The human skeletal system is made up of 206 bones in the average adult, along with cartilage, ligaments, tendons, and the connective tissues that hold everything together. These parts are organized into two major divisions, built from several types of bone, connected at joints, and filled with living tissue that constantly rebuilds itself.
The Two Main Divisions
Every bone in your body belongs to one of two groups: the axial skeleton or the appendicular skeleton.
The axial skeleton contains 80 bones and forms the central axis of your body. It includes the skull, the vertebral column (spine), the ribs, and the sternum (breastbone). These bones protect your brain, spinal cord, heart, and lungs while keeping your torso upright.
The appendicular skeleton contains 126 bones and covers everything else: your arms, legs, hands, feet, and the girdles that attach your limbs to the axial skeleton. The shoulder girdle connects your arms; the pelvic girdle connects your legs. Together, these bones handle movement and weight-bearing.
Why Babies Have More Bones Than Adults
A newborn has roughly 275 to 300 bones. Many of those start as cartilage, which is tough but flexible. As a child grows, smaller bones gradually fuse and harden through a process called ossification. This fusion continues through puberty or shortly after, eventually leaving the standard 206 bones of an adult skeleton.
Five Types of Bones
Not all bones look alike, and their shape reflects the job they do.
- Long bones are longer than they are wide, with a shaft and two bulky ends. Your thigh bone (femur), shin bone, and the bones of your arms and forearms are long bones. They act as levers for movement.
- Short bones are roughly cube-shaped, with vertical and horizontal dimensions about equal. The small bones in your wrists and ankles are short bones, built to absorb shock and allow fine, complex movements.
- Flat bones are thin, flattened, and often curved. Most of the skull bones are flat bones. They protect internal organs and provide broad surfaces for muscle attachment.
- Irregular bones don’t fit neatly into the other categories. Vertebrae and certain skull bones are irregular, shaped specifically for the unique mechanical demands placed on them.
- Sesamoid bones are small, round bones embedded within tendons. The kneecap is the largest and most well-known example.
What’s Inside a Bone
A living bone is far from solid. It has distinct layers, each with a specific role.
The outermost layer is the periosteum, a thin membrane packed with blood vessels and nerves. It delivers the blood supply your bones need, gives bones their sense of feeling, and contains specialized cells that drive bone growth and repair after a fracture. The periosteum’s outer portion is made of thick collagen fibers for protection, while its inner layer houses the cells responsible for producing new bone tissue.
Beneath the periosteum sits compact bone, the dense, hard outer shell that gives bone its rigidity and strength. This is the white, smooth material you picture when you think of bone.
Deeper inside, especially at the ends of long bones, you’ll find spongy bone. It looks like a honeycomb and is lighter than compact bone, but its lattice-like structure is excellent at absorbing impact. Short and irregular bones are made primarily of spongy bone with only a thin shell of compact bone on the outside.
At the center of most bones is the medullary cavity, the hollow interior that houses bone marrow.
Bone Marrow: Red and Yellow
Bone marrow comes in two types. Red marrow is found mostly at the ends of bones, within the spongy bone. It contains blood stem cells that produce red blood cells (which carry oxygen), white blood cells (which fight infection), and platelets (which help blood clot). In adults, red marrow concentrates in the hip bones, vertebrae, ribs, sternum, and skull.
Yellow marrow fills the central cavities of long bones and is made mostly of fat. It serves as an energy reserve and can convert back to red marrow if the body urgently needs more blood cell production.
The Cells That Maintain Bone
Bone is not a static structure. It constantly tears itself down and rebuilds through a process called remodeling, driven by three key cell types.
Osteoblasts are the builders. They line the bone surface, making up about 4 to 6 percent of all bone cells, and secrete the raw material that becomes new bone tissue. Osteoclasts are the demolition crew. These large, multinucleated cells dissolve old or damaged bone by releasing acids and enzymes, clearing the way for fresh bone to take its place.
Osteocytes are the most abundant bone cells, comprising 90 to 95 percent of the total, with lifespans of up to 25 years. They sit embedded within the bone itself, connected to each other through a network of tiny channels. This network lets osteocytes sense mechanical stress, like the impact of walking or lifting, and signal osteoblasts and osteoclasts to adjust bone strength where it’s needed. They’re essentially the skeleton’s built-in sensor system.
Mineral Storage
Your skeleton doubles as a mineral bank. Over 99 percent of your body’s total calcium is stored in bones and teeth, locked into a crystalline compound that makes up almost 40 percent of bone’s weight. When blood calcium levels drop, your body withdraws calcium from bone. When levels are adequate, it deposits calcium back. This constant exchange is one reason nutrition matters for bone health throughout life, not just during childhood.
Joints: Where Bones Meet
Joints are the points where two or more bones come together, and they vary dramatically in how much movement they allow.
Fibrous joints are held together by dense connective tissue and allow little to no movement. The sutures of the skull are fibrous joints, locked tight to protect the brain. The joints anchoring teeth into the jawbone are also fibrous.
Cartilaginous joints connect bones with cartilage and permit limited movement. The discs between your vertebrae are cartilaginous joints, flexible enough to let you bend and twist but strong enough to resist pulling and compression forces. The pubic symphysis, the joint at the front of the pelvis, is another example. In women, its slight mobility is critical during childbirth.
Synovial joints are the body’s main functional joints, freely movable and defined by a fluid-filled joint cavity. Most of the joints you think of in everyday life are synovial. They come in several subtypes:
- Hinge joints bend and straighten along one axis, like a door hinge. Your elbows, knees, and ankles are hinge joints.
- Ball-and-socket joints allow the widest range of motion, rotating in nearly every direction. Your shoulders and hips are ball-and-socket joints.
- Condyloid joints permit movement in two axes: bending, straightening, and side-to-side motion. Your knuckles are condyloid joints.
- Saddle joints also move in two axes. The base of your thumb, where it meets the wrist, is a saddle joint, giving the thumb its distinctive ability to oppose the other fingers.
- Planar joints allow small gliding movements between two flat bone surfaces. The small joints between the bones of the wrist and the ankle are planar joints.
Connective Tissues: Ligaments, Tendons, and Cartilage
Bones alone can’t do much. The skeleton depends on soft connective tissues to function.
Tendons connect muscles to bones. They’re made predominantly of type I collagen fibers bundled into increasingly larger units, creating a structure that efficiently transmits the pulling force of a muscle contraction to the bone it needs to move.
Ligaments connect bones to other bones, stabilizing joints and preventing excessive movement. The anterior cruciate ligament (ACL) in the knee, for example, is essential for rotational stability and keeps the shinbone from sliding forward under the thighbone. Tendons and ligaments are structurally similar, both dense and fibrous, but they serve different mechanical roles.
Cartilage is a firm, flexible tissue that cushions joints, shapes structures like the nose and ears, and provides the template from which many bones originally form during development. At joint surfaces, a smooth layer of cartilage reduces friction and absorbs shock so bones don’t grind against each other during movement.