The skeletal system is made up of 206 bones in most adults, along with cartilage, ligaments, tendons, bone marrow, and joints that connect everything together. It’s not just a frame that holds you upright. Your skeleton produces blood cells, stores minerals, and protects your brain, spinal cord, and organs from impact.
The Two Main Divisions of Bone
Every bone in your body belongs to one of two groups: the axial skeleton or the appendicular skeleton.
The axial skeleton runs down the center of your body and contains 80 bones. It includes 28 skull bones (the protective plates around your brain, your facial bones, and six tiny bones deep inside your ears), one small horseshoe-shaped bone in your throat, 26 vertebrae that form your spinal column, and the 25 bones of your rib cage and breastbone. This central core protects your most vital organs: the brain, spinal cord, heart, and lungs.
The appendicular skeleton makes up the remaining 126 bones. These are the bones of your arms, legs, shoulders, and hips. Everything that lets you reach, grab, walk, or kick belongs here. The appendicular skeleton is built for movement, while the axial skeleton is built for protection.
Five Types of Bone
Not all bones look or function the same way. They come in five structural categories, each suited to a different job.
- Long bones are cylindrical and longer than they are wide. Your upper arm bone, thigh bone, and the small bones in your fingers and toes all fall into this group. They act as levers for movement.
- Short bones are roughly cube-shaped and about equal in length, width, and thickness. The only short bones in your body are in your wrists and ankles, where they provide stability and allow small, precise movements.
- Flat bones are thin and often curved. Your skull plates, shoulder blades, ribs, and breastbone are flat bones. They protect internal organs and provide broad surfaces for muscle attachment.
- Irregular bones don’t fit neatly into any other shape category. Your vertebrae and many of your facial bones, particularly the ones containing sinus cavities, are irregular bones.
- Sesamoid bones are small, round, and shaped like sesame seeds. They form within tendons where those tendons pass over a joint. Your kneecaps are the only sesamoid bones that every person has, though smaller ones can develop in the hands and feet.
Compact and Spongy Bone Tissue
If you could slice a bone open, you’d see two distinct layers of tissue. The outer shell is compact bone: dense, heavy, and tightly organized. It’s built from tiny cylindrical units, each made of concentric rings of bone material surrounding a central canal that carries blood vessels. This architecture gives compact bone its extraordinary strength.
Beneath that dense shell lies spongy bone, which looks like a honeycomb of thin plates and bars with small cavities in between. Spongy bone is lighter and less dense, but it’s not weak. The lattice structure distributes stress efficiently, much like the steel framework inside a building. The cavities within spongy bone are filled with red bone marrow, which is where your body produces blood cells.
Three Cell Types That Build and Rebuild Bone
Bone is living tissue, constantly being broken down and rebuilt by three specialized cell types working together.
Osteoblasts are the builders. They produce the raw material of bone and are responsible for hardening it with minerals. They also send chemical signals that control the activity of the cells that break bone down. Osteocytes are osteoblasts that have become embedded within the bone they built. They sit inside the hard matrix and extend long, branching arms that connect them to neighboring cells, forming a communication network throughout the bone. When you exercise or put weight on your skeleton, osteocytes sense that mechanical force and relay signals that trigger more bone building where it’s needed.
Osteoclasts do the opposite: they dissolve and absorb old or damaged bone. They’re the only cells in the body capable of breaking down hardened bone tissue. This demolition work is essential. Without it, your skeleton couldn’t release stored calcium into your bloodstream, repair micro-damage from daily activity, or reshape itself in response to changing demands. The balance between osteoblast building and osteoclast breakdown determines whether your bones stay strong, get denser, or gradually weaken over time.
Cartilage, Ligaments, and Tendons
Bones alone would be rigid and brittle at every connection point. Three types of connective tissue make the system functional.
Cartilage is the strong, flexible tissue that cushions your joints. It acts like a shock absorber, preventing bone surfaces from grinding against each other when you move. You also have cartilage in your nose, ears, and the discs between your vertebrae.
Ligaments are bands of tough tissue that connect bones to other bones. They hold joints together and prevent them from bending in directions they shouldn’t. Your knee alone has four major ligaments keeping the joint stable. Tendons serve a different purpose: they connect muscles to bones, transmitting the force of a muscle contraction into actual movement of the skeleton.
Joints: Where Bones Meet
Every point where two or more bones come together is a joint, and there are three structural types with very different levels of mobility.
Fibrous joints contain dense connective tissue and allow little to no movement. The joints between your skull plates are fibrous. They’re essentially locked together, which is exactly what you want protecting your brain. Cartilaginous joints allow some movement but not much. The joints between your vertebrae and the connection at the front of your pelvis fall into this category. Synovial joints are the most mobile. Your knees, shoulders, hips, elbows, and knuckles are all synovial joints. They’re enclosed in a fluid-filled capsule that lubricates the joint surfaces and allows a wide range of motion.
Bone Marrow
The interior of your bones contains marrow, which comes in two forms. Red bone marrow is where your body produces red blood cells, white blood cells, and platelets. In infants, red marrow fills most of the bone cavities. As you age, a large portion of it converts to yellow marrow, which is primarily fat. In adults, about half of all bone marrow remains red, concentrated in flat bones like the hip, breastbone, skull, ribs, vertebrae, and shoulder blades, as well as the ends of large leg and arm bones.
Yellow marrow fills the hollow shafts of long bones and mostly serves as fat storage. But it retains stem cells capable of producing cartilage, bone, and fat cells. If your body faces a sudden demand for blood cells, such as after significant blood loss, yellow marrow can convert back to red marrow and resume blood cell production.
Mineral Storage and Calcium Regulation
Your bones contain more calcium than any other organ. The hard matrix of bone is loaded with calcium salts, primarily calcium phosphate, which give bone its rigidity. But this calcium isn’t locked away permanently. When calcium levels in your blood drop below normal, bone releases calcium into the bloodstream to meet metabolic needs. When blood calcium rises, the excess gets deposited back into the bone matrix. This exchange happens almost continuously, making your skeleton a dynamic mineral reservoir rather than a static structure.
How the Skeleton Changes Over a Lifetime
A newborn has roughly 275 to 300 bones. That number decreases as the child grows because many smaller bones gradually fuse together into larger ones through a process called ossification. By adulthood, the typical count settles at 206. Bone density continues to increase into your late twenties, peaks around age 30, and then slowly declines. A bone density scan measures this decline using a T-score: a score of negative 1 or higher is considered healthy, negative 1 to negative 2.5 indicates mild bone loss (osteopenia), and negative 2.5 or lower suggests osteoporosis, a condition where bones become fragile enough to fracture more easily.