Endochondral and intramembranous ossification

Introduction

Bone forms in two main ways: intramembranous (direct from mesenchyme) and endochondral (through a cartilage model). Both produce the same type of bone in the end, but the steps are quite different.

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Intramembranous Ossification

Where it happens

• Flat bones of skull
• Mandible
• Clavicle
• Maxilla
• Growth of short bones, thickning of long bones

Steps

1. Mesenchymal condensation Mesenchymal cells cluster and differentiate into osteoblasts.
2. Osteoid secretion Osteoblasts secrete unmineralised matrix (osteoid).
3. MineralisationCalcium phosphate crystals deposit into the osteoid.
4. Formation of trabeculae Osteoblasts trap themselves → become osteocytes. Network of woven bone appears.
5. Periosteum formation Outer mesenchymal cells form periosteum; bone thickens.
6. Remodelling to lamellar bone Woven bone → lamellar bone. Spaces become marrow cavities.

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Endochondral Ossification

Where it happens

• Long bones (femur, tibia, humerus)
• Vertebrae
• Base of skull

Steps

1. Cartilage model formation Hyaline cartilage template forms first.
2. Cartilage grows Chondrocytes divide and matrix expands.
3. Hypertrophy and matrix calcification Chondrocytes enlarge, release alkaline phosphatase → matrix calcifies.
4. Blood vessels invade VEGF attracts vessels. Perichondrium becomes periosteum. Osteoblasts arrive.
5. Primary ossification centre Osteoblasts deposit bone on calcified cartilage. Happens in the diaphysis.
6. Secondary ossification centres Appear later in epiphyses (ends of bone).
7. Formation of growth plate Layer of cartilage remains between diaphysis and epiphysis → enables bone lengthening.
8. Remodelling Woven bone is replaced by lamellar bone; marrow cavity expands.

Bibriography

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RNDr. Lucie Fraser Lantová, Ph.D. Ossification