Endochondral Ossification - Primary Center of Ossification

Primary Center of Ossification

The first site of ossification occurs in the primary center of ossification, which is in the middle of diaphysis (shaft). Then:

1. Formation of periosteum

   The perichondrium becomes the periosteum. The periosteum contains a layer of undifferentiated cells (osteoprogenitor cells) which later become osteoblasts.

2. Formation of bone collar

   The osteoblasts secrete osteoid against the shaft of the cartilage model (Appositional Growth). This serves as support for the new bone.

3. Calcification of matrix

   Chondrocytes in the primary center of ossification begin to grow (hypertrophy). They stop secreting collagen and other proteoglycans and begin secreting alkaline phosphatase, an enzyme essential for mineral deposition. Then calcification of the matrix occurs and apoptosis of the hypertrophic chondrocytes occurs. This creates cavities within the bone. The exact mechanism of chondrocyte hypertrophy and apoptosis is currently unknown.

4. Invasion of periosteal bud

   The hypertrophic chondrocytes (before apoptosis) secrete Vascular Endothelial Cell Growth Factor that induces the sprouting of blood vessels from the perichondrium. Blood vessels forming the periosteal bud invade the cavity left by the chondrocytes and branch in opposite directions along the length of the shaft. The blood vessels carry hemopoietic cells, osteoprogenitor cells and other cells inside the cavity. The hemopoietic cells will later form the bone marrow.

5. Formation of trabeculae

   Osteoblasts, differentiated from the osteoprogenitor cells that entered the cavity via the periosteal bud, use the calcified matrix as a scaffold and begin to secrete osteoid, which forms the bone trabecula. Osteoclasts, formed from macrophages, break down spongy bone to form the medullary (bone marrow) cavity.