Danielle has a PhD in Natural Resource Sciences and a MSc in Biological Sciences
What Is Ossification?
Osteoblasts are cells that create new bone. Ossification is the formation of bone by osteoblasts. The process begins about six weeks after fertilization during an embryo's development and continues to about the age of 25. There are two primary processes that create bone: intramembranous ossification and endochondral ossification. But for this lesson, we'll focus on the intramembranous kind.
Intramembranous ossification is when bone develops from fibrous connective tissue called mesenchyme. This process produces flat bones, including bones in the skull and face, the pelvic bone, and the clavicle, to name a few.
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.
First, stem cells in the mesenchyme develop into osteoblasts, which act as ossification centers, which form when mesenchyme cells can differentiate into either calcium-secreting osteoblasts or bone matrix-secreting osteoblasts, but both become like a bone-creation headquarters.
Next, the bony, extracellular matrix is formed, called an osteoid. The osteoblasts developed during Step 1 secrete proteins that form this osteoid matrix. As it forms, osteoblasts become trapped inside it. Cells that reside within fully formed bone are called osteocytes. In other words, the difference between osteoblasts and osteocytes are just their location. During this step, osteoid combines with calcium to make up calcified bones.
Osteoblasts continue to make osteoid, laying down multiple layers of the matrix. Osteoid develops around blood vessels, eventually forming trabeculae, thin columns and plates of bone, that become 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.
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.
Osteoblasts are cells that create new bone. Ossification is the formation of bone by osteoblasts. Intramembranous ossification is when flat bones are developed from connective tissue called mesenchyme. This process begins about six weeks after fertilization in developing embryos and lasts until about 25 years of age. The steps involved are:
- Ossification centers form as stem cells in the mesenchyme develop into osteoblasts.
- Osteoblasts form osteoid, a bony matrix. Cells that reside within fully formed bone are called osteocytes.
- Osteiod develops around blood vessels, eventually forming trabeculae, thin columns and plates of bone that become known as spongy bone. The blood vessels that are left outside spongy bone condense to form periosteum, which is a membrane made up of irregular connective tissue that surrounds bone.
- Compact bone forms and bone marrow appears.
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