Bone Development Human bones grow until about age 2

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Bone Development

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

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Calcification

• The process of depositing calcium salts
• Occurs during bone ossification and in other
  tissues

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Ossification

• The 2 main forms of ossification are
• intramembranous ossification
• endochondral ossification

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Intramembranous Ossification Step 1

• Mesenchymal cells aggregate
• differentiate into osteoblasts
• begin ossification at the ossification center
• develop projections called spicules

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Intramembranous Ossification Step 2
Figure 611 (Step 2)
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Intramembranous Ossification Step 2

• Blood vessels grow into the area
• to supply the osteoblasts
• Spicules connect
• trapping blood vessels inside bone

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Intramembranous Ossification Step 3
Figure 611 (Step 3)
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Intramembranous Ossification Step 3

• Spongy bone develops and is remodeled into
• osteons of compact bone
• periosteum
• or marrow cavities

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Endochondral Ossification

• Ossifies bones that originate as hyaline
  cartilage
• Most bones originate as hyaline cartilage

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Endochondral Ossification

• Growth and ossification of long bones occurs in 6
  steps

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Endochondral Ossification Step 1

• Chondrocytes in the center of hyaline cartilage
• enlarge
• form struts and calcify
• die, leaving cavities in cartilage

Figure 69 (Step 1)
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Endochondral Ossification Step 2
Figure 69 (Step 2)
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Endochondral Ossification Step 2

• Blood vessels grow around the edges of the
  cartilage
• Cells in the perichondrium change to osteoblasts
  
• producing a layer of superficial bone around the
  shaft which will continue to grow and become
  compact bone (appositional growth)

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Endochondral Ossification Step 3

• Blood vessels enter the cartilage
• bringing fibroblasts that become osteoblasts
• spongy bone develops at the primary ossification
  center

Figure 69 (Step 3)
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Endochondral Ossification Step 4

• Remodeling creates a marrow cavity
• bone replaces cartilage at the metaphyses

Figure 69 (Step 4)
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Endochondral Ossification Step 5

• Capillaries and osteoblasts enter the epiphyses
• creating secondary ossification centers

Figure 69 (Step 5)
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Endochondral Ossification Step 6
Figure 69 (Step 6)
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Endochondral Ossification Step 6

• Epiphyses fill with spongy bone
• cartilage within the joint cavity is articulation
  cartilage
• cartilage at the metaphysis is epiphyseal
  cartilage

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Endochondral Ossification

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

Endochondral Ossification
PLAY
Figure 69 (Step 2)
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What are 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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How does the skeletal system remodel and maintain
homeostasis, and what are the effects of
nutrition, hormones, exercise, and aging on bone?
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Remodeling

• The adult skeleton
• maintains itself
• replaces mineral reserves
• Remodeling
• recycles and renews bone matrix
• involves osteocytes, osteoblasts, and osteoclasts

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KEY CONCEPTS

• Bone continually remodels, recycles, and replaces
• Turnover rate varies
• If deposition is greater than removal, bones get
  stronger
• If removal is faster than replacement, bones get
  weaker

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Bone Degeneration

• Bone degenerates quickly
• Up to 1/3 of bone mass can be lost in a few weeks
  of inactivity

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KEY CONCEPTS

• What you dont use, you lose
• Stresses applied to bones during physical
  activity are essential to maintain bone strength
  and mass

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Effects of Hormones and Nutrition on Bone

• Normal bone growth and maintenance requires
  nutritional and hormonal factors

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Calcitriol

• The hormone calcitriol
• is made in the kidneys
• helps absorb calcium and phosphorus from
  digestive tract
• synthesis requires vitamin D3 (cholecalciferol)

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Vitamins

• Vitamin C is required for collagen synthesis, and
  stimulates osteoblast differentiation
• Vitamin A stimulates osteoblast activity
• Vitamins K and B12 help synthesize bone proteins

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Other Hormones

• Growth hormone and thyroxine stimulate bone
  growth
• Estrogens and androgens stimulate osteoblasts
• Calcitonin and parathyroid hormone regulate
  calcium and phosphate levels

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Calcium Regulation

• Calcium ions in body fluids
• must be closely regulated
• Homeostasis is maintained
• by calcitonin and parathyroid hormone
• which control storage, absorption, and excretion

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Calcitonin and Parathyroid Hormone Control

• Bones
• where calcium is stored
• Digestive tract
• where calcium is absorbed
• Kidneys
• where calcium is excreted

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Parathyroid Hormone (PTH)

• Produced by parathyroid glands in neck
• Increases calcium ion levels by
• stimulating osteoclasts
• increasing intestinal absorption of calcium
• decreases calcium excretion at kidneys

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Calcitonin

• Secreted by C cells (parafollicular cells) in
  thyroid
• Decreases calcium ion levels by
• inhibiting osteoclast activity
• increasing calcium excretion at kidneys

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Fractures

• Fractures
• cracks or breaks in bones
• caused by physical stress
• Fractures are repaired in 4 steps

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Fracture Repair Step 1
Figure 615 (Step 1)
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Fracture Repair Step 1

• Bleeding
• produces a clot (fracture hematoma)
• establishes a fibrous network
• Bone cells in the area die

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Fracture Repair Step 2
Figure 615 (Step 2)
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Fracture Repair Step 2

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

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Fracture Repair Step 3
Figure 615 (Step 3)
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Fracture Repair Step 3

• Osteoblasts
• replace central cartilage of external callus
• with spongy bone

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Fracture Repair Step 4
Figure 615 (Step 4)
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Fracture Repair Step 4

• Osteoblasts and osteocytes remodel the fracture
  for up to a year
• reducing bone calluses

Steps in the Repair of a Fracture
PLAY
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The Major Types of Fractures

• Potts fracture

Figure 616 (1 of 9)
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The Major Types of Fractures

• Comminuted fractures

Figure 616 (2 of 9)
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The Major Types of Fractures

• Transverse fractures

Figure 616 (3 of 9)
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The Major Types of Fractures

• Spiral fractures

Figure 616 (4 of 9)
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The Major Types of Fractures

• Displaced fractures

Figure 616 (5 of 9)
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The Major Types of Fractures

• Colles fracture

Figure 616 (6 of 9)
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The Major Types of Fractures

• Greenstick fracture

Figure 616 (7 of 9)
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The Major Types of Fractures

• Epiphyseal fractures

Figure 616 (8 of 9)
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The Major Types of Fractures

• Compression fractures

Figure 616 (9 of 9)
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What are the effects of aging on the skeletal
system?
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Age and Bones

• Bones become thinner and weaker with age
• Osteopenia begins between ages 30 and 40
• Women lose 8 of bone mass per decade, men 3

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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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Osteoporosis

• Severe bone loss
• Affects normal function
• Over age 45, occurs in
• 29 of women
• 18 of men

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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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Cancer and Bone Loss

• Cancerous tissues release osteoclast-activating
  factor
• that stimulates osteoclasts
• and produces severe osteoporosis