Which sequence correctly describes intramembranous ossification?

• A. Mesenchymal cells differentiate into osteoblasts; periosteum forms around the new bone; ossification centers form; osteoid is secreted; woven bone forms.
• B. Mesenchymal cells differentiate into osteoblasts; ossification centers form; osteoid is secreted; woven bone forms and is later replaced by lamellar bone; periosteum forms around it.
• C. Chondrocytes hypertrophy; cartilage model develops; periosteal bud invades; primary ossification center forms.
• D. Osteoblasts differentiate into osteoclasts; direct formation of lamellar bone without woven bone.

Answer: (B)

Intramembranous ossification builds bone directly from mesenchyme without a cartilage template. Mesenchymal cells cluster and differentiate into osteoblasts, which begin secreting osteoid. The mineralization of this osteoid forms woven bone with a disorganized collagen pattern, creating early bone tissue. As this woven bone forms, a periosteum develops around the newly formed bone surface, and the woven bone is later remodeled into mature lamellar bone. This sequence—mesenchyme to osteoblasts, ossification centers, osteoid secretion, woven bone formation, then replacement by lamellar bone with periosteum formation—is exactly what option describing the correct order conveys. The other descriptions mix in cartilage models or incorrect cell transitions and skip the woven bone stage, which is not how intramembranous ossification proceeds.