Intramembranous ossification eg the bones of the cranium

The flat bones of the skull begin as highly vascular-ized sheets of embryonic tissue. Undifferentiated cells within these sheets differentiate directly into osteo-blasts and form a radiating network of bony spicules lying parallel to the surface of the brain. During growth, successive generations of osteoblasts add new bone to the outside and periphery of this structure, while osteoclasts resorb from the inner surface to maintain proportional thickness and shape.

Cartilage

Rapidly dividing cell layer

Rapidly dividing cell layer

Cartilage

Hypertrophied chondrocytes

Primary bone collar

Marrow cavity

Periosteum

Osteoclasts

Hypertrophied chondrocytes

Primary bone collar

Marrow cavity

Periosteum

(C)

Figure 2 The principal stages of embryonic long-bone growth.

Rapidly dividing chondrocytes add new cells to growth plate

Maturing chondrocytes expand and synthesize matrix

Hypertrophic chondrocytes calcify their matrix

Osteoclasts invade calcified cartilage, leaving vertical bars of material as a framework for subsequent bone formation

Figure 3 Continued longitudinal growth at the growth plate.

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