BIO121 Intramembranous Ossification

Step 3 - Formation of the Trabeculae

As the bone extracellular matrix forms, it develops into trabeculae that fuse with one another to form spongy bone around the network of blood vessels in the tissue. Connective tissue that is associated with blood vessels in the trabeculae differentiates into red bone marrow.

Step 1 - Development of the Ossification Center

At the site where bone will develop, specific chemical messages cause the mesenchymal cells to cluster together and differentiate, first into osteogenic cells and then into osteoblasts. The site of such a cluster is called an Ossification Center. Osteoblasts secret the organic extracellular matrix of bone until they are surrounded by it.

Step 2 - Calcification

Next, the secretion of extracellular matrix stops, and the cells, now called osteocytes, lie in lacunae and extend their narrow cytoplasmic processes into canaliculi that radiate in all directions. Within a few days, calcium and other mineral salts are deposited and the extracellular matrix hardens or calcifies (calcification).

Intramembranous Ossification Overview

Simpler of the two methods of bone formation. The flat bones of the skull, most of the facial bones, mandible, and medial parts of the clavicle are formed in this way. Also, the "soft spots" that help the fetal skull pass through the birth canal later harden as they undergo intramembranous occification.

Step 4 - Development of the Periosteum

In conjuction with the formation of the trabeculae, the mesenchyme condenses at the periphery of the bone and develops into the periosteum. Eventually, a thin layer of compact bone replaces the surface layers of the spongy bone, but spongy bone remains in the center. Much of teh newly formed bone is remodeled (destroyed and reformed) as the bone is transformed into adult size and shape.