Bone: Growth and Development

Ossification (Osteogenesis)

Process of bone tissue formation which forms the bony skeleton

When does ossification start

Begins in the 2nd month of development

When does postnatal bone growth end

Early adulthood

How long does bone remodeling and repair go on

Its life long

Endochondral ossification

Bone forms by replacing hyaline cartilage

Bones began as cartilage (endochondral) bones

What ossification process forms most of the skeleton

Endochondral

Intramembranous Ossification

Bone develops from fibrous membrane

What bones are formed by intramembranous ossification

Flat bones of skull (frontal, parietal, occipital, and temporal), clavicle, and part of the mandible in the fetus

What is something that intramembranous ossification does

It thickens long bones throughout life

What must happen prior to endochondral ossification taking place

The breakdown of the hyaline cartilage model

What is fibrous connective tissue made by for intramembranous ossification

Mesenchymal cells

What happens in intramembranous ossification fic this

Ossification centers appear, osteoid is secreted, woven bone and periosteum form, lamellar bone replaces woven bone and red marrow appears.

Postnatal bone growth: Interstitial (longitudinal) growth

Increase in length of long bones

Postnatal bone growth: Interstitial (longitudinal) growth

Increase in the length of long bones

Postnatal bone growth: Appositional growth

Increase in bone thickness

What does interstitial growth require

The presence of epiphyseal cartilage

What does the epiphyseal plate maintain

Constant thickness

• The rate of cartilage growth on one side balanced by bone replacement on the other.

What does the concurrent remodeling of epiphyseal ends do

It maintains its proportion

What are the 5 growth zones within cartilage

Resting (quiescent) zone

Proliferation (growth) zone

Hypertrophic zone

Calcification zone

Ossification (osteogenic) zone

Resting (quiescent) zone

Cartilage on epiphyseal side of epiphyseal plate

Relatively intact

Proliferation (growth) zone

Cartilage on diaphysis side of epiphyseal plate

Rapidly divide pushing epiphysis away from diaphysis= lengthening

Hypertrophic Zone

Older chondrocytes closet to diaphysis and their lacune enlarge and erode= interconnecting space

Calcification zone

Surrounding cartilage matrix calcifies, chondrocytes die and deteriorate.

Ossification Zone

Chondrocyte deterioration leaves long spicules of calcified cartilage at epiphysis-diaphysis junction

Spicules eroded by osteoclast

Ultimately replaced with spongy bone

What happens near the end of adolescence for interstitial growth

Chondroblast divide less often

Epiphyseal plate closure

Bone lengthening ceases

• Requires the presence of cartilage

Bone of epiphysis and diaphysis fuses

• Female: About 18 years old

• Male: About 21 years old

Growth Hormone

Hormone most important in stimulating epiphyseal plate activity in infancy and childhood

Thyroid Hormone

Modulates 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

Glucocorticoids

High amount increases bone loss (impairs growth at epiphyseal plates)

Serotonin

High levels prevent osteoblast differentiation (could lead to low bone density)

What can excess or deficits of any hormone cause

Abnormal skeletal growth

Achondroplastic dwarfism

Long bones stop growing in childhood

• Normal torso, short limbs

Failure of cartilage growth in metaphysis

Spontaneous mutation produced mutant dominant allele

Pituitary dwarfism

Lack of growth hormone

Normal proportions with short stature