OSSEOUS TISSUE

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OSSEOUS TISSUE

• SKELETAL STRUCTURE

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Skeletal System

• 206 bones, cartilage, ligaments, and connective
  tissues
• Functions
• support
• provides a rigid framework
• storage
• calcium phosphorus
• lipids
• production of blood cells
• formed in red marrow
• protection
• brain is encased in skull
• heart and lungs are surrounded by boney sternum
  and rib cage
• leverage
• allows for movement due to interaction of
  muscular skeletal systems
• acid-base balance
• absorbs or releases alkaline salts

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Divisions of the Skeletal System

• Axial skeleton
• consists of bones forming axis of the body
• Skull
• Hyoid
• Sternum
• Ribs
• Vertebrae
• sacrum cocyx
• auditory ossicles (not a part of either but put
  here by convention)
• Appendicular skeleton
• consists of bones that anchor appendages to axial
  skeleton
• upper lower extremities, shoulder and pelvic
  girdles

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Types of Bones

• Long
• Flat
• Short
• Irregular
• Sesamoid
• Sutural

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Long Bones

• longer than wide
• function as levers
• act on skeletal muscles to produce movements
• found in appendages
• fingers toes

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Short Bones

• boxy small
• nearly cube-shape
• found in wrist-carpals
• ankle-tarsals
• limited movements

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Flat Bones

• thin
• roughly parallel surfaces
• found in the roof of the skull
• sternum, ribs scapula
• enclose protect soft organs
• provide broad surfaces for muscle attachment

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Irregular Bones

• bones that do not fall into any other category
• varied, complex shapes, sizes surface features
• vertebrae, sacrum, coccyx, temporal, sphenoid,
  ethmoid, zygomatic, maxilla, mandible, palatine,
  inferior nasal concha, hyoid

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Sesamoid Bones

• shaped like sesame seeds develop in areas where
  there is a great deal of friction
• most only a few mms number in each person
  differs
• patella present in everyone

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Sutural Bones

• also called Wormian bones
• small
• located in sutures
• classified by location-not by shape

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Bone Composition

• Osseous Tissue
• supporting connective tissue
• Composed of an extra cellular matrix and
  specialized cells
• give flexibility
• two types
• compact or dense bone
• dense, hard, relatively solid
• forms protective exterior of all bones
• spongy or cancellous bone
• found inside most compact bone
• very porous
• full of tiny holes forming open networks of
  struts plates
• lighter than compact bone
• reduces skeletal weight
• makes it easier for muscles to move bones

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Extracellular Matrix

• composed of collagen fibers ground substance
• hardened by inorganic calcium phosphate deposits
• called mineralization or calcification
• solid calcium phosphate salts deposited around
  protein fibers
• Calcium phosphate makes up 2/3rd of bone weight
• Calcium phosphate calcium hydroxide ?
  hydroxyapatite-Ca10(PO4)6(OH)2
• Calcium phosphate is hard, brittle inflexible
• can withstand compression
• Collagen fibers are stronger than steel,
  flexible- can be twisted bent
• not good at being compressed
• collagen makes a frame around which calcium
  minerals deposit
• combination makes bone flexible, strong
  resistant to shattering

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Bone Cell Types

• Osteogenic cells
• stem cells ? produce other bone cells
• found in cellular layer of periosteum, endosteum
  central canals
• continually divide
• only bone cell that can divide
• Osteoblasts
• bone-forming cells
• make organic matter of bone matrix
• Osteocytes
• mature bone cells
• most of bone cell population
• former osteoblasts that have become trapped in
  matrix they have deposited
• Osteoclasts
• bone destroying cells

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Osteocytes

• cannot divide
• function to maintain monitor protein mineral
  content of matrix
• participate in bone repair by converting back
  into osteoblasts or osteogeneic cells at the site
  of injury
• sense strain regulate bone remodeling

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Osteoclasts

• bone dissolving cells
• function to remove bone by osteolysis
• secrete acids proteolytic enzymes which degrade
  minerals fibers and dissolve boney matrix
• releases matrix components into the blood
  restoring calcium and phosphorus concentrations
  in body fluids

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Types of Bone Tissue

• Compact Bone
• dense
• covers exterior of all bones
• Spongy Bone
• cancellous
• trabecular
• inside compact bone
• lighter

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Compact Bone

• basic functional unit -osteon or Haversian
  system.
• osteocytes are arranged in concentric circles or
  layers-lamellae
• around a central or Haversian canal
• runs parallel to surface
• contains blood vessels
• perforating central canal are Volkmanns canals
• run perpendicular to surface
• canaliculi run through layers
• connect osteocytes to each other
• interstitial lamellae fill spaces between

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Spongy Bone

• matrix composition-same
• osteocytes, canalicui lamellae-different
  arrangements
• has no osteons
• matrix forms plates or struts called trabeculae
  (little beams)
• form a thin, branching open network filled with
  red bone marrow
• makes bone lighter

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Bone Type Bone Tissue Type Location

• the relationship between compact spongy bone
  and the relative proportions of each varies with
  bone shape with the function of the bone

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Long Bone Structure

• Diaphysis or shaft-long cylindrical
• Outside made of dense bone
• medullary canal or marrow cavity is filled with
  marrow
• Yellow bone marrow is dominated by fat cells
  red marrow is responsible for forming blood cells
• Epiphysis-expanded extremities at either end of
  the bone
• articulates with other bones-forming joints
• have broad surfaces for muscle attachment.
• filled with cancellous tissue surrounded by thin
  layer of compact bone
• Metaphysis
• connects diaphysis to epiphysis

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Flat Bone Composition

• function
• provide protection for underlying structures
• broad surfaces for muscle attachment
• function can be seen by structure
• resembles a spongy bone sandwich
• composed of 2 thin layers of compact bone
  covering a layer of spongy bone
• bone marrow is present
• there is no marrow cavity

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Periosteum Endosteum

• Periosteum
• covers all portions of compact bone except at
  joint cavities
• has fibrous outer layer an inner cellular
  layer
• isolates bones from surrounding tissues
• provides route for blood vessels nerves
• participates in bone growth repair
• continuous with other connective tissues that
  mesh with-tendons ligaments
• perforating or Sharpeys fibers bond tendons
  ligaments into the general structure of bone
• endosteum
• consists of an incomplete cellular layer
• lines marrow cavities
• covers trabeculae of spongy bones
• lines inner surfaces of central canals
• active during bone growth, repair, and remodeling

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Blood Nerve Supply

• bone tissue is highly vascular
• Vessels pass into the bone through the
  periosteum
• Periosteal arteries enter via perforating canals
• nutrient artery vein
• enter through a nutrient foramen located in
  middle of the bone

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Bone Growth

• new bone matrix is made through osteogenesis or
  ossification
• process makes releases proteins other organic
  components of matrix
• substance is osteoid
• bone matrix before calcium salts have been added
• calcium salts are laid down in a process called
  calcification

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Bone Development Growth

• skeleton begins to form at 6 weeks post
  fertilization
• does not stop until around age 25
• develops by two methods
• intramembranous ossification
• endochondral ossification

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Intramembranous Ossification

• bone forms from mesenchyme or fibrous connective
  tissue
• produces flat bones of skull, most of the facial
  bones, mandible medial part of the clavicle
• bone develop within a fibrous sheet similar to
  dermis of the skin
• bones are called dermal bones

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Intramembranous Ossification Steps

• Step1 Development of Ossification Center
• Step 2 Calcification
• Step 3 Formation of Trabeculae
• Step 4 Development of Periosteum

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Step1 Development of Ossification Center

• at site where the bone is to form, chemical
  messages cause mesenchymal cells (embryonic
  connective tissue) to cluster together into a
  layer of soft tissue
• cells enlarge differentiate into osteogenic
  cells and then into osteoblasts.
• site is the ossification center
• osteoblasts begin to secrete organic matrix
• eventually become trapped become osteocytes

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Step 2 Calcification

• Calcium other salts deposit on organic
  extracellular matrix made by osteoblasts
• As trabeculae continue to grow calcium phosphate
  is deposited
• causes matrix to harden or calcify

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Step 3 Formation of Trabeculae

• osteoblasts continue to deposit matrix
• continue to be calcified producing struts of
  trabeculae
• connective tissue present differentiates into red
  bone marrow

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Step 4 Development of the Periosteum

• Mesenchyme condenses at periphery of the
  bone?periosteum.
• Trabeculae at surface continue to calcify until
  spaces between them are filled in converting
  spongy bone to compact bone
• process gives rise to sandwich like arrangement
  of flat bones

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Intramembranous Ossification
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Endochondral Ossification

• bone forms by replacing pre-existing hyaline
  cartilage model with bone
• most bones are made this way
• begins around sixth week of fetal development
• continues into the 20s

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Endochondral Ossification Steps

• Step 1 Development of Hyaline Cartilage Model
• Step 2 Growth of Cartilage Model
• Step 3 Development of Primary Ossification
  Center
• Step 4 Development of Medullary Cavity
• Step 5 Development of Secondary Ossification
  Centers
• Step 6 Formation of Articular Cartilage
  Epiphseal Growth

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Step 1 Development of Hyaline Cartilage Model

• at site when bone will form chemical messengers
  cause mesenchymal cells to crowed together in
  general shape of future bone
• cells develop into chondroblasts.
• begin to secrete cartilage extracellular matrix
  which develops into a hyaline cartilage bone
  covered with a perichondrium

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Step 2 Growth of Cartilage Model

• once chondroblasts become embedded in
  extracellular matrix ? become chrondrocytes.
• cartilage model continues to grow longer from
  either end via interstitial or endogenous
  growth.
• grows in diameter or thickness via appositional
  or exogenous growth
• new cartilage is laid on the outside of model by
  chondroblasts
• as model continues to grow chondrocytes in area
  get larger in the mid-region area the cartilage
  matrix begins to calcify
• enlarged chondrocytes are deprived of nutrients
  due to their size and calcification diffusion
  cannot occur
• die and disintegrate
• dying leaves spaces which merge into small
  cavities called lacunae

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Step 3 Development of Primary Ossification
Center

• ossification continues inward from surface of
  bone to inside in the middle of model- primary
  ossification center
• a nutrient artery penetrates perichondrium
• stimulates osteogenic cells there to become
  osteoblasts
• once this occurs perichondrium is termed
  periosteum
• in the primary ossification center most of
  cartilage will be replaced with bone
• osteoblasts begin to deposit a thin collar of
  boney matrix around middle of cartilage model
  forming trabeculae of spongy bone
• primary ossification spreads from central area
  toward both ends of the cartilage model

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Step 4 Development of Medullary Cavity

• as primary ossification center grows osteoclast
  cells break down some newly formed spongy bone
  trabeculae
• leaves a cavity
• capillaries fibroblasts migrate to the inside
  of the cartilage and take over the spaces left by
  the dying chondrocytes
• as center is hollowed out filled with blood and
  stem cells, it becomes primary marrow cavity.
• region of transition from cartilage to bone at
  the end of the primary marrow cavity is called
  the metaphysis

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Step 5 Development of SecondaryOssification
Centers

• when branches of the epiphyseal artery enter the
  epiphyses the secondary ossification centers form
• bone formation is similar to as described in the
  center of the bone
• here however spongy bone remains in the epiphyses
• secondary ossification proceeds outward from
  center of each epiphysis toward outer surface of
  the bone

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Step 6 Formation of Articular Cartilage
Epiphseal Growth

• hyaline cartilage covering epiphyses develop into
  articular cartilages
• during infancy childhood epiphyses fill with
  spongy bone
• cartilage is limited to articular cartilages
• prior to adulthood there is some hyaline
  cartilage that remains between the diaphysis and
  the epiphysis
• called epiphyseal or growth plate
• area where bone will continue to grow in length
  until it becomes adult sized

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Endochondral Ossification
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Endochondral Ossification
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Bone Growth

• bone increases in length width
• increases in length at epiphyseal plate
• interstitital growth
• diameter of bone increases through appositional
  growth
• new tissues is deposited at surface of the bone

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Interstitital Growth

• occurs at epiphyseal plate
• consists of hyaline cartilage in middle with a
  transitional zone on either side
• in transitional zone cartilage is turning into
  bone
• epiphysis makes cartilage ostoblasts try to
  overtake it by making bone
• osteoblasts cannot catch up ?bone gets longer

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Interstitital Growth

• epiphyseal plate consists of four zones
• zone of resting cartilage
• zone of proliferating cartilage
• zone of hypertrophic cartilage
• zone of calcified cartilage

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Interstitital Growth

• In zone of resting cartilage small chondrocytes
  present
• do not participate in bone growth
• cells anchor plate to the epiphysis
• in zone of proliferating cartilage contains
  slightly larger chondrocytes
• undergo interstitial growth
• cells divide replacing those that die on
  diaphysis side of plate
• in zone of hypertrophy there are large, maturing
  chondrocytes arranged in columns
• zone of calcified cartilage contains few cells
• cells are mostly dead due to extracellular
  matrix around them having been calcified and no
  blood or nutrients can reach them

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Interstitital Growth

• at puberty rising levels of sex thyroid
  hormones cause osteoblasts to outpace
  manufacture of cartilage at epiphyseal end
• growth plate eventually fuses shut, leaving an
  epiphyseal line
• completes length of bone

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Appositional Growth

• way diameter of bone increases
• new tissues is deposited at surface of bone
• at surface periosteal cells differentiate into
  osteoblasts
• begin to secrete organic parts of matrix.
• oteoid tissue is calcified
• as osteoblasts become trapped ?osteocytes
• lay down matrix in layers parallel to surface
• produce circumferential lamellae of bone

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Bone Dynamics

• bones constantly adapt to demands placed on them
  and are continually remodeled throughout life
• part of normal growth maintenance
• 10 of skeleton tissue is replaced each year
• organic and mineral components are continuously
  recycled removed through remodeling
• gives bone the ability to adapt to new stresses

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Bone Dynamics

• activities of both cells types are continuous
• activities must be balanced
• when osteoclasts remove calcium faster than
  osteoblasts can deposit it?bone weakens
• when osteoblast activity predominates ?bones get
  stronger and more massive

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Wolffs law

• bones structure is determined by mechanical
  stresses placed on it
• one such stress is exercise
• when bone is stressed?mineral crystals generate
  small electrical fields which attract osteoblasts
• bony landmarks or bumps and ridges on surface of
  bone where tendons attach may become more
  pronounced as muscles work to withstand increased
  forces
• regular exercise is needed to maintain normal
  bone structure
• bone degeneration results from inactivity
• changes in mineral content does not necessarily
  change shape of bones because boney matrix
  contains protein fibers
• bones can okay but may be soft due to no mineral
  deposition
• this is called osteomalacia
• one form of this is rickets
• typically due to a vitamin D3 deficiency
• not properly mineralized bones are flexible
• legs will bend under the weight of the body

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Nutritional Needs

• bone growth and maintenance requires
• calcium
• phosphorous
• magnesium
• fluoride
• manganese
• Calcitriol
• from kidneys
• absorption of calcium phosphate from GI tract
• synthesis of calcitriol depends on Vitamin D3
• therefore Vitamin D3 is needed for proper bone
  growth
• Vitamin C
• needed for enzymatic reactions
• needed for collagen synthesis
• needed to stimulate osteoblast differentiation
• without vitamin C there is a loss of bone
  strength and mass-scurvy
• Vitamin A
• stimulates osteoblast activity
• especially important for bone growth in children

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Hormonal Needs

• Growth hormone
• Thyroxine
• Sex hormones
• androgens in males
• estrogens in females
• help to close epiphyseal plates
• stimulate osteoblasts to produce bone at rate
  faster than epiphyseal cartilage can expand

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Calcium Balance

• most abundant mineral in the body
• 90 is in bones
• crucial to membrane functions
• needed for activities of neurons muscle cells
• for homeostatic balance three hormones are needed
• Calcitriol
• Calcitonin
• Parathyroid hormone

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Calcitriol

• active form of vitamin D
• principle function?raise blood calcium
• increases absorption of calcium by small
  intestine

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Calcitonin Parathyroid Hormones

• opposite effects
• Targets
• bones where calcium is stored
• digestive tract where calcium is absorbed
• kidneys where calcium is excreted

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Calcitonin

• made in thyroid gland
• blood calcium levels rise? parafollicular or C
  cells?release calcitonin?lowers blood calcium
• inhibits osteoclast activity ? slowing rate of
  calcium release from bone
• stimulates osteoblasts?
• encourages calcium to be deposited into bones
• more important during childhood
• also important in reducing loss of bone mass
  during prolonged starvation during late stages
  of pregnancy
• role in healthy adults is unknown

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Parathyroid Hormone

• made by parathryroid gland
• calcium levels fall ?parathyroid glands?secrete
  parathyroid hormone
• raises blood calcium levels
• increases osteoclast acitivty ? increases release
  of calcium from bones
• promotes calcium reabsorption by kidneys
• promotes final step of calcitriol synthesis in
  kidneys ?enhancing calcium uptake by intestine
• inhibits collagen synthesis by osteoblasts?calcium
  deposition into bone decreases

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Calcium Balance