Bone Ossification & Growth

Ossification (osteogenesis)

The process of bone formation.

Formation of bony skeleton starts and ends

2 month of fetal development, continues through early adulthood

remodeling: throughout life

Two major types of ossification

Endochondral ossification and intramembranous ossification.

Endochondral ossification

Bone develops by replacing a hyaline cartilage model.

Intramembranous ossification

Bone develops within a fibrous connective tissue membrane.

Primary ossification center

First major region of bone formation; develops in the diaphysis.

Secondary ossification centers

Ossification centers that later develop in the epiphyses.

Fetal primary ossification centers

Primary centers appearing prenatally in long bones, ribs, and skull.

What does endochondral ossification begin with?

A hyaline cartilage model of the future bone.

Step 1 of endochondral ossification

A bone collar forms around the diaphysis of hyaline cartilage.

Bone collar

Early bone tissue forming around the diaphysis of cartilage model.

Location of primary ossification center

In the diaphysis.

Step 2 of endochondral ossification

Cartilage in central diaphysis calcifies and develops cavities.

What happens to diaphysis cartilage in Step 2?

Its matrix calcifies and develops cavities.

Step 3 of endochondral ossification

Periosteal bud invades internal cavities and spongy bone forms.

Periosteal bud

Vascular structure invading developing bone cavities in endochondral ossification.

What forms after periosteal bud enters internal cavities?

Spongy bone.

Step 4 of endochondral ossification

Diaphysis elongates, medullary cavity forms, and secondary centers appear.

Medullary cavity

hollow space inside the shaft of a long bone- forms during step 4 of endochondral ossification

Where do secondary ossification centers form?

In the epiphyses.

Where does the medullary cavity develop?

Within the diaphysis.

Step 5 of endochondral ossification

The epiphyses ossify.

Where does hyaline cartilage remain after epiphyses ossify?

In epiphyseal plates and articular cartilages.

Endochondral ossification sequence

Bone collar → Cavities calcify → Periosteal bud → Medullary cavity → Epiphyses ossify.

Secondary ossification center location

Epiphysis.

Which appears first: primary or secondary center?

Primary ossification center.

What remains between diaphysis and epiphysis during growth?

Epiphyseal plate cartilage.

What remains over the surface of the epiphysis?

Articular cartilage.

Initial tissue: Endochondral vs. Intramembranous

Endochondral starts with cartilage; intramembranous starts with fibrous connective tissue membrane

Step 1 of intramembranous ossification

Ossification centers appear in fibrous connective tissue membrane.

Early cells at intramembranous ossification center

Mesenchymal cells and osteoblasts.

Structures at early intramembranous ossification center

Collagen fibers and osteoid.

Step 2 of intramembranous ossification

Osteoid is secreted within fibrous membrane and calcifies.

What cells secrete osteoid?

Osteoblasts.

What happens to secreted osteoid?

It calcifies.

Mature bone cell trapped after matrix calcifies

Osteocyte.

Step 3 of intramembranous ossification

Woven bone and periosteum form.

Structures forming early woven bone

Trabeculae of woven bone.

What forms periosteum during intramembranous ossification?

Condensed surrounding mesenchyme.

Step 4 of intramembranous ossification

Lamellar bone replaces woven bone and red marrow appears.

What replaces woven bone during intramembranous ossification?

Lamellar bone.

Type of bone developing just deep to periosteum

A plate of compact bone.

What do spongy bone cavities contain?

Red marrow.

Intramembranous ossification sequence

Ossification centers → Osteoid calcifies → Woven bone/periosteum → Lamellar bone.

What is the first bone that develops during intramembranous ossification

Woven bone.

Where does long bone growth in length occur?

At the epiphyseal plate.

Resting zone of epiphyseal plate

Inactive cartilage zone anchoring plate to epiphysis.

Proliferation zone of epiphyseal plate

Zone where cartilage cells rapidly undergo mitosis.

Hypertrophic zone of epiphyseal plate

Zone where older cartilage cells enlarge.

Calcification zone of epiphyseal plate

Matrix calcifies, chondrocytes die, and blood vessels invade.

Ossification zone of epiphyseal plate

Osteoblasts deposit bone matrix over calcified cartilage spicules.

Epiphyseal plate zones in order

Resting → Proliferation → Hypertrophic → Calcification → Ossification.

Mechanism of longitudinal bone growth

Cartilage grows on epiphyseal side while diaphyseal cartilage is replaced by bone.

Appositional growth

Growth adding bone to the outside, increasing bone diameter/thickness.

Bone remodeling

continuous process where old bone is broken down and replaced with new bone throughout life. Bone is living tissue, not a fixed structure.

Bone resorption

Removal or breakdown of existing bone tissue.

Cell responsible for bone resorption

Osteoclast.

Cell responsible for forming new bone

Osteoblast.

Primary ossification center

shaft/diaphysis

Secondary ossification centers

ends/epiphyses

Articular cartilage

Cartilage not replaced by bone at ends of bone where joint forms

Epiphyseal plate (growth plate)

cartilage stays, allows long bone to keep growing in length