Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
Intramembranous ossification is one of two types of bone formation and is the process responsible for the development of flat bones, especially those found in the skull and clavicles. Unlike endochondral ossification, cartilage is not involved or present in this process.
The first step in the process is the formation of bone spicules which eventually fuse with each other and become trabeculae. The periosteum is formed and bone growth continues at the surface of trabeculae. Much like spicules, the increasing growth of trabeculae result in interconnection and this network is called woven bone. Eventually, woven bone is replaced by lamellar bone.
- Mesenchyme cell in the membrane become osteochondral progenitor cell
- osteochondral progenitor cell specialized to become osteoblast
- Osteoblast produce bone matrix and surrounded collagen fiber and become osteocyte
- As the result process trabeculae will develop
- Osteoblast will trap trabeculae to produce bone
- Trabeculae will join together to produce spongy cell
- Cells in the spongy cell will specialize to produce red bone marrow
- Cells surrounding the developing bone will produce periosteum
- Osteoblasts from the Periosteum on the bone matrix will produce compact bone
Formation of bone spicules
Embryologic mesenchymal cells (MSC) condense into layers of vascularized primitive connective tissue. Certain mesenchymal cells group together, usually near or around blood vessels, and differentiate into osteogenic cells which deposit bone matrix constitutively. These aggregates of bony matrix are called bone spicules. Separate mesenchymal cells differentiate into osteoblasts, which line up along the surface of the spicule and secrete more osteoid, which increases the size of the spicule.
Formation of woven bone
As the spicules continue to grow, they fuse with adjacent spicules and this results in the formation of trabeculae. When osteoblasts become trapped in the matrix they secrete, they differentiate into osteocytes. Osteoblasts continue to line up on the surface which increases the size. As growth continues, trabeculae become interconnected and woven bone is formed. The term primary spongiosa is also used to refer to the initial trabecular network.
Primary centre of ossification
The periosteum is formed around the trabeculae by differentiating mesenchymal cells. The primary centre of ossification is the area where bone growth occurs between the periosteum and the bone. Osteogenic cells that originate from the periosteum increase appositional growth and a bone collar is formed. The bone collar is eventually mineralized and lamellar bone is formed.
Formation of osteon
Osteons are units or principal structures of compact bone. During the formation of bone spicules, cytoplasmic processes from osteoblasts interconnect. This becomes the canaliculi of osteons. Since bone spicules tend to form around blood vessels, the perivascular space is greatly reduced as the bone continues to grow. When replacement to compact bone occurs, this blood vessel becomes the central canal of the osteon.