Chapter 6: Osseous Tissue and Bone Structure

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Chapter 6 Osseous Tissue and Bone Structure
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The Skeletal System

• Skeletal system includes
• bones of the skeleton
• cartilages, ligaments, and other connective
  tissues that stabilize the bones

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Skeletal System

• Functions
• 1. Support framework structure of body
• 2. Storage of minerals and lipids
• Minerals calcium and phosphate
• - for osmotic regulation, enzyme
  function,
• nerve impulses
• Yellow marrow triglycerides
• 3. Blood cell production all formed elements
• - red marrow stem cells ? hematopiesis
• 4. Protection surround soft tissues
• 5. Leverage for movement
• - levers upon which skeletal muscles act

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Classification of Bones

• Bone are identified by
• shape
• internal tissues
• bone markings
• SHAPE
• Long bones
• Flat bones
• Sutural bones
• Irregular bones
• Short bones
• Sesamoid bones

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Shape of Bones

• Long Bones
• Longer than wide, consist of shaft and 2 ends
• e.g. bones of appendages
• Short Bones
• Approx. equal in all dimensions
• e.g. carpals, tarsals
• Flat Bones
• Thin, 2 parallel surfaces
• e.g. skull, sternum, ribs, scapula

Figure 61a
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Shape of Bones

• Irregular Bones
• Complex shapes
• E.g. vertebrae, os coxa
• Sesamoid Bones
• Seed shaped, form in tendon
• E.g. patella, total number can vary
• Sutural Bones
• - Extra bones in sutures of skull

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Bone Structure

• A bone is an organ consisting of many tissue
  types
• Osseous, nervous, cartilage, fibrous CT, blood,
  etc.
• All bones consist of 2 types of bone tissue
• Compact bone
• - solid, dense bone, makes up surfaces and
  shafts
• Spongy Bone/Cancellous bone
• - meshy, makes up interior of bones, houses red
  marrow in spaces

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Bone Markings

• Bones are not flat on the surface
• Have projections, depressions, and holes for
  muscle attachment, blood nerve supply
• Depressions or grooves
• along bone surface
• Projections
• where tendons and ligaments attach
• at articulations with other bones
• Tunnels
• where blood and nerves enter bone

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Bone Markings
Table 61 (2 of 2)
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Long Bones Structure

• Diaphysis
• - Hollow shaft of compact bone
• Medullary (marrow) cavity
• Center of diaphysis, contains yellow marrow
• Triglycerides for energy reserve
• Epiphysis
• Expanded end of bone, surface of compact bone
• Center filled with spongy bone with red marrow in
  spaces
• Produces blood cells

Figure 62a
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Long Bones Structure

• Epiphyseal line or plate
• Cartilage that marks connection of diaphysis with
  epiphysis
• Line adults, narrow (aka metaphysis)
• Plate thick, allows growth during childhood
• Periosteum
• 2 layer covering around outside of bone
• Outer Fibrous Layer
• Inner Cellular Layer
• Endosteum
• Cellular layers, covers all inside surfaces

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• Articular Cartilage
• Hyaline cartilage on end where bone contacts
  another, no periosteum or perichondrium
• Joint/Articulation
• - connection between two bones, surrounded by
  CT capsule, lined with synovial membrane
• Joint cavity filled with synovial fluid to reduce
  friction on articular cartilage

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Flat Bone Structure

• Thin layer of spongy bone with red marrow between
  two layers of compact bone
• Covered by periosteum and endosteum
• Site of most hematopoiesis
• Production of blood cells and cell fragments that
  are suspended in plasma (RBC, WBC, and platelets

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Characteristics of Bone Tissue

• Periosteum
• covers outer surfaces of bones
• consist of outer fibrous and inner cellular
  layers
• Endosteum
• Inner, cellular layer of periosteum

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Bone Histology

• Bone osseous tissue, supporting CT
• Consists of specialized cells in a matrix of
  fibers and ground substance
• Characteristics of bone
• Dense matrix packed with calcium salts
• Osteocytes in lacunae
• Canaliculi for exchange of nutrients and waste
• Two layer periosteum, covers bone except at
  articular surfaces

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Bone Histology

• Matrix 98 of bone tissue
• 1/3 osteoid organic part
• Collagen fibers ground substance
• Tough and flexible
• 2/3 densely packed crystals of hydroxyapatite
  (calcium salts, mostly calcium phosphate)
• Hard but brittle
• Cells only 2 of bone
• Osteocytes
• Osteoblasts
• Osteoprogenitor cells
• Osteoclasts

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Cells located in Bones

• Osteocytes mature bone cells
• -no cell division
• -located in lacunae between layers of matrix
  called lamellae
• -canaliculi link lacunae to each other and blood
  supply
• -osteocytes linked to each other via gap
  junctions on cell projections in canaliculi
• - allow exchange of nutrients and wastes
• -Function
• 1. To maintain protein and mineral content of
  matrix
• 2. Can also participate in bone repair
• -become stem cell like when broken free of
  lacuna

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Cells located in Bones

• Osteoblasts - Immature bone cells
• Perform osteogenesis
• Formation of new bone matrix
• Produce osteoid
• Organic components of matrix that is not yet
  calcified to form bone
• Promote deposit of calcium salts which
  spontaneously form hydroxyapatite
• Once enclosed in lacuna by matrix, osteoblast
  differentiates into osteocyte and no longer
  produces new matrix

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Cells located in Bones

• 3. Osteoprogenitor Cells mesenchymal cells
• - bone stem cell that produces daughters
• - daughters become osteoblasts for repair
  and growth
• - located in endosteum and inner periosteum

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Cells located in Bones

• 4. Osteoclasts
• - large, multinuclear
• - derived from monocytes (macrophages)
• - perform osteolysis
• - digest and dissolve bone matrix
• - release minerals
• 1. For use in blood or
• 2. Recycling during bone remodeling

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Cells located in Bones
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Homeostasis

• Bone building (by osteocytes) and bone recycling
  (by osteoclasts) must balance
• more breakdown than building, bones become weak
• exercise causes osteocytes to build bone

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How would the strength of a bone be affected if
the ratio of collagen to hydroxyapatite increased?

1. Strength increases, flexibility increases.
2. Strength increases, flexibility decreases.
3. Strength decreases, flexibility. decreases.
4. Strength decreases, flexibility increases.

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If the activity of osteoclasts exceeds the
activity of osteoblasts in a bone, how will the
mass of the bone be affected?

1. stable mass, but re-positioned matrix
2. mass will not be affected
3. more mass
4. less mass

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The difference between compact bone and spongy
bone.
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Structure of Compact Bone

• Consists of osteons
• Parallel to surface
• Each osteon is around a central canal
• Contains blood vessels and nerves
• Perforating canals perpendicular to osteons act
  to connect the osteons
• Osteon is built of layers of matrix secreted by
  osteoblasts
• Each layer concentric lamella
• Osteocytes are located in lacunae between
  lamellae
• Ostocytes are connected to neighboring cells and
  central canal via canaliculi

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Structure of Compact Bone

• Interstitial lamellae fill spaces between osteons
• Circumferiential lamellae run perimeter inside
  and out in contact with
• endosteum and periosteum
• Compact bone is designed to receive stress from
  one direction
• Very strong parallel to osteons
• Weak perpendicular to osteons

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Compact Bone
Figure 65
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Structure of Spongy Bone

• Lamellae meshwork called trabeculae (no
  osteons)
• Red marrow fills spaces around trabeculae
• Osteocytes in lacunae are linked by canaliculi
• No direct blood supply (no central canals)
• Nutrients diffuse into canaliculi in trabeculae
  from red marrow
• Spongy bone make up
• low stress bones
• Areas of bone where stress comes from multiple
  directions
• Provide light weigh strength

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Bone Marrow

• Red Marrow
• Located in space between trabeculae
• Has blood vessels
• Forms red blood cells
• Supplies nutrients to osteocytes
• Yellow Marrow
• In some bones, spongy bone holds yellow bone
  marrow
• is yellow because it stores fat

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Structure of Spongy Bone
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Periosteum and Endosteum

• Compact bone is covered with membrane
• periosteum on the outside
• endosteum on the inside

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Periosteum

• Fibrous outer layer
• - Dense irregular CT
• Cellular Inner layer
• Osteoprogenitor cells
• Functions
• Isolate bone from surrounding tissues
• Site for attachment for tendons and ligaments
• Route for nerves and blood vessels to enter bone
• Participates in bone growth and repair

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Endosteum

• Thin cellular layer
• Lines medullary cavity, central canals, and
  covers trabeculae
• Consists of
• osteoblasts, osteoprogenitor cells, and
  osteoclasts
• Cells become active during bone growth and repair

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Endosteum
Figure 68b
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Bone Growth

• Begins 6-8 weeks post fertilization
• Continues through puberty (18-25 y)
• Osteogenesis ossification formation of bone
• Not calcification
• Hardening of matrix or cytoplasm with calcium
• Can happen to many tissues
• Two types of Ossification
• Intramembranous forms flat bones
• Endochondrial forms long bones

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Bone Development

• Human bones grow until about age 25
• Osteogenesis
• bone formation
• Ossification Deposition of calcium salts
• the process of replacing other tissues with bone

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The difference between intramembranous
ossification and endochondral ossification.
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Intramembranous Ossification

• Bone develops from mesenchyme or fibrous CT in
  deep layers of dermis
• Also called dermal ossification
• because it occurs in the dermis
• produces dermal bones such as mandible and
  clavicle
• Produces skull bones
• There are 4 main steps in intramembranous
  ossification

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Intramembranous Ossification Step 1

• Ossification center appears in the fibrous CT
  membrane
• Mesenchymal cells aggregate
• Differentiate into osteoblasts
• Begin ossification at the ossification center

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Intramembranous Ossification Step 2

• Bone matrix (osteoid) is secreted within the
  fibrous membrane
• Osteoblasts begin to secrete osteoid, which is
  mineralized within a few days
• Trapped osteoblasts become osteocytes

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Intramembranous Ossification Step 3

• Woven bone and periosteum form
• Accumulating osteoid is laid down between
  embryonic blood vessels, which form a random
  network
• Vascularized mesenchyme condenses on the external
  face of the woven bone and becomes periosteum
  around spongy bone

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Intramembranous Ossification Step 4

• Bone collar of compact bone forms and red marrow
  appears
• Trabeculae just deep to the periosteum thickens,
  forming a woven bone collar that is later
  replaced with mature lamellar bone
• Spongy bone, consisting of distinct trabeculae,
  persists internally and its vascular tissue
  becomes red marrow

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Endochondral Ossification

• Ossifies bones that originate as hyaline
  cartilage
• Most bones originate as hyaline cartilage
• Cartilage grows by interstitial and appositional
  growth
• Cartilage is slowly replaced from the inside out

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Endochondral Ossification

• Growth and ossification of long bones occurs in 6
  steps

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Endochondral Ossification Step 1

• Primary ossification center begins to form
• Chondrocytes in the center of hyaline cartilage
• Enlarge in diaphysis
• Surrounding matrix calcifies killing the enclosed
  chondrocytes
• die, leaving cavities in cartilage

Figure 69 (Step 1)
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Endochondral Ossification Step 2

• Blood vessels grow around the edges of the
  cartilage
• Cells in the perichondrium change to osteoblasts
  
• Secrete osteoid
• Osteiod is mineralized and produces a layer of
  superficial bone around the shaft which will
  continue to grow around the diaphysis and become
  compact bone (appositional growth)

Figure 69 (Step 2)
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Endochondral Ossification Step 3

• Capillaries and fibroblast migrate into the
  primary ossification center
• Blood vessels enter the cartilage
• Bringing fibroblasts that become osteoblasts and
  secrete osteoid
• Mineralized into rebeculae
• Spongy bone develops at the primary ossification
  center and continues to growth toward the
  epiphysis

Figure 69 (Step 3)
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Endochondral Ossification Step 4

• Remodeling creates a marrow cavity
• Osteoclasts degrade trabeculae in the center to
  create the marrow cavity
• Bone increases in length by interstital growth of
  the epiphyseal plate followed by replacement of
  plate cartilage by spongy bone
• Cartilage continues to grow on epiphyseal side
  and is replaced by bone on diaphysis side
• Bone increases in diameter by appositional growth
  from cellular layers of peristeum

Figure 69 (Step 4)
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Endochondral Ossification Step 5

• Secondary ossification centers form in epiphyses
• Capillaries and osteoblasts enter the epiphyses
• creating secondary ossification centers

Figure 69 (Step 5)
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Endochondral Ossification Step 6

• Epiphyses become ossified with spongy bone
• Hyaline cartilage remains on articular surfaces
  (not calcified or ossified)
• Ossification continues at both 1and 2
  ossification centers until all epiphyseal
  cartilage has been replaced with bone ?
  epiphyseal closure
• Adult bone retains the epiphyseal line

Figure 69 (Step 6)
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Endochondral Ossification

• Appositional growth
• compact bone thickens and strengthens long bone
  with layers of circumferential lamellae

Figure 69 (Step 2)
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During intramembranous ossification, which
type(s) of tissue is/are replaced by bone?

1. hyaline cartilage
2. fibrous connective tissue
3. mesenchymal connective tissue
4. osteoid tissue

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In endochondral ossification, what is the
original source of osteoblasts?

1. de novo synthesis
2. cells brought with via the nutrient artery
3. cells of the inner layer of the perichondrium
4. chondrocytes from the original model

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The characteristics of adult bones.
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Epiphyseal Lines
Figure 610
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Epiphyseal Lines

• When long bone stops growing, after puberty
• epiphyseal cartilage disappears
• is visible on X-rays as an epiphyseal line

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A child who enters puberty several years later
than the average age is generally taller than
average as an adult. Why?

1. Epiphyseal plates fuse during puberty.
2. Bone growth continues throughout childhood.
3. Growth spurts usually occur at the onset of
   puberty.
4. All of the above.

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The skeletal system remodels and maintains
homeostasis.The effects of nutrition,
hormones, exercise, and aging on bone.
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Bone Remodeling

• Bones are not static constantly recycled and
  renewed
• 5-7 of skeleton is recycled/week
• Osteoclasts secrete
• Lysosomal enzymes digest osteoid
• Hydrochloric acid solubilize calcium salts
• Osteoblasts secrete
• Osteoid (organic matrix)
• Alkaline phosphatase induces mineralization of
  osteoid
• - Complete mineralization takes 1 week

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Bone Remodeling

• Bones Adapt
• Stressed bones grow thicker
• Bumps and ridges for muscle attachment enlarge
  when muscles are used heavily
• Bones weaken with inactivity up to 1/3 or mass
  is lost with few weeks of inactivity
• Heavy metals can get incorporated
• Condition of bones depends on interplay between
  osteoclast and osteoblast activity

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Skeleton as a Calcium Reserve

• Calcium is important for normal function of
  neurons and muscle
• Blood calcium 9-11 mg/100ml
• If blood levels are too high
• Nerve and muscle cells are non responsive
• If blood levels are too low
• Nerve and muscle cells are hyper-excitable ?
  convulsions, death

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The Skeleton as Calcium Reserve

• Bones store calcium and other minerals
• Calcium is the most abundant mineral in the body
• Calcium ions are vital to
• membranes
• neurons
• muscle cells, especially heart cells

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Skeleton as a Calcium Reserve

• Calcium homeostasis depends on
• Storage in the Bones
• Absorption in the GI
• Excretion at the Kidneys
• These factors are controlled by hormones to
  regulate blood calcium levels

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If blood calcium levels Low

• Parathyroid hormone (from parathyroid gland)
  triggers
• Increase osteoclast activity
• - decrease storage
• Enhanced calcitriol action
• - increase absorption
• Decreased calcium excretion at the kidneys

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If Blood Calcium levels High

• Calcitonin (from thyroid gland) triggers
• Inhibition of osteoclast activity
• Increased calcium excretion at the kidneys

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Nutritional and Hormone Effects on Bone

• Many nutrients and hormones are required for
  normal bone growth and maintenance
• Calcium and phosphate salts
• Calcitriol
• Vitamin C
• Vitamin A
• Vitamin K and B12
• Growth Hormones
• Thyroxin
• Estrogens and Androgens
• Calcitonin
• Parathyroid Hormone

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Nutritional and Hormone Effects on Bone

• Calcium and phosphate salts
• - From food, for mineralization of matrix
• Calcitriol
• - From kidneys, for absorption of calcium and
  phosphate
• Vitamin C
• - From food, for collagen synthesis and
  osteoblast differentiation
• Vitamin A
• - From carotene in food, for normal bone growth
  in children
• Vitamin K and B12
• - From food, for synthesis of osteoid proteins

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Nutritional and Hormone Effects on Bone

• Growth Hormones
• - From pituitary gland, for protein synthesis and
  cell growth
• Thyroxin
• - From thyroid gland, for cell metabolism and
  osteoblast activity
• Estrogens and Androgens
• - From gonads, for epiphyseal closure
• Calcitonin
• - From thyroid gland AND
• Parathyroid Hormone
• From parathyroid gland, to regulate calcium and
  phosphate levels in body fluids
• Affects bone composition

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Hormones for Bone Growth and Maintenance
Table 62
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Abnormalities

• Genetic/Physiological Abnormalities1. Giantism
  
• too much Growth hormone prior to epiphyseal
  closure, bones grow excessively large
• 2. Acromegaly
• - too much GH after closure, bones dont
• grow but all cartilage does
• - ribs, nose, ears, articular cartilage
• 3. Pituitary Dwarfism
• - not enough GH, bones fail to elongate

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Abnormalities

• Diet Related Abnormalities
• 1. Scurvy
• - lack of Vit. C
• - causes low collagen content, reduced bone
• mass, bones brittle
• 2. Osteomalacia
• - lack calcitriol, osteoid produced but
• not mineralized, bones flexible
• -Called Rickets in children and leads to
• permanent deformity

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A seven-year-old child has a pituitary tumor
involving the cells that secrete growth hormone
(GH), resulting in increased levels of GH. How
will this condition affect the childs growth?

1. The individual will be taller.
2. The individual will be shorter.
3. Growth of the individual will be erratic and
   slow.
4. Excessive growth will be limited to axial
   skeleton.

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Why does a child who has rickets have difficulty
walking?

1. Joints become fused, preventing movement.
2. Bones are brittle and break under body weight.
3. Bones are flexible and bend under body weight.
4. Motor skills are impaired.

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What effect would increased PTH secretion have on
blood calcium levels?

1. higher level of calcium
2. lower level of calcium
3. uncontrolled level of calcium
4. no effect on blood calcium, PTH effects calcium
   in the bones

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How does calcitonin help lower the calcium ion
concentration of blood?

1. by inhibiting osteoclast activity
2. by increasing the rate of calcium excretion at
   the kidneys
3. by increasing the rate of calcium uptake by
   intestinal cells
4. 1 and 2

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Types of fractures and how do they heal.
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Fractures

• Fractures
• cracks or breaks in bones
• caused by physical stress
• Bones break in response to excessive stress
• Bones are designed to heal
• Fractures are repaired in 4 steps

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Fracture Repair Step 1

• Bleeding
• produces a clot (fracture hematoma)
• Seals off dead osteocytes and broken blood vessels

Figure 615 (Step 1)
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Fracture Repair Step 2

• Cells of the endosteum and periosteum
• Divide and migrate into fracture zone
• Cells of Periosteum
• create external callus of fibrocartilage
• Cells of Endosteum
• create internal callus of spongy bone
• Calluses stabilize the break
• external callus of cartilage and bone surrounds
  break
• internal callus develops in marrow cavity

Figure 615 (Step 2)
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Fracture Repair Step 3

• Osteoblasts
• replace cartilage with spongy bone
• Fracture gap is now filled with all spongy bone

Figure 615 (Step 3)
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Fracture Repair Step 4

• A bulge from the callus marks the fracture point
• Osteoblasts and osteocytes remodel the fracture
  for up to a year
• Spongy bone is replaced with compact bone and
  excess callus material is removed

Figure 615 (Step 4)
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The effects of aging on the skeletal system.
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Effects of Aging

• Bones become thinner and weaker with age
• 1. Osteopenia reduction in bone mass
• All adults suffer in some degree
• Osteoclasts out-work osteoblast
• sex hormones in youth inhibit osteoclasts
• Women 8/decade after 40
• Men 3/decade after 40

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Effects of Aging

• 2. Osteoporosis reduction in bone mass that
  compromises function
• More common in women
• Over age 45, occurs in
• 29 of women
• 18 of men
• Thinner bones to start
• Greater rate of osteopenia

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Effects of Bone Loss

• The epiphyses, vertebrae, and jaws are most
  affected
• resulting in fragile limbs
• reduction in height
• tooth loss

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Hormones and Bone Loss

• Estrogens and androgens help maintain bone mass
• Bone loss in women accelerates after menopause

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Why is osteoporosis more common in older women
than in older men?

1. Testosterone levels decline in post-menopausal
   women.
2. Older women tend to be more sedentary than older
   men.
3. Declining estrogen levels lead to decreased
   calcium deposition.
4. In males, androgens increase with age.

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SUMMARY (1 of 2)

• Bone shapes, markings, and structure
• The matrix of osseous tissue
• Types of bone cells
• The structures of compact bone
• The structures of spongy bone
• The periosteum and endosteum
• Ossification and calcification
• Intramembranous ossification
• Endochondrial ossification

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SUMMARY (2 of 2)

• Blood and nerve supplies
• Bone minerals, recycling, and remodeling
• The effects of exercise
• Hormones and nutrition
• Calcium storage
• Fracture repair
• The effects of aging