Proportion five zones in cartilage resting quiescent

proportion
•
Five zones in cartilage
–
Resting (quiescent) zone
–
Proliferation (growth) zone
–
Hypertrophic zone
–
Calcification zone
–
Ossification (osteogenic) zone

Interstitial Growth:
Growth in Length of Long Bones
•
Resting zone
–
Cartilage on epiphyseal side of epiphyseal plate
–
Relatively inactive
•
Proliferation (growth) zone
–
Cartilage on diaphysis side of epiphyseal plate
–
Rapidly divide pushing epiphysis away from diaphysis
 lengthening
•
Hypertrophic zone
–
Older chondrocytes closer to diaphysis and their
lacunae enlarge and erode  interconnecting spaces

Interstitial Growth:
Growth in Length of Long Bones
•
Calcification zone
–
Cartilage matrix calcifies, chondrocytes die
and deteriorate
•
Ossification zone
–
Chondrocyte deterioration leaves spikes of
calcified cartilage at epiphysis-diaphysis
junction
–
Spikes eroded by osteoclasts
–
Covered with new bone by osteoblasts
–
Then replaced with spongy bone

Figure 6.10 Growth in length of a long bone occurs at the epiphyseal plate.
Resting zone
1 Proliferation zone
Cartilage cells undergo
mitosis.
2 Hypertrophic zone
Older cartilage cells
enlarge.
3 Calcification zone
Matrix calcifies; cartilage
cells die; matrix begins
deteriorating; blood
vessels invade
cavity.
4 Ossification zone
New bone forms.
Calcified
cartilage spicule
Osteoblast
depositing bone
matrix
Osseous tissue
(bone) covering
cartilage spicules

Interstitial Growth:
Growth in Length of Long Bones
•
Near end of adolescence chondroblasts
divide less often
•
Epiphyseal plate thins then is replaced by
bone
•
Epiphyseal plate closure
–
Bone lengthening ceases
•
Requires presence of cartilage
–
Bone of epiphysis and diaphysis fuses
–
Females – about 18 years
–
Males – about 21 years

Appositional Growth: Growth in Width
•
Allows lengthening bone to widen
•
Occurs throughout life
•
Osteoblasts beneath periosteum secrete
bone matrix on external bone
•
Osteoclasts remove bone on endosteal
surface
•
Usually more building up than breaking
down

 Thicker, stronger bone but not too heavy

Figure 6.11 Long bone growth and remodeling during youth.
Bone growth
Bone remodeling
Cartilage
grows here.
Bone
replaces
cartilage
here.
Cartilage
grows here.
Bone replaces
cartilage here.
Articular cartilage
Epiphyseal plate
Bone that was
here has been
resorbed.
Appositional
growth adds
bone here.
Bone that was
here has been
resorbed.

Hormonal Regulation of Bone Growth
•
Growth hormone (GH)
–
Most important in stimulating epiphyseal plate activity in infancy
and childhood
•
Thyroid hormone (TH)
–
Controls activity of growth hormone
–
Ensures proper proportions
•
Testosterone (males) and estrogens (females) at
puberty
–
Promote adolescent growth spurts
–
End growth by inducing epiphyseal plate closure
•
Excesses or deficits of any of these cause abnormal
skeletal growth

Bone Homeostasis: Bone Remodeling
•
Consists of both bone deposit and bone
resorption
•
Occurs at surfaces of both periosteum and
endosteum
•
Remodeling units
–
Adjacent osteoblasts and osteoclasts

Bone Deposit
•