Under the appropriate morphogenetic signals, osteogenesis begins within the core of this membrane. The instructed cells differentiate into osteoblasts which in turn secrete and initiate the mineralization of a matrix rich in type I collagen.
Ossification begins at predefined sites of the membrane and radiates outward.
In the calvarium, separate osteogenic fronts meet at the cranial sutures. During early post-natal life these sutures remain patent, allowing the cranial vault to grow and expand to accommodate the enlarging brain. Growth of the calvarial bones occurs through the proliferation and differentiation of osteoblasts and the deposition of bone matrix at the margins of the suture. Altered signaling within the regulatory pathways controlling osteoblast growth and differentiation in the calvarium results in dysmorphic facial features. FGFs have potent effects on the survival, proliferation and differentiation of osteoblasts and their precursors.