Intramembranous ossification is one of the two processes involved in the development of bones within an embryo. The flat bones of the face, most of the cranial bones, and the clavicles are formed via this process. During intramembranous ossification, the bones develop directly from sheets of undifferentiated mesenchymal connective tissue.

The process begins when mesenchymal cells in the embryonic skeleton gather together and differentiate into osteogenic cells, which then develop into osteoblasts. These early osteoblasts assemble in clusters termed ossification centers.

Formation of Bone Matrix

The osteoblasts secrete the uncalcified osteoid matrix, which calcifies within a few days as mineral salts are deposited. As the matrix hardens, it traps some of the osteoblasts within it. Once entrapped, the osteoblasts mature into osteocytes, and the osteogenic cells in the surrounding connective tissue differentiate into new osteoblasts. Thus the bone grows outward from these ossification centers.

Formation of Spongy and Compact Bone

The osteoid secreted around the capillaries results in a trabecular matrix forming the spongy bone. As the trabeculae form around the blood vessels, the intertrabecular spaces are filled with red marrow derived from the vasculature. Meanwhile, the osteoblasts surrounding the spongy bone produce a protective layer of compact bone. The osteogenic cells at the surface, together with the mesenchymal membrane, form the periosteum, thus shaping the final bone structure.

Bone Development with Age

Intramembranous ossification begins in the uterus during fetal development and continues into adolescence. At birth, the skull and clavicles are not fully ossified, nor are the skull sutures closed. This allows the skull and shoulders to deform during passage through the birth canal. The last bones to ossify via intramembranous ossification are the flat bones of the face, which reach their adult size at the end of the adolescent growth spurt.

This text is partially adapted from Openstax, Anatomy and Physiology 2e, Section 6.4: Bone Formation and Development