysis remove bone
• There is normally slightly less breaking down
  than building up
• This unequal process produces a thicker, stronger
  bone but prevents it from becoming too heavy

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Appositional Growth
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Hormonal Regulation of Bone Growth

• During infancy and childhood, the most important
  stimulus of epiphyseal plate activity is growth
  hormone from the anterior pituitary, whose
  effects are modulated by thyroid hormone,
  ensuring that the skeleton has proper proportions
  as it grows
• At puberty, male and female sex hormones
  (testosterone and estrogen) are released in
  increasing amounts
• Initially these sex hormones promote the growth
  spurt typical of adolescence, as well as the
  masculinization or feminization of specific parts
  of the skeleton
• Ultimately these hormones induct the closure of
  the epiphyseal plate ending longitudinal bone
  growth

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BONE HOMEOSTASIS

• Every week we recycle 5 to 7 of our bone mass,
  and as much as half a gram of calcium may enter
  or leave the adult skeleton each day
• Spongy bone is replaced every 3-4 years
• Compact bone, is replaced approximately every 10
  years
• This is fortunate because when bone remains in
  place for long periods the calcium crystallizes
  and becomes very brittleripe conditions for
  fracture
• When we break bones (most common disorder of
  bones), they undergo a remarkable process of
  self-repair

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Bone Remodeling

• In the adult skeleton, bone deposit and bone
  resorption (removal) occur BOTH at the surface of
  the periosteum and the surface of the endosteum
• These two processes constitute bone remodeling
• They are coupled and coordinated by remodeling
  units (osteoblasts and osteoclasts)
• Osteoblast bone forming cells
• Osteoclast large cells that resorb or break down
  bone matrix
• In adult skeletons, bone remodeling is balanced
  bone deposit and removal, bone deposit occurs at
  a greater rate when bone is injured, and bone
  resorption allows minerals of degraded bone
  matrix to move into the blood

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Bone Remodeling

• Bone deposit involves osteoblasts
• Occurs wherever bone is injured or added bone
  strength is required
• Optimal bone deposit requires
• Healthy diet rich in proteins
• Vitamin C
• Vitamin D
• Vitamin A
• Minerals calcium, phosphorus, magnesium, and
  manganese

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Bone Remodeling

• Bone Resorption accomplished by osteoclasts
• Move along a bone surface, digging grooves called
  resorption bays as they break down the bone
  matrix
• Secretes
• Lysosomal enzymes that digest the organic matrix
• Hydrochloric acid that converts the calcium salts
  into soluble forms that pass easily into solution
• May also phagocytize the demineralized matrix and
  dead osteocytes

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Control of Remolding

• Regulated by two control loops
• A negative feedback hormonal mechanism that
  maintains Ca2 homeostasis in the blood
• Calcium is important in many physiological
  processes
• Nerve impulses
• Muscle contraction
• Blood coagulation
• Secretion by glands, nerve cells
• Cell division
• Responses to mechanical and gravitational forces
  acting on the skeleton
• Daily calcium requirement is
• 400-800 mg from birth until the age of 10
• 1200-1500 mg from ages 11 to 24

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Hormonal Mechanism

• Mostly used to maintain blood calcium
  homeostasis, and balances activity of parathyroid
  hormone (PTH) and calcitonin (thyroid)

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Hormonal Mechanism

• Increased parathyroid hormone (PTH) level
  stimulates osteoclasts to resorb bone, releasing
  calcium to the blood
• Osteoclasts are no respectors of matrix age
• They break down both old and new matrix
• ONLY osteoid (unmineralized matrix), which lacks
  calcium salts, escapes digestion
• As blood concentrations of calcium rise, the
  stimulus for PTH release ends

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HORMONE CONTROL
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Hormonal Mechanism

• Calcitonin (Thyroid)
• Secreted when blood calcium levels rise
• Inhibits bone resorption
• Encourages calcium salt deposit in bone matrix,
  effectively reducing blood calcium levels
• As blood calcium levels fall, calcitonin release
  wanes

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HORMONE CONTROL
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REMODELING
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Hormonal Mechanism

• These hormonal controls act not to preserve the
  skeletons strength or well-being but rather to
  maintain blood calcium homeostasis
• In fact, if blood calcium levels are low for an
  extended time, the bones become so demineralized
  that they develop large, punched-out-looking
  holes
• Thus, the bones serve as a storehouse from which
  ionic calcium is drawn as needed

112
Response to Mechanical Stress and Gravity

• Wolffs Law Response to mechanical stress
  (muscle pull) and gravity serves the needs of the
  skeleton by keeping the bones strong where
  stressors are acting
• A bones anatomy reflects the common stresses it
  encounters
• Example a bone is loaded (stressed) whenever
  weight bears down on it or muscles pull on it
• Tends to bend the bone
• Compresses the bone on one side and subjects it
  to tension (stretching) on the other side
• Both forces are minimal toward the center of the
  bone (cancel each other out)

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BONE STRESS
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Wolffs Law

• 1. Long bones are thickest midway along the
  diaphysis, exactly where bending stresses are
  greatest (bend a stick and it will split near the
  middle
• 2. Curved bones are thickest where they are most
  likely to buckle
• 3. Trabeculae of spongy bone form trusses, or
  struts, along lines of compression
• 4. Large, bony projections occur where heavy,
  active muscles attach
• Bones of weight lifters have enormous thickenings
  at the attachment sites of the most used muscles
• Also explains the featureless bones of the fetus
  and the atrophied bones of bedridden
  peoplesituations in which bones are not stressed

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Control of Remolding

• Skeleton is continuously subjected to both
  hormonal influences and mechanical forces
• The hormonal loop determines whether and when
  remodeling occurs in response to changing blood
  calcium levels
• Mechanical stress determines where it occurs
• Example
• When bone must be broken down to increase blood
  calcium levels, PTH is released and targets the
  osteoclasts
• Mechanical forces determine which osteoclasts are
  most sensitive to PTH stimulation, so that bone
  in the least stressed areas (temporarily
  dispensable) is broken down

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Bone Repair

• Fractures are breaks in bones
• Due to trauma to bones or thin, weaken bones

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Classification of Fracture

• Position of the bone ends after fracture
• Nondisplaced bone ends retain their normal
  position
• Displaced bone ends are out of normal alignment
• Completeness of break
• Complete bone is broken through
• Incomplete bone is not broken through
• Greenstick bone breaks incompletely (like green
  twig breaks)
• Only one side of the shaft breaks the other side
  bends
• Orientation of the break relative to the long
  axis of the bone
• Linear parallel fracture
• Transverse break is perpendicular to the bones
  long axis

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Classification of Fracture

• Whether the bone ends penetrate the skin
• Open (compound) penetrates the skin
• Closed (simple) does not penetrate the skin
• Location
• Arm, leg, etc.
• Epiphyseal epiphysis separates from the
  diaphysis along the epiphyseal plate
• Depressed skull bones pushed in
• External appearance
• Nature of break
• Comminuted bone fractures into 3 or more pieces
• Spiral angular

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Bone Repair

• Repair of fractures involves four major stages
• 1. Hematoma formation mass of clotted blood
• Because blood vessels are damaged
• Bone cells deprived of nutrients die at the site
• Tissue at the site become swollen, painful, and
  inflamed

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BONE HEALING
121
Bone Repair

• 2. Fibrocartilaginous callus formation
• Formation of soft granulation tissue (soft
  callus)
• Capillaries grow into the hematoma
• Phagocytes invade the area
• Fibroblasts
• Produce collagen fibers that span the break and
  connect the bone ends
• Osteoblasts
• Begin forming spongy bone

122
BONE HEALING
123
Bone Repair

• 3. Bony callus formation
• New bone trabeculae begins to form and is
  gradually converted to bony (hard) callus

124
BONE HEALING
125
Bone Repair

• 4. Remodeling of the bony callus
• Excess material on the diaphysis exterior and
  within the medullary cavityis removed
• Compact bone is laid down to reconstruct the
  shaft walls

126
BONE HEALING
127
Bone RepairNew Methods

• 1. Electrical stimulation of fracture
• 2. Ultrasound treatments
• 3. Free Vascular fibular graft
• 4. VEGF vascular endothelial growth factor
• 5. Nanobiotechnology
• 6. Bone Substitutes

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HOMEOSTATIC IMBALANCE

• Imbalances between bone deposit and bone
  resorption underline nearly every disease that
  affects the adult skeleton

129
Osteomalacia

• Soft bones
• Includes a number of disorders in adults in which
  the bone is inadequately mineralized
• Osteoid is produced, but calcium salts are not
  deposited, so bones are soft and weak
• Main symptom is pain when weight is put on the
  affected bones
• Cause insufficient calcium or by a vitamin D
  deficiency (helps to absorb calcium)
• Treatment drink vitamin D-fortified milk and
  exposing the skin to sunlight which stimulates
  production of vitamin D

130
Rickets

• Inadequate mineralization of bones in children
  caused by insufficient calcium or vitamin D
  deficiency
• Treatment drink vitamin D-fortified milk and
  exposing the skin to sunlight which stimulates
  production of vitamin D
• Because young bones are still growing rapidly,
  rickets is much more severe than adult
  Osteomalacia
• Bowed legs, deformities of the pelvis, skull, and
  rib cage are common

131
Osteoporosis

• Refers to a group of disorders in which the rate
  of bone resorption exceeds the rate of formation
• Bones become so fragile that something as simple
  as a hearty sneeze or stepping off a curb can
  cause them to break
• Bones have normal bone matrix (intercellular
  material of a tissue), but bone mass is reduced
  and the bones become more porous and lighter
  increasing the likelihood of fractures
• Spongy bone of the spine is most vulnerable, and
  compression fractures of the vertebrae are common
• Femur, particular the neck, is also very
  susceptible to fracture (broken hip)

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Osteoporosis

• Older women are especially vulnerable to
  osteoporosis, due to the decline in estrogen
  after menopause
• Other factors that contribute to osteoporosis
  include
• A petite body form
• Insufficient exercise or immobility to stress the
  bones
• A diet poor in calcium and vitamin D
• Abnormal vitamin D receptors
• Smoking
• Reduces estrogen levels
• Hormone-related conditions
• Hyperthyroidism
• Diabetes mellitus

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OSTEOPOROSIS(a) Normal Bone(b) Osteoporotic
Bones
134
Pagets disease

• Is characterized by excessive bone deposition and
  resorption, with the resulting bone abnormally
  high in spongy bone
• High ratio of spongy bone to compact bones
• It is a localized condition that results in
  deformation of the affected bone
• Weaken of a region of a bone
• Cause unknown

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DEVELOPMENTAL ASPECTS OF BONESTIMING OF EVENTS

• The skeleton derives from embryonic mesenchymal
  cells, with ossification occurring at precise
  times
• Most long bones have obvious primary ossification
  centers by 12 weeks
• At birth, most bones are well ossified, except
  for the epiphyses, which form secondary
  ossification centers
• Throughout childhood, bone growth exceeds bone
  resorption in young adults, these processes are
  in balance in old age, resorption exceeds
  formation

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FETAL OSSIFICATION
137
BONE REPAIR