Intramembrános

Intramembranous ossification, sometimes referred to by the base term intramembranous bone formation, is a mode of bone development in which bone tissue forms directly from mesenchymal tissue without an intermediate cartilage stage. It primarily makes the flat bones of the skull vault, the facial bones, and the clavicles, and it contrasts with endochondral ossification, in which bones develop by replacing a cartilage template.

The process begins when mesenchymal cells in a region condense and differentiate into osteoblasts, creating one or more ossification centers. Osteoblasts secrete osteoid, which mineralizes to form woven bone. As the centers expand, surrounding mesenchyme differentiates into periosteum. Vessels invade the forming tissue, delivering nutrients and cells required for growth. Over time, woven bone is remodeled by osteoclasts and osteoblasts into mature lamellar bone, producing compact tables at the outer surfaces and a spongy inner architecture.

Regulation of intramembranous ossification involves transcription factors and signaling pathways that promote osteoblast differentiation and maturation. RUNX2 is a key transcription factor, and signaling through BMPs, Wnt, and FGF families guides mesenchymal cells toward an osteogenic fate. Mechanical forces and vascularization also influence the pattern and rate of formation.

Clinical relevance includes conditions affecting early skull and clavicle development. Defects in intramembranous ossification can contribute to cranial deformities such as craniosynostosis and to clavicular agenesis or hypoplasia seen in certain congenital disorders, for example cleidocranial dysostosis.