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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. Calcification. The process of depositing calcium salts Occurs during bone ossification and in other tissues. Ossification.

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bone development
Bone Development
  • Human bones grow until about age 25
  • Osteogenesis:
    • bone formation
  • Ossification:
    • the process of replacing other tissues with bone
  • The process of depositing calcium salts
  • Occurs during bone ossification and in other tissues
  • The 2 main forms of ossification are:
    • intramembranous ossification
    • endochondral ossification
intramembranous ossification step 1
Intramembranous Ossification: Step 1
  • Mesenchymal cells aggregate:
    • differentiate into osteoblasts
    • begin ossification at the ossification center
    • develop projections called spicules
intramembranous ossification step 21
Intramembranous Ossification: Step 2
  • Blood vessels grow into the area:
    • to supply the osteoblasts
  • Spicules connect:
    • trapping blood vessels inside bone
intramembranous ossification step 31
Intramembranous Ossification: Step 3
  • Spongy bone develops and is remodeled into:
    • osteons of compact bone
    • periosteum
    • or marrow cavities
endochondral ossification
Endochondral Ossification
  • Ossifies bones that originate as hyaline cartilage
  • Most bones originate as hyaline cartilage
endochondral ossification1
Endochondral Ossification
  • Growth and ossification of long bones occurs in 6 steps
endochondral ossification step 1
Endochondral Ossification: Step 1
  • Chondrocytes in the center of hyaline cartilage:
    • enlarge
    • form struts and calcify
    • die, leaving cavities in cartilage

Figure 6–9 (Step 1)

endochondral ossification step 21
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)
endochondral ossification step 3
Endochondral Ossification: Step 3
  • Blood vessels enter the cartilage:
    • bringing fibroblasts that become osteoblasts
    • spongy bone develops at the primary ossification center

Figure 6–9 (Step 3)

endochondral ossification step 4
Endochondral Ossification: Step 4
  • Remodeling creates a marrow cavity:
    • bone replaces cartilage at the metaphyses

Figure 6–9 (Step 4)

endochondral ossification step 5
Endochondral Ossification: Step 5
  • Capillaries and osteoblasts enter the epiphyses:
    • creating secondary ossification centers

Figure 6–9 (Step 5)

endochondral ossification step 61
Endochondral Ossification: Step 6
  • Epiphyses fill with spongy bone:
    • cartilage within the joint cavity is articulation cartilage
    • cartilage at the metaphysis is epiphyseal cartilage
endochondral ossification2
Endochondral Ossification
  • Appositional growth:
    • compact bone thickens and strengthens long bone with layers of circumferential lamellae


Endochondral Ossification

Figure 6–9 (Step 2)

epiphyseal lines
Epiphyseal Lines

Figure 6–10

epiphyseal lines1
Epiphyseal Lines
  • When long bone stops growing, after puberty:
    • epiphyseal cartilage disappears
    • is visible on X-rays as an epiphyseal line
How does the skeletal system remodel and maintain homeostasis, and what are the effects of nutrition, hormones, exercise, and aging on bone?
  • The adult skeleton:
    • maintains itself
    • replaces mineral reserves
  • Remodeling:
    • recycles and renews bone matrix
    • involves osteocytes, osteoblasts, and osteoclasts
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
bone degeneration
Bone Degeneration
  • Bone degenerates quickly
  • Up to 1/3 of bone mass can be lost in a few weeks of inactivity
key concepts1
  • What you don’t use, you lose
  • Stresses applied to bones during physical activity are essential to maintain bone strength and mass
effects of hormones and nutrition on bone
Effects of Hormones and Nutrition on Bone
  • Normal bone growth and maintenance requires nutritional and hormonal factors
  • The hormone calcitriol:
    • is made in the kidneys
    • helps absorb calcium and phosphorus from digestive tract
    • synthesis requires vitamin D3(cholecalciferol)
  • Vitamin C is required for collagen synthesis, and stimulates osteoblast differentiation
  • Vitamin A stimulates osteoblast activity
  • Vitamins K and B12 help synthesize bone proteins
other hormones
Other Hormones
  • Growth hormone and thyroxine stimulate bone growth
  • Estrogens and androgens stimulate osteoblasts
  • Calcitonin and parathyroid hormone regulate calcium and phosphate levels
calcium regulation
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
calcitonin and parathyroid hormone control
Calcitonin and Parathyroid Hormone Control
  • Bones:
    • where calcium is stored
  • Digestive tract:
    • where calcium is absorbed
  • Kidneys:
    • where calcium is excreted
parathyroid hormone pth
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
  • Secreted by C cells(parafollicular cells) in thyroid
  • Decreases calcium ion levels by:
    • inhibiting osteoclast activity
    • increasing calcium excretion at kidneys
  • Fractures:
    • cracks or breaks in bones
    • caused by physical stress
  • Fractures are repaired in 4 steps
fracture repair step 1
Fracture Repair: Step 1

Figure 6–15 (Step 1)

fracture repair step 11
Fracture Repair: Step 1
  • Bleeding:
    • produces a clot (fracture hematoma)
    • establishes a fibrous network
  • Bone cells in the area die
fracture repair step 2
Fracture Repair: Step 2

Figure 6–15 (Step 2)

fracture repair step 21
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
fracture repair step 3
Fracture Repair: Step 3

Figure 6–15 (Step 3)

fracture repair step 31
Fracture Repair: Step 3
  • Osteoblasts:
    • replace central cartilage of external callus
    • with spongy bone
fracture repair step 4
Fracture Repair: Step 4

Figure 6–15 (Step 4)

fracture repair step 41
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

the major types of fractures1
The Major Types of Fractures
  • Comminuted fractures

Figure 6–16 (2 of 9)

the major types of fractures2
The Major Types of Fractures
  • Transverse fractures

Figure 6–16 (3 of 9)

the major types of fractures3
The Major Types of Fractures
  • Spiral fractures

Figure 6–16 (4 of 9)

the major types of fractures4
The Major Types of Fractures
  • Displaced fractures

Figure 6–16 (5 of 9)

the major types of fractures5
The Major Types of Fractures
  • Colles’ fracture

Figure 6–16 (6 of 9)

the major types of fractures6
The Major Types of Fractures
  • Greenstick fracture

Figure 6–16 (7 of 9)

the major types of fractures7
The Major Types of Fractures
  • Epiphyseal fractures

Figure 6–16 (8 of 9)

the major types of fractures8
The Major Types of Fractures
  • Compression fractures

Figure 6–16 (9 of 9)

age and bones
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%
effects of bone loss
Effects of Bone Loss
  • The epiphyses, vertebrae, and jaws are most affected:
    • resulting in fragile limbs
    • reduction in height
    • tooth loss
  • Severe bone loss
  • Affects normal function
  • Over age 45, occurs in:
    • 29% of women
    • 18% of men
hormones and bone loss
Hormones and Bone Loss
  • Estrogens and androgens help maintain bone mass
  • Bone loss in women accelerates after menopause
cancer and bone loss
Cancer and Bone Loss
  • Cancerous tissues release osteoclast-activating factor:
    • that stimulates osteoclasts
    • and produces severe osteoporosis