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
• The 2 main forms of ossification are:
– intramembranous ossification
– endochondral ossification
Intramembranous
Ossification: Step 1
• Mesenchymal cells aggregate:
– differentiate into osteoblasts
– begin ossification at the ossification center
– develop projections called spicules
Intramembranous
Ossification: Step 2
Figure 6–11 (Step 2)
Intramembranous
Ossification: Step 2
• Blood vessels grow into the area:
– to supply the osteoblasts
• Spicules connect:
– trapping blood vessels inside bone
Intramembranous
Ossification: Step 3
Figure 6–11 (Step 3)
Intramembranous
Ossification: Step 3
• Spongy bone develops and is remodeled
into:
– osteons of compact bone
– periosteum
– or marrow cavities
Endochondral Ossification
• Ossifies bones that originate as hyaline
cartilage
• Most bones originate as hyaline cartilage
Endochondral Ossification
• Growth and ossification of long bones
occurs in 6 steps
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 2
Figure 6–9 (Step 2)
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
• 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
• Remodeling creates a marrow
cavity:
– bone replaces cartilage at the
metaphyses
Figure 6–9 (Step 4)
Endochondral
Ossification: Step 5
• Capillaries and osteoblasts
enter the epiphyses:
– creating secondary
ossification centers
Figure 6–9 (Step 5)
Endochondral
Ossification: Step 6
Figure 6–9 (Step 6)
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 Ossification
• Appositional growth:
– compact bone thickens and
strengthens long bone with
layers of circumferential
lamellae
PLAY
Endochondral Ossification
Figure 6–9 (Step 2)
What are the
characteristics of adult
bones?
Epiphyseal Lines
Figure 6–10
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?
Remodeling
• 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 degenerates quickly
• Up to 1/3 of bone mass can be lost in a
few weeks of inactivity
KEY CONCEPTS
• 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
• Normal bone growth and maintenance
requires nutritional and hormonal factors
Calcitriol
• The hormone calcitriol:
– is made in the kidneys
– helps absorb calcium and phosphorus from
digestive tract
– synthesis requires vitamin D3 (cholecalciferol)
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
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 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
• Bones:
– where calcium is stored
• Digestive tract:
– where calcium is absorbed
• Kidneys:
– where calcium is excreted
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
Calcitonin
• Secreted by C cells (parafollicular cells) in
thyroid
• Decreases calcium ion levels by:
– inhibiting osteoclast activity
– increasing calcium excretion at kidneys
Fractures
• Fractures:
– cracks or breaks in bones
– caused by physical stress
• Fractures are repaired in 4 steps
Fracture Repair: Step 1
Figure 6–15 (Step 1)
Fracture Repair: Step 1
• Bleeding:
– produces a clot (fracture hematoma)
– establishes a fibrous network
• Bone cells in the area die
Fracture Repair: Step 2
Figure 6–15 (Step 2)
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
Figure 6–15 (Step 3)
Fracture Repair: Step 3
• Osteoblasts:
– replace central cartilage of external callus
– with spongy bone
Fracture Repair: Step 4
Figure 6–15 (Step 4)
Fracture Repair: Step 4
• Osteoblasts and osteocytes remodel the
fracture for up to a year:
– reducing bone calluses
PLAY
Steps in the Repair of a Fracture
The Major Types of Fractures
• Pott’s fracture
Figure 6–16 (1 of 9)
The Major Types of Fractures
• Comminuted fractures
Figure 6–16 (2 of 9)
The Major Types of Fractures
• Transverse fractures
Figure 6–16 (3 of 9)
The Major Types of Fractures
• Spiral fractures
Figure 6–16 (4 of 9)
The Major Types of Fractures
• Displaced fractures
Figure 6–16 (5 of 9)
The Major Types of Fractures
• Colles’ fracture
Figure 6–16 (6 of 9)
The Major Types of Fractures
• Greenstick fracture
Figure 6–16 (7 of 9)
The Major Types of Fractures
• Epiphyseal fractures
Figure 6–16 (8 of 9)
The Major Types of Fractures
• Compression fractures
Figure 6–16 (9 of 9)
What are the effects of
aging on the skeletal
system?
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
• The epiphyses, vertebrae, and jaws are
most affected:
– resulting in fragile limbs
– reduction in height
– tooth loss
Osteoporosis
• Severe bone loss
• Affects normal function
• Over age 45, occurs in:
– 29% of women
– 18% of men
Hormones and Bone Loss
• Estrogens and androgens help maintain
bone mass
• Bone loss in women accelerates after
menopause
Cancer and Bone Loss
• Cancerous tissues release osteoclastactivating factor:
– that stimulates osteoclasts
– and produces severe osteoporosis