Ossification is the process by which fontanelles close over by 18 to 24 months. What process causes the fontanelles to close by 18-24 months?

• A. Calcification
• B. Tendon formation
• C. Fracture
• D. Ossification

Answer: (D)

The closing of fontanelles is driven by ossification of the cranial membranes, specifically intramembranous ossification. In this process, the mesenchymal tissue within the fontanelle differentiates directly into osteoblasts and lays down bone matrix that mineralizes to form solid bone. As the cranial bones ossify and the sutures fuse, the fontanelles gradually disappear, typically by about 18 to 24 months. Calcification is part of bone hardening that accompanies ossification, but the essential mechanism that converts the soft membranous area into bone is ossification. Tendon formation or fractures do not account for this normal developmental change.