Bone forms in a process called ossification. There are two main types of ossification, including intramembranous. Read this lesson to learn about intramembranous ossification and the steps involved in this process.

What Is Ossification?

Ossification is the formation of bone by osteoblasts, and it begins about six weeks after fertilization during an embryo’s development. Ossification continues to about the age of 25. Osteoblasts are cells that create new bone. There are two primary processes that create bone, and intramembranous ossification is one of them. The other is endochondral ossification, but for this lesson we’ll focus on intramembranous ossification.

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What Is Intramembranous Ossification?

Intramembranous ossification is when bone develops from fibrous connective tissue called mesenchyme. Through this process flat bones are made, including those in the skull and face, the pelvic bone, and the clavicle, to name a few.

Intramembranous ossification forms flat bones, like those found in the skull.
flat bone

Steps of Intramembranous Ossification

Different sources categorize the steps slightly differently, but the process is the same. Let’s outline the four steps of intramembranous ossification.

  1. First, ossification centers form when stem cells in the mesenchyme develop into osteoblasts. Mesenchyme cells can differentiate into either calcium-secreting osteoblasts or bone matrix-secreting osteoblasts, but the important thing to remember is that as they become osteoblasts they form an ossification center, like a bone creation headquarters.
  2. Next, the bony, extracellular matrix is formed. The osteoblasts developed during step one secrete proteins that form this matrix called osteoid. As the matrix forms, osteoblasts become trapped inside it; at this point they become osteocytes. Osteocytes are different from osteoblasts in that they are found inside the bone and are made by osteoblasts. During this step, osteoid combines with calcium to make up calcified bones.
  3. Osteoblasts continue to make osteoid, laying down multiple layers of the matrix. Osteoid develops around blood vessels, eventually forming trabeculae around the vessels, which becomes known as spongy bone. The blood vessels that are left outside of spongy bone condense to form periosteum, which is a membrane made up of irregular connective tissue that surrounds bone.
  4. Finally, compact bone forms. After the spongy bone is developed, osteoid continues to form around it. As layers are added, it compacts to form lamellar, or compact bone outside of the spongy bone. During this phase, bone marrow appears where the blood vessels in the spongy bone are located.
The process of intramembranous ossification is shown here, starting with panel a.
intramembranous ossification

Lesson Summary

Wow! What have we learned about bone formation (or ossification) in this lesson? Well, we have learned that intramembranous ossification is one type of ossification where flat bones are developed from connective tissue called mesenchyme. This process begins about six weeks after fertilization in developing embryos. Let’s quickly review the steps involved. First, ossification centers form when stem cells in the mesenchyme develop into osteoblasts (cells that form new bone). In turn, osteoblasts form osteoid, a bony matrix. As osteoid is laid down, spongy bone develops around the underlying blood vessels. Blood vessels left outside of spongy bone become periosteum, the membrane surrounding bone. Osteoid continues to develop outside of the spongy bone, forming a compressed layer that turns into compact bone. During this time, the blood vessels in the spongy bone become bone marrow. And this is one way bones are formed!