The human skeletal system is made up of four main components: bones, cartilage, ligaments, and tendons. Together, they account for about 20 percent of your total body weight. While most people think of the skeleton as just bones, the system also includes the connective tissues that hold bones together, the cartilage that cushions them, and the living cells inside them that constantly rebuild and repair.
Bones: The Framework
An adult skeleton typically contains 206 named bones, but you weren’t born with that number. Newborns have between 275 and 300 bones. As a child grows, many of those smaller bones fuse together and harden through a process called ossification. A good example is the skull: babies are born with five major skull bones separated by soft spots (fontanelles), which gradually close within the first year or two of life. The fusing process continues throughout childhood and wraps up around puberty.
Those 206 adult bones are divided into two groups. The axial skeleton forms the central core of your body and contains 80 bones: the skull, vertebral column, ribs, and sternum (breastbone). The appendicular skeleton makes up the remaining 126 bones, covering your arms, legs, shoulders, and hips. Think of the axial skeleton as the trunk and the appendicular skeleton as everything that branches off from it.
Five Types of Bones
Not all bones look or function the same way. They fall into four major categories based on their shape:
- Long bones are longer than they are wide. Your thighbone (femur), shinbone, forearm bones, and finger bones all qualify. They act as levers for movement.
- Short bones are roughly cube-shaped and built for stability. The small bones in your wrists and ankles are short bones, along with the kneecap.
- Flat bones are thin and broad, providing protection and large surfaces for muscle attachment. Your skull, shoulder blades, sternum, and ribs are flat bones.
- Irregular bones don’t fit neatly into the other categories. The vertebrae in your spine, the sacrum at the base of the spine, and the tiny hyoid bone in your throat are all irregular.
What Bone Tissue Looks Like Inside
If you could cut a bone open, you’d see two distinct types of tissue. The outer layer is compact bone, a dense, solid-looking material made of tightly packed cylindrical units. Each unit has a central canal carrying blood vessels, surrounded by concentric rings of bone matrix, with bone cells nestled in tiny pockets between the rings. Microscopic channels connect these cells to the blood supply, keeping them alive despite being locked inside hard tissue.
Deeper inside, especially at the ends of long bones and within flat bones, you’d find spongy bone. It looks like a lattice or honeycomb, with thin plates and bars of bone surrounding small cavities filled with red bone marrow. Despite its appearance, spongy bone isn’t random. Those plates are precisely arranged along the lines of mechanical stress your body places on them, and they can even realign if the pattern of stress changes. This makes spongy bone remarkably strong for its weight.
The Cells That Keep Bone Alive
Bone is living tissue, and three specialized cell types work together to maintain it throughout your life. Osteoblasts are the builders. They produce new bone material and are responsible for growth and repair. Osteocytes are mature bone cells that were once osteoblasts but became embedded in the bone they created. They act as sensors, detecting mechanical forces like the impact of walking or lifting. When osteocytes sense these forces, they release signaling molecules that tell osteoblasts to ramp up production.
Osteoclasts handle demolition. They break down old or damaged bone so it can be replaced. This constant cycle of breakdown and rebuilding is called bone remodeling, and it keeps your skeleton strong and responsive to the demands you place on it. The balance between osteoblast building and osteoclast breakdown is critical. When that balance tips toward too much breakdown, conditions like osteoporosis develop.
Cartilage, Ligaments, and Tendons
Cartilage is a tough, rubbery tissue that cushions joints, shapes structures like your nose and ears, and serves as a template for bone growth in children. Much of a baby’s skeleton starts as cartilage before gradually hardening into bone. In adults, cartilage persists at the ends of bones where they meet at joints, absorbing shock and reducing friction.
Ligaments and tendons are both dense, fibrous connective tissues built primarily from collagen fibers. Ligaments connect bone to bone, stabilizing joints and limiting movement to safe ranges. The ligaments in your knee, for example, prevent the joint from bending sideways. Tendons connect muscle to bone, transmitting the force your muscles generate so you can actually move. Despite their similar composition, ligaments and tendons develop through different biological pathways, which is part of why they heal differently after injury. Both tissues contain specialized cells that sit between parallel chains of collagen fibers, maintaining and repairing the tissue over time.
Joints: Where Bones Meet
The places where two or more bones come together are called joints, and they vary widely in how much movement they allow. Some joints are completely immovable, like the seams between the bones of your adult skull. Others allow only slight movement, like the joints between your vertebrae, where small amounts of flex at each level add up to the full range of motion in your spine. The most familiar joints are freely movable ones: your shoulders, hips, knees, elbows, and knuckles. These are lined with cartilage, lubricated by fluid, and held together by ligaments.
What the Skeleton Actually Does
Beyond providing structure and enabling movement, the skeletal system handles several functions you might not expect. Over 99 percent of your body’s calcium is stored in your bones and teeth. When blood calcium levels drop, your body pulls calcium from bone to keep critical processes like nerve signaling and muscle contraction running. Bone mineral makes up almost 40 percent of bone’s weight, which gives you a sense of how much mineral storage is packed into the skeleton.
The red bone marrow inside spongy bone is where your body produces red blood cells, white blood cells, and platelets. In adults, this blood cell production is concentrated in bones like the vertebrae, sternum, ribs, and pelvis. Without this function, your body couldn’t replace the millions of blood cells it uses up every day.
The skeleton also protects your most vulnerable organs. The skull encases the brain, the ribcage shields the heart and lungs, and the vertebral column surrounds the spinal cord. Flat and irregular bones are especially well suited for this protective role because of their broad, shield-like shapes.