Final answer:
Intramembranous ossification forms flat bones like the skull, mandible, and clavicles. Mesenchymal cells differentiate into osteoblasts, leading to the hardening of bone through the deposition of calcium. Spongy bone is remodeled into compact bone.
Step-by-step explanation:
Intramembranous ossification is a process of bone development from fibrous membranes. It is involved in the formation of flat bones like the skull, mandible (jawbone), and clavicles (collarbones). During this process, mesenchymal cells differentiate into osteoblasts at the ossification center, which secrete the extracellular matrix and deposit calcium, leading to the hardening of the bone. The spongy bone formed around blood vessels is remodeled into a thin layer of compact bone on the surface.