Which bones are formed by intramembranous ossification?
A.
Long bones arms and legs
B.
Bones of the pelvic girdle
C.
Vertebrae
D.
Flat bones of the skull
The correct answer and Explanation is :
The correct answer is D. Flat bones of the skull.
Intramembranous ossification is a process by which bone develops directly from mesenchymal (undifferentiated) connective tissue, bypassing a cartilage stage. This type of ossification is responsible for forming several bones, primarily those in the skull and face.
Here’s a more detailed explanation:
Process of Intramembranous Ossification
- Mesenchymal Condensation: The process begins with mesenchymal cells aggregating in the area where bone formation is to occur. These cells differentiate into osteoblasts, which are bone-forming cells.
- Formation of Osteoid: Osteoblasts secrete an extracellular matrix called osteoid, which is initially unmineralized. This osteoid forms the foundation for bone development.
- Mineralization: The osteoid gradually becomes mineralized with calcium salts, transforming it into mature bone tissue. This process leads to the formation of trabecular (spongy) bone within the developing structure.
- Development of Compact Bone: As ossification progresses, the trabecular bone is remodeled and compact bone develops on the outer edges of the newly formed bone.
- Formation of Periosteum: The outer layer of mesenchymal cells condenses to form the periosteum, a membrane that covers the outer surface of bones.
Bones Formed by Intramembranous Ossification
Intramembranous ossification predominantly forms the flat bones of the skull, such as:
- The frontal bone
- The parietal bones
- The occipital bone
- The temporal bones
Additionally, parts of the clavicle and some facial bones are also formed through this process.
Contrast with Endochondral Ossification
In contrast, endochondral ossification is the process responsible for forming long bones, such as those in the arms and legs, as well as the bones of the pelvic girdle and vertebrae. In endochondral ossification, a cartilage model is first formed and later replaced by bone.
Summary
Intramembranous ossification is crucial for the formation of the flat bones of the skull and some facial bones, which develop directly from mesenchyme, avoiding the cartilage stage seen in endochondral ossification.