Final answer:
Intramembranous ossification begins in utero during fetal development and is vital for forming flat bones such as the skull. It continues into adolescence, with the bones of the face being the last to ossify at the end of this period.
Step-by-step explanation:
Intramembranous ossification begins in utero during fetal development and continues into adolescence. This process is responsible for the development of flat bones such as the skull, mandible, and clavicles. During fetal development, ossification starts as mesenchymal cells group into clusters and form an ossification center. The secreted osteoid traps osteoblasts, which then become osteocytes, and this process is critical for bone formation.
At birth, bones that develop through intramembranous ossification are not fully ossified. Skull sutures remain open to facilitate birth, and ossification continues as the child grows. Interestingly, the last bones to complete ossification via this process are the flat bones of the face, which achieve their adult size at the end of the adolescent growth spurt.