Intramembranous Ossifcation

Intramembranous ossification occurs as follows:

1) Development of the ossification center 2) Calcification 3) Formation of trabeculae 4) Development of the periosteum

2) Calcification

+After secretion of extracellular matrix stops, 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 as it calcifies (calcification). +The mesenchyme continue to produce osteogenic cells, which form a peripheral layer of osteoblasts.

3) Formation of trabeculae

+As the bone extracellular matrix forms, it develops into trabeculae that fuse with one another to form sponge bone tissue. +Blood vessels grow into the spaces between the trabeculae and deposit connective tissue cells that differentitate into red bone marrow.

4) Development of the periosteum

+At the periphery, the mesenchyme condenses and develops into periosteum. +Within the periosteum, osteoblasts produce a thin layer of compact bone tissue, but spongy bone tissue remains in the center. +Much of the newly formed bone is remodeled as the bone is transformed into its adult size and shape.

1) Development of the ossification center

+At the site where the bone will develop, specific chemical messages cause the cells in mesenchyme 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 secrete bone extracellular matrix until they are surrounded by it.

The bones formed in this way:

+the flat bones of the skull +the mandible (lower jawbone), and clavicle (collar bone) +the "soft spots" that help the fetal skull pass through the birth canal later harden as they undergo intramembranous ossification

Mnemonic for the steps:

OCT-P (like octopus) +Development of the *ossification center* +*Calcification* +Formation of *trabeculae* +Development of the *periosteum*

Intramembranous ossification

is the simpler of the two methods of bone formation (intramembranous and endochondral ossifcation)