CHAPTER 6

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CHAPTER 6Bones Tissue

• Functions of the skeletal system
• Classification of bones based on shape
• General features of bone
• Bone cells and matrix of bone
• Compact and spongy bone
• Bone marrows
• Bone development
• Overview of bone growth and remodeling

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SKELETAL SYSTEM

• Defined Includes all of the bones of the human
  body (total of 206), and their associated
  cartilages and joints.
• Functions
• Support supporting framework for body
• Protection protects vital organs (brain and
  thoracic cavity)
• Movement / levers- allows movement and flexion as
  well as levers for different movements
• Mineral storage- calcium and phosphate
• Hematopoiesis- principal site for blood cell
  formation in red marrow of flat bones (e.g.
  sacrum, sternum, etc.)
• Electrolyte balance of calcium and phosphorus
• Acid/Base balance buffers the blood calcium
  phosphate
• Detoxification by absorbing heavy minerals

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Classification based on the shape

• Long bones bones that are longer than they are
  wide Ex. Femur, tibia, fibula, ulna and humerus
• Short bones bones that are shaped like a cube
  there is also a special class of short bones
  called sesamoid bones. Ex. Tarsal, carpal, and
  patella.
• Flat bones bones that are thin and flat.
  Ex.Scapula, skull, ribs, and sternum.
• Irregular bones bones that do not fit in any of
  the prior categories because they have irregular
  shapes. Ex. Vertebrae and hip bones.

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Shapes of bones

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GROSS structure of a typical bone

• Articular cartilage Consists of Hyaline
  cartilage covering the end of the bone surface
  where it articulates with another bone,
• (e.g. femur and tibia, humerus and scapula).
  Fibrocartilage makes up the menisci of the knee
  joints.
• Epiphyses The end of the bone. One at each end
  of long bones.
• Epiphyseal line Remnant of the cartilaginous
  growth plate or epiphyseal plate.

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Typical bone structure

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Bone structure

• Periosteum Tough outer connective tissue
  covering on bone. Consists of 2 layers outside
  is dense irregular CT and deeper layer lined
  with osteoblast and osteoclast cells. It is
  richly supplied by blood vessels and nerves and
  secured to bone by Sharpeys fibers.
• Endosteum connective tissue covering on inside
  of bone cavities. Is osteogenic in that it
  contains osteoblasts and osteoclasts.

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Typical bone structure

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Bone structure continued

• Diaphysis The shaft of the bone between the two
  epiphyses. Contains the medullary cavity and is
  filled with yellow marrow in adults.
• Sharpeys fibers Bundles of collagenous fibers
  that tightly attach the periosteum to bony matrix.

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Types of Bones cells

• Bone is formed and metabolized by specific cells
• and is in constant state of remodeling.
• Osteoclasts Bone destroying cells
• C means chewing
• Osteoblasts Bone generating cells
• B means building
• 3. Osteocytes Mature bone cells, spider shaped
  and maintain bone tissue

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Bone Cells

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Bone matrix

• The matrix of bone is made up of organic and
  inorganic matter.
• The organic portion is of collagen fibers and
  various proteoglycans, glycosaminoglycans and
  glycoproteins.
• The inorganic portion is calcium phosphate salts
  hydroxyapetite and calcium carbonate
• The combination of these makes for a bone that is
  very strong and yet flexible.

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Chemical Composition of Bone

• In addition to bone cells, the majority of
  compact and spongy bone is composed of inorganic
  molecules.
• These molecules are called hydroxyapatitie and
  are made of calcium and phosphate.
• The combination of these form a cement like
  material that gives bone its hardness and
  strength. In combination with collagen fibers
  that form the matrix of bones and allows for
  elasticity and flexibility.

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Compact bone terms

• Osteon/Haversian System structural unit of
  compact bone. Oriented parallel to shaft and
  forming a group of hollow tubes through which an
  artery, vein and nerve pass into and through
  bone.
• Lacunae small cavities (halos) containing
  osteocyte
• Osteocyte true bone cell, spider shaped and
  found in lacunae at the junctions of the lamellae
• Lamellae layers of the collagen fiber matrix
  with each layer going in opposite direction to
  the adjacent layer.

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Compact bone terms

• Lamellae may be concentric (forming rings like a
  tree)
• or circumferential (encircling the entire bone
  structure).
• Canaliculi Hair like canales that connect each
  lacunae
• and in turn connect to the Central canal. Remove
  wastes
• and bring nutrients into osteocytes
• Volkmans canal/perforating canal Canals running
• perpendicular to the Haversian canals, but
  connecting to
• them. They bring in the artery, vein and nerves
  to the
• bone structure.

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Spongy Bone

• Spongy bone composes the inner portion of the
  bone lining the marrow cavity. It has a
  honeycomb appearance of trabeculae and spicules.
  Although it looks poorly organized it is designed
  to withstand the specific stresses put on each
  bone because of their trabeculae.
• Trabeculae are tiny bone struts or plates that
  form very strong support structure for the spongy
  bones. They are irregularly arranged lamellae and
  osteocytes, but contain no osteons per se as it
  receives it nutrients from the marrow tissue.

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Spongy bone histology

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Bone marrow

• Yellow marrow is found in medullary cavity of
  long bones and is not hematopoietic in adults.
  Yellow marrow replaces red marrow as we mature
  and is made up mainly of fat.
• Red marrow is found in the axial skeleton and
  girdles and in the epiphyses of the femur and
  humerus and is very active hematopoietically.

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Classification based on bone formation

• During embryonic development a blue-print for
  each of our bones is formed from fibrous
  membranes and/or hyaline cartilage.
• Cartilage is replaced (the cartilage does NOT
  magically become bone) by bone in one of two
  ways 1). Endochondral ossification and 2).
  Intramembranous ossification

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Bone development

• Ossification or osteogenesis
• - is the process of forming new bone
• Two methods of ossification
• Endochondral ossification
• Intramembranous ossification

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Endochondral ossification

• The process by which bone is formed from hyaline
  cartilage
• Most bones in the body are formed by this method
  (including the vertebrae, pelvic bones and limb
  bones).
• Consists of 3 sites of ossification.
• The primary ossification center
• The metaphysis
• The secondary ossification center

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Endochondral ossification

• During the first 8 weeks of fetal development,
  hyaline cartilage forms a model of future bone
  formation. The center (diaphysis) of the
  cartilage is primary ossification site which
  contains numerous chondrocytes in lacunae.
  Surrounding the cartilage model is an outer layer
  chondrocytes called the perichondrium which
  differentiate into osteoblast cells and begin
  to lay down a bony collar around the site.
• Once the collar is formed the perichondrium
  becomes periosteum.

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Endochondral ossification

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Endochondral ossification

• Buds of connective tissue grow from the
  periosteum into the caritlage and transform the
  primary ossification site into the primary marrow
  space. This space is lined with spongy bone.
• The metaphysis forms between the marrow space and
  the cartilaginous epiphyseal end.
• -It is considered as a transitional zone where
  cartilage is formed into bone at each end of the
  epiphysis. The metaphysis is just beneath the
  epiphyseal plate where bone growth continues
  until after adulthood.

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The metaphysis

• Consists of five distinct zones
• Zone of reserve cartilage
• Zone of cell proliferation
• Zone of cell hypertrophy
• Zone of calcification
• Zone of bone deposition

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The secondary ossification center

• Begins at the time of birth.
• Forms in the epiphysis and develops similarly to
  the primary ossification center.
• The bone formed in the secondary ossification
  site persists as spongy bone and growth occurs
  beneath the outer covering of hyaline cartilage
  which persists as articular cartilage within the
  joint cavity on each end of the epiphysis.

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Intramembranous ossification

• Bone formed by replacing a fibrous membrane and
  not from cartilage. Ex. Skull and clavicle
• Basic Overview of Process
• during the first 8 wks of embryonic development,
  fiberous membranes (CT) form in the areas of
  future flat bones
• beginning around 8 wks, an ossification center
  forms in the membrane. This center is composed
  of osteoblasts.
• the osteoblasts begin to secrete hydroxy apetite
• the internal spongy bone forms
• the external compact bone forms

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Bone Growth

• Once the cartilage models of embryonic
  development are replaced by bone, they must
  continue to grow through infancy, childhood and
  adolescence.
• Increased length bones continue to lengthen
  because hyaline cartilage remaining in the
  epiphyseal plates continues to grow. As
  adulthood approaches, this cartilage becomes less
  active and is eventually replaced by bone.

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Bone growth

• Appositional growth - Increased Width bones
  continue to widen as osteoblasts form more layers
  of bone around the outside and osteoclasts break
  down some of the bony matrix inside.
• Why would bone need to be broken down inside as
  it grows outside?

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Control of Bone Growth

• Bones increase in length and width because of the
  influence of minerals, vitamins, and hormones in
  the body.
• Calcium and phosphate are necessary for
  calcification
• Vitamins A, C and D promote bone growth.
• The specific hormones which affect growth are
  growth hormone (GH), thyroid hormone (T3 and T4),
  and the sex steroids (estrogen and tesstosterone).

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BONE REMODELING

• Even though the bones in an adult do not continue
  to grow as described above, they are constantly
  being remodeled. This means that bone is always
  being broken down by osteoclasts and reformed by
  osteoblasts (really no different from remodeling
  ones home).
• Each week we turn over about 5 of our bone mass.

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Control of Remodeling

• Two major factors influence remodeling.
• Calcium levels - our bodies need a homeostatic
  level of calcium in the blood for all cells to
  function properly.
• Mechanical stress - the varied activities of life
  puts different stresses on each bone as we age,
  which requires slight adjustments to compensate
  for these stresses.