Bone Development & Growth
Bone Growth Video Segment
Hyaline Cartilage – the most common cartilage
Chondrocytes =
cartilage cells
Lacunae =
pockets that
house
chondrocytes
Cartilage is
avascular
(without a blood
supply)
The Early Embryonic Skeleton
Bone
develops
later →
• First skeleton composed of cartilage
• Replaced with bone (ossification); starting 6 weeks
after fertilization
• More than 300 bones at birth → fuse to 206
Bone Growth
2 types:
• Interstitial – in length
at epiphyseal plate; when
growth ceases, bone
shows epiphyseal lines
• Appositional – in width;
new bone is added to the
outer surface by
osteoblasts from
periosteum
Bone Remodeling
= replacement of old bone by new
• bone is very metabolically (chemically) active
• bone is remodeled along the lines of mechanical stress
(remember to exercise; exercise increases bone
mass)
• different rates in different regions
• distal head of the femur is replaced ~ every 4
months
Bone Nutrition
• Minerals
– Calcium (dairy, leafy vegetables, tofu; the most
abundant mineral in the human body)
– Phosphorus (dairy, lean meat, fish)
• Vitamins
– A – stimulates osteoblasts
– C – needed for collagen synthesis
– D – needed to make calcitriol (hormone that increases
intestinal absorption of Ca++)
– B12 & K – needed for synthesis of bone proteins
Homeostatic Imbalance:
Rickets
Homeostatic
Imbalances
• Disease of children due to
lack of vitamin D
• Calcium is not deposited in
bones
• Bones become soft &
deformed
Rickets
Hormonal Regulation
• Growth hormone
• responsible for general growth of all body tissues
• Sex hormones (testosterone and estrogen) stimulate
osteoblasts
• Calcitonin & parathyroid hormone regulate Ca++ level
(calcitonin ↓ [Ca++ ] while parathyroid hormone ↑ [Ca++ ] in the
blood)
• Calcitriol ↑ Ca++ level
Calcium Homeostasis
• Bone serves as a “buffer” to prevent sudden changes
in blood Ca++ levels
• too much blood Ca++ (hypercalcemia) or too little
(hypocalcemia) can cause the heart to stop working
Another Example of Homeostatic Imbalance:
Osteoporosis
Homeostatic Imbalances
• Decline in bone density
• Increased risk of fracture
Possible causes:
–
–
–
–
–
Lack of estrogen in women
Lack of exercise to stress the bones
Inadequate intake of calcium and phosphorus
Abnormalities of vitamin D metabolism
Loss of muscle mass
Osteoporosis
Bone Fractures
• Fracture = break of the bone
• Repair of bone:
– Spongy bone forms in first few days
– Blood vessels regrow and spongy bone hardens
– Full healing takes 1-2 months
Steps in the
Repair of a
Fracture
Fig. 6.9