2.2. Bone development

What is endochondral ossification?

The replacement of hyaline cartilage with bone tissue. Forms all bones below the skull except the clavicle. Begins month 2 in utero.

What is the blueprint used in endochondral ossification?

Hyaline cartilage serves as the template/blueprint for ossification.

What happens in step 1 of endochondral ossification?

The perichondrium becomes vascularised. Blood vessels supply nutrients causing mesenchymal cell differentiation into osteoblasts. Osteoblasts form a bone collar at the diaphysis wall and deposit osteoid. This is the primary ossification centre.

What happens in step 2 of endochondral ossification?

Chondrocytes in the central cavity enlarge, causing the matrix to calcify. Calcified matrix becomes impermeable to nutrients, causing cell death and forming a central clearing, supported by the bone collar. Healthy chondrocytes elsewhere cause elongation.

What happens in step 3 of endochondral ossification?

A periosteal bud (artery, vein, lymphatics, nerves, and osteogenic cells) invades the cavity. Osteoclasts degrade cartilage matrix while osteoblasts deposit new spongy bone. Bone continues to elongate.

What happens in step 4 of endochondral ossification?

The primary ossification centre enlarges. Osteoclasts break down newly formed spongy bone to form the medullary cavity. Cartilaginous growth continues only in the epiphyses, and bony epiphyseal surfaces begin to form.

What happens in step 5 of endochondral ossification?

A secondary ossification centre appears at one or both epiphyses (usually after birth). Epiphyses ossify. Cartilage remains only on the bone surface and at epiphyseal plates as articular cartilage.

Where do secondary ossification centres appear and when?

At one or both epiphyses, usually after birth. Larger bones may have two (e.g. humerus); short bones have one; irregular bones may have several.

What cartilage remains after endochondral ossification is complete?

Articular cartilage (on joint surfaces) and cartilage at the epiphyseal plates.

What is intramembranous ossification?

Direct ossification of mesenchymal cells within a membrane. Forms cranial bones and the clavicle — mostly flat bones. Occurs before week 8 in utero.

What is the key difference between endochondral and intramembranous ossification?

Endochondral ossification replaces a hyaline cartilage template. Intramembranous ossification forms bone directly from mesenchymal cells with no cartilage intermediate.

What happens in step 1 of intramembranous ossification?

Mesenchymal stem cells aggregate and differentiate into osteoblasts, forming an ossification centre. Osteoblasts begin secreting osteoid (unmineralised bone).

What happens in step 2 of intramembranous ossification?

Peripheral mesenchymal cells continue differentiating into osteoblasts and secrete osteoid inward. Osteoblasts become trapped and differentiate into osteocytes. Osteoid calcifies after several days to form hardened bone matrix.

What happens in step 3 of intramembranous ossification?

Osteoid is deposited randomly around embryonic blood vessels, forming finely woven trabeculae. Mesenchyme begins to differentiate into periosteum.

What happens in step 4 of intramembranous ossification?

Lamellar (compact) bone replaces woven bone at the outer edge. Internal spongy bone remains. Vascular tissue within trabecular spaces forms red bone marrow. Osteoblasts remain on the surface for remodelling.

When does ossification begin in embryonic development?

Week 6 or 7 of development.

What bones are formed by intramembranous ossification?

Cranial bones and the clavicle — mostly flat bones.

What bones are formed by endochondral ossification?

All bones below the skull except the clavicle.

What is osteoid?

Unmineralised bone matrix secreted by osteoblasts before it calcifies and hardens.

What is the primary ossification centre?

The starting point of ossification in the diaphysis during endochondral ossification, where osteoblasts first deposit osteoid.

What is the secondary ossification centre?

An ossification centre that appears in the epiphyses, usually after birth, during endochondral ossification.