Bone Development and Growth: Intramembranous & Endochondral Ossification, Epiphyseal Plates, Hormonal Regulation

intramembranous ossification

Bone develops from a fibrous membrane

Endochondral ossification

Bone develops from a cartilage model

Mesenchyme

Embryonic tissue that gives rise to bone during ossification

Bone formed via intramembranous ossification

Flat bones of the skull, part of the mandible, most of the clavicle

Bone formed via endochondral ossification

Most bones in the body (especially long bones)

First step of intramembranous ossification

Mesenchyme condenses and osteoblasts secrete osteoid

Second step of intramembranous ossification

Osteoid calcifies and traps osteocytes

Third step of intramembranous ossification

Periosteum forms on bone surface

Fourth step of intramembranous ossification

Compact bone is deposited on surface layers

First step of endochondral ossification

Mesenchyme forms hyaline cartilage model

Second step of endochondral ossification

Bone collar forms around diaphysis of cartilage model

Third step of endochondral ossification

Central cartilage in diaphysis calcifies and forms cavities

Fourth step of endochondral ossification

Periosteal bud invades cavities, forming spongy bone

Fifth step of endochondral ossification

Diaphysis elongates, medullary cavity forms, secondary ossification centers appear

Sixth step of endochondral ossification

Epiphyses ossify

Interstitial growth

Bone growth in length

Appositional growth

Bone growth in width or thickness

Location of interstitial growth

At the epiphyseal plate

Epiphyseal plate

Growth zone in long bones made of hyaline cartilage

Epiphyseal plate closure

Occurs when epiphysis and diaphysis fuse, stopping bone growth

Typical epiphyseal plate closure age in females

Around 18 years

Typical epiphyseal plate closure age in males

Around 21 years

Zone 1 of epiphyseal plate

Resting zone

Zone 2 of epiphyseal plate

Proliferation (growth) zone

Zone 3 of epiphyseal plate

Hypertrophic zone

Zone 4 of epiphyseal plate

Calcification zone

Zone 5 of epiphyseal plate

Ossification (osteogenic) zone

Main hormone regulating childhood bone growth

Growth hormone (GH)

Gland that produces growth hormone

Anterior pituitary

Hormone that modulates growth hormone

Thyroid hormone

Hormones responsible for adolescent growth spurts

Estrogen and testosterone

Effect of sex hormones on epiphyseal plates

Cause closure and end of growth

Too much growth hormone

Gigantism (e.g., Robert Wadlow)

Too little growth hormone

Dwarfism or stunted growth

Achondroplastic dwarfism cause

Chondrocytes in proliferation and hypertrophic zones fail to multiply

Why achondroplasia affects limbs but not head

Skull bones grow via intramembranous ossification

Appositional growth process

Osteoblasts lay down matrix on bone surface, forming circumferential lamellae

Function of periosteal bud

Invades cartilage cavity and brings in blood vessels and osteogenic cells

Compact bone

Dense outer layer of bone

Spongy bone

Inner honeycomb-like structure made of trabeculae

Diaphysis

Shaft of a long bone

Epiphysis

Ends of a long bone

Epiphyseal line

Remnant of closed epiphyseal plate in adults

Periosteum

Outer membrane surrounding bone

Endosteum

Inner membrane lining bone cavities

Ossification

The process of bone formation

Postnatal bone growth stops

When epiphyseal plates close

Circumferential lamellae

Surface layers of bone formed during appositional growth

Mechanical stress on bone

Stimulates appositional growth

Hormones that regulate postnatal bone growth

Growth hormone, thyroid hormone, sex hormones