What is a Flat Bone?

A flat bone is a type of bone that is thin, flattened, and often curved. Unlike long bones, they don't have a central marrow cavity. Instead, they feature a layer of spongy bone sandwiched between two thin layers of compact bone. This structure provides strength and support while remaining relatively lightweight.

• Shape: Broad, flat, and typically curved.
• Structure: Composed of a layer of spongy bone (also known as cancellous bone) enclosed by two thin layers of compact bone (also known as cortical bone). This spongy bone layer, called diploë in flat bones of the skull, contains bone marrow.
• Marrow: Contains bone marrow, but it's not contained within a central cavity like in long bones.
• Function: Protection of internal organs and providing surfaces for muscle attachment.

Examples of Flat Bones

Several important bones in the human body are classified as flat bones. These include:

• Skull bones: The cranium (frontal, parietal, temporal, occipital bones) protects the brain.
• Ribs: Protect the heart and lungs.
• Sternum (breastbone): Protects the heart and major blood vessels.
• Scapulae (shoulder blades): Provide attachment points for numerous muscles.
• Pelvic bones (ilium, ischium, pubis): Form the pelvis, protecting organs and supporting the weight of the upper body.

How Flat Bones Develop

Flat bones primarily form through a process called intramembranous ossification, where bone tissue develops directly from mesenchymal connective tissue. This is in contrast to endochondral ossification, which involves cartilage as an intermediate step, and is how long bones are formed.

Further Considerations

While the primary characteristic of flat bones is their shape and structure, their specific composition and functions can vary depending on their location and role in the body. For example, the skull bones are specialized to protect the brain, while the ribs are designed to protect the thoracic cavity.