Skull bone development.
Some of the craniofacial bones are formed through intramembranous ossification, a process in which mesenchymal cells condense without forming a cartilage intermediate.
In mice, mesenchymal condensations on the lateral side of the head start on E12.5, to eventually yield the frontal bones. The cellular mass bearing osteogenic activity then spreads upward toward the top of the skull. At E14.5, the osteoblasts in these osteogenic fronts differentiate to produce a bone matrix. At E15.5, the ends of the cellular masses meet at the midline, where a suture is formed.
At the suture site, cells in mesenchymal condensations differentiate to form osteoprogenitor cells, which later differentiate into bone-matrix molecules-producing osteoblasts. These osteoblasts later differentiate into osteocytes, which become embedded within the bone matrix. All panels are frontal views; bottom panel shows a schematic frontal view of the mouse skull (i.e., frontal and parietal bones).