Ch 6.4-Bone Formation
When does bone formation occur?
1) During embryonic development
2) During infancy, childhood, and adolescence
3) Remodeling of bone
4) Repair of fractures
Embryonic Bone Formation
Two Patterns
Intramembranous Ossification
Forms directly within mesenchyme
(Simple; few # of bones)
Endochondral Ossification
Forms within hyaline cartilage that
develops from mesenchyme
(Complex; most bones)
Intramembranous Ossification
STEP 1: Development of ossification center
Blood capillary
Ossification center
Mesenchymal cells cluster
Mesenchymal cell
Form osteogenic cells → osteoblasts
Osteoblast
Secrete organic bone EC matrix
Collagen fiber
Intramembranous Ossification
STEP 2: Calcification
Osteocyte
Osteocytes become fully developed
Extend cytoplasmic projections
Osteoblast
Newly calcified
bone EC matrix
Surrounding osteoblasts deposit
calcium and mineral salts
Intramembranous Ossification
STEP 3: Formation of trabeculae
Mesenchyme
condenses
Blood vessel
Ossification centers fuse to
form trabeculae
Spongy bone
trabeculae
Osteoblast
Blood vessels grow in
between
Intramembranous Ossification
STEP 4: Development of the periosteum
Periosteum
Mesenchyme condenses
Spongy bone tissue
Forms periosteum
Compact bone replaces surface
Compact bone tissue
layer of spongy bone
Endochondral Ossification
STEP 1: Development of cartilage model
Perichondrium
Proximal
epiphysis
Hyaline
cartilage
Mesenchymal cells form shape of
future bone
Diaphysis
Develop into chondroblasts
Distal
epiphysis
Secrete cartilage EC Matrix
Endochondral Ossification
STEP 2: Growth of cartilage model
Perichondrium
Chondroblasts deeply buried in cartilage
Uncalcified
extracellular matrix
Calcified
extracellular
matrix
Now called chondrocytes
Mid-region chondrocytes grow;
Surrounding EC matrix calcifies
Chrondrocytes begin to die,
leaving small cavities
Endochondral Ossification
STEP 3: Development of primary ossification center
Nutrient artery penetrates perichondrium
Periosteum
Osteogenic cells become osteoblasts
Nutrient artery
Primary
ossification center
Spongy bone
Blood vessels induce growth of primary
ossification center
Osteoblasts turn cartilage into bone
Endochondral Ossification
STEP 4: Development of the medullary cavity
Uncalcified
extracellular
matrix
Calcified
extracellular
matrix
Periosteum
Medullary cavity
Osteoclasts breakdown newly
formed spongy bone
Endochondral Ossification
STEP 5: Development of secondary ossification centers
Secondary
ossification
center
Epiphyseal
artery and vein
Uncalcified
extracellular
matrix
Blood vessels enter epiphyses
Secondary ossification centers develop
Proceeds outward
Nutrient artery and
vein
Endochondral Ossification
STEP 6: Formation of articular cartilage and epiphyseal plate
Articular cartilage
Spongy bone
Epiphyseal plate
Outer cartilage becomes articular cartilage
Cartilage remains between diaphysis and
epiphysis
Forms epiphyseal plate
Epiphyseal Plate
- Helps bones grow in length
- Contains a group of young
chondrocytes
- New chondrocytes form on
epiphyseal side
- Old chondrocytes form bone on
diaphyseal side
Epiphyseal Line
Bone Remodeling
- The ongoing replacement of old bone tissue by new bone tissue
Bone Resorption
Destruction of EC matrix
Bone Deposition
Formation of EC matrix
Fractures
Partial or Complete
Closed or Open
Fractures
Phagocytes remove
dead bone tissue
Chondroblasts form
fibrocartilage bridge
Fibrocartilage turned
to spongy bone
Bone Remodeling
Factors Affecting Bone Growth
- Mineral supply
- Calcium
- Phosphorus
- Magnesium
- Vitamins A, C, and D
- Hormones
- Human growth hormone (hGH): Before puberty
- Estrogens and Androgens: At puberty
- Weight-bearing exercise
Some stimulus disrupts
homeostasis by
Calcium Homeostasis
Decreasing
Blood calcium (Ca2+) level
Receptors
- Bones store 99% of body’s calcium
Parathyroid gland
cells detect lowered
Ca2+ level
Input
- Acts as a “buffer” for blood calcium levels
Control center
PTH gene “turned on”
- Ca released into blood by osteoclasts and removed
from blood by osteoblasts
Output
- Negative feedback loop
- Receptor: Parathyroid gland
- Control Center: Parathyroid Hormone (PTH) gene
- Effector: Osteoclasts and Kidneys
Increased production
of cyclic AMP
Increased
release of PTH
Effectors
Osteoclasts
Kidneys retain
increase bone Ca2+ in blood,
resorption
excrete
phosphate in
urine, and
produce
calcitriol
Increase in blood
Ca2+ level
Return to homeostasis
when response brings
blood Ca2+ level back to
normal
Osteoporosis
Risk Factors
-
Over 50 yrs old
Female
Low estrogen/testosterone
Excess alcohol consumption
Smoking
Inactivity
Soft drinks