Yes, the sternum (breastbone) is a flat bone. It shares the same structural blueprint as other flat bones in the body: a layer of spongy bone sandwiched between two thin layers of compact bone. This internal architecture gives the sternum its strength without adding bulk, and it’s the defining feature that separates flat bones from long bones, short bones, and irregular bones.
What Makes a Bone “Flat”
Flat bones have a specific internal structure. Two thin outer shells of dense, compact bone enclose a middle layer of spongy (cancellous) bone. This sandwich design creates a lightweight shield that absorbs impact well. Flat bones have marrow inside that spongy layer, but unlike long bones such as the femur or humerus, they do not have a hollow bone marrow cavity running through the center.
The sternum fits this definition precisely. Its broad, relatively thin shape and layered internal structure are textbook flat bone characteristics. Other flat bones in the human body include the skull bones, ribs, shoulder blades, and hip bones. All of these share a common purpose: protecting soft organs underneath while providing broad surfaces for muscle attachment.
Structure of the Sternum
The sternum sits at the front center of your chest and is made up of three distinct parts. The manubrium is the wide, handle-shaped section at the top. Your collarbones and first pair of ribs connect here. Below it is the body (sometimes called the gladiolus), the longest section, where ribs two through seven attach either directly or through cartilage. At the very bottom is the xiphoid process, a small, pointed piece of bone that starts out as cartilage and hardens over time.
These three parts don’t all fuse together the way you might expect. The body’s segments typically fuse into one solid piece by age 25, and the xiphoid process begins hardening around age 6. But the joint between the manubrium and the body contains fibrocartilage that usually prevents full fusion, even late in life. This slight flexibility at that joint actually helps the chest wall expand during breathing.
How the Sternum Develops
The sternum begins forming remarkably early. During the sixth week of fetal development, two bars of tissue appear on either side of the chest and migrate toward the midline, fusing together to create a cartilage template by around the tenth week. Bone then gradually replaces this cartilage through a process called ossification, which unfolds over decades.
The manubrium’s ossification center appears between the third and sixth month of pregnancy, and its centers typically merge before birth. The body of the sternum develops four segments (called sternebrae) from paired ossification centers that appear during the first year of life. These segments merge into a single center between ages 6 and 12, with full calcification and fusion of the sternebrae usually complete by age 25. The xiphoid process is the last to finish, with its ossification centers appearing sometime during the first decade of life. This drawn-out timeline is one reason the sternum can look different on imaging depending on a person’s age.
Why the Sternum’s Flat Shape Matters
The sternum acts as the front wall of the rib cage, and its flat, broad shape is directly related to its protective role. Sitting right over the heart, major blood vessels, and portions of the lungs, it forms a bony shield against blunt force to the chest. Together with the ribs and spine, it creates the thoracic cage, a structure rigid enough to guard vital organs but flexible enough to allow your lungs to expand with each breath.
The flat shape also provides a wide attachment surface. Several muscles involved in breathing and arm movement anchor to the sternum, including chest muscles and muscles between the ribs. A long bone or irregular bone couldn’t offer the same broad, stable platform.
The Sternum’s Role in Blood Cell Production
One of the most important functions of flat bones, and a reason their classification matters beyond anatomy class, is that they house red bone marrow. Red marrow is where your body produces red blood cells, white blood cells, and platelets. In adults, red marrow retreats from most of the skeleton and concentrates in flat bones: the sternum, hip bones, skull, ribs, vertebrae, and shoulder blades, along with the ends of some long bones.
This is why doctors sometimes perform a bone marrow biopsy on the sternum or hip bone when evaluating blood disorders or cancers like leukemia. The sternum’s position close to the skin surface and its reliable concentration of active marrow make it a practical site for sampling. For most people, the sternum quietly produces billions of blood cells every day without any awareness that it’s happening.