Following chondrocyte hypertrophy and calcification in endochondral ossification, which event typically follows?

• A. Osteoid is secreted by osteoblasts.
• B. Primary ossification center forms.
• C. Periosteal bud invades.
• D. Growth continues with secondary centers.

Answer: (C)

When cartilage shaped by chondrocyte hypertrophy and calcification is ready to become bone, the next crucial move is vascular invasion. Periosteal buds—blood vessels from the periosteum along with osteoprogenitor cells—penetrate the calcified cartilage. This invasion brings osteoblasts to lay down osteoid and osteoclasts to remove calcified cartilage, kicking off the formation of the primary ossification center in the diaphysis. Without this vascular invasion, bone formation can’t begin in that region. After this step, bone tissue gradually replaces the cartilage, and later secondary centers form in the ends as growth continues.