Bone Development: Intramembranous Ossification

There are two major processes that create bone during embryonic development, intramembranous and endochondral ossification. This packet will focus on intramembranous ossification.

Intramembranous ossification is the embryonic development of flat bones from an embryonic tissue called the mesenchyme. The word mesenchyme is a more or less a term used to describe embryonic connective tissue. There are four major steps that occur throughout this process that I will highlight here and elaborate more on in the video presentation below:

1) Formation of ossification centers: centrally located stem cells within the mesenchyme differentiate (specialize) into osteoblasts, forming the ossification center

2) Matrix formation: osteoblasts start to secrete the fibers (proteins) that make up the bony matrix, called osteoid. Soon after the osteoid will combine with calcium to form calcified bone. This calcified bone will engulf the osteoblasts which will make them form into osteocytes.

3) Periosteum and Weaving: The osteoid is continually laid down randomly around blood vessels. Structures called trabeculae (little beams) form around the vessels and pores are found where the blood vessels are located, thus forming spongy bone. The blood vessels on the outside of the spongy bone condense and form into the periosteum (outer vascular CT outside of bone)

4) Compact bone formation: as the trabeculae thicken within the spongy bone the osteoblasts on the periphery will continue to lay down osteoid. This osteoid will condense to form lamellar bone (compact bone) around the spongy bone (on both sides). As this process contines red bone marrow will start to appear where the blood vessels were located in the spongy spaces