A bone is a living organ made of minerals, proteins, water, and several types of specialized cells that constantly build, maintain, and repair its structure. The adult human skeleton contains 206 bones, though newborns start with 275 to 300 separate bones that gradually fuse together during childhood and adolescence. Far from being dry, static structures, bones grow, heal, store minerals, produce blood cells, and adapt their shape in response to the physical forces placed on them every day.
What Bone Is Made Of
By weight, bone tissue is roughly 60% mineral, 30% protein, and 10% water. The mineral portion is primarily hydroxyapatite, a crystalline form of calcium and phosphate that gives bone its hardness and rigidity. These crystals are tiny, plate-shaped structures only 20 to 50 nanometers long. The protein portion is mostly collagen, which forms a flexible framework that the mineral crystals embed into. This combination is what makes bone both hard and slightly flexible. Without the minerals, bone would bend like rubber. Without the collagen, it would shatter like chalk.
Your skeleton also serves as the body’s mineral vault. Over 99% of your total body calcium is stored in your bones and teeth. When blood calcium levels drop, your body pulls calcium from bone to keep your heart, muscles, and nerves functioning properly. When calcium is plentiful, bone stores the surplus.
The Two Types of Bone Tissue
If you could slice a bone open, you’d see two distinct types of tissue working together.
Compact bone forms the dense, smooth outer layer. It’s built from tightly packed cylindrical units called osteons, each containing a central canal with blood vessels running parallel to the bone’s length. These canals interconnect through smaller perforating canals that link to vessels on the bone’s surface. Compact bone handles most of the load-bearing work and gives bone its solid appearance.
Spongy bone (also called cancellous bone) sits inside, particularly at the ends of long bones and within flat bones like the skull and ribs. Instead of solid tissue, it consists of a lattice of thin plates and bars called trabeculae, with small cavities between them filled with bone marrow. This lattice looks random at first glance, but the trabeculae align precisely along the lines of stress the bone experiences, similar to the structural braces inside a building. If the direction of stress changes over time, the trabeculae can actually realign to match.
Four Shapes of Bone
Not every bone in your body looks like the classic leg bone you’d picture. Bones are classified into four main categories based on their shape, and each shape reflects a different job.
- Long bones act as levers for movement. These include the thighbone, shinbone, upper arm bone, forearm bones, and the small bones in your fingers and toes.
- Short bones provide stability with limited motion. The wrist bones, ankle bones, and kneecaps fall into this group.
- Flat bones protect internal organs and provide broad surfaces for muscle attachment. Your skull, ribs, breastbone, and shoulder blades are flat bones.
- Irregular bones have complex shapes that don’t fit the other categories. The vertebrae of your spine, the sacrum, and the small hyoid bone in your throat are all irregular.
The Cells That Keep Bone Alive
Bone contains four main cell types, each with a specific role. Understanding these cells is key to understanding why bone can heal, strengthen, and weaken over a lifetime.
Osteoblasts are the builders. They make up about 4 to 6% of all bone cells and sit along bone surfaces, where they secrete new bone material in two steps: first laying down a protein framework, then mineralizing it. They originate from stem cells in the bone marrow.
Osteocytes are by far the most abundant bone cells, comprising 90 to 95% of all bone cells. They can live up to 25 years. Osteocytes are former osteoblasts that became embedded in the bone matrix they helped create. They sit inside tiny cavities and extend long, branch-like projections through microscopic channels that connect them to neighboring cells. This interconnected network allows osteocytes to sense mechanical pressure and loading, essentially acting as the bone’s built-in strain gauges. When they detect changes in force, they signal osteoblasts and osteoclasts to add or remove bone where needed.
Osteoclasts are the demolition crew. They break down old or damaged bone by creating an acidic environment that dissolves the mineral content, leaving scooped-out pits in the bone surface. This controlled destruction is essential because it allows damaged bone to be replaced with fresh, stronger tissue.
Bone lining cells are flat, quiet cells that cover bone surfaces where no building or breaking down is happening. They act as a protective barrier, preventing osteoclasts from accessing bone when resorption isn’t needed.
How Bone Constantly Rebuilds Itself
Your skeleton is never truly finished. Through a process called remodeling, your body continuously removes old bone and replaces it with new tissue. This happens in a repeating cycle. First, osteoclasts dissolve a section of aging or damaged bone. Then, during a reversal phase, immune cells clean up the debris and release growth factors that recruit osteoblasts to the site. Finally, osteoblasts fill in the cleared area with fresh collagen and minerals. At any given moment, multiple sites across your skeleton are at different stages of this cycle.
Remodeling is what allows bones to repair microdamage from daily activity, adapt to new physical demands, and release stored minerals when the body needs them. It’s also why bones weaken with age or inactivity. If the rate of bone removal outpaces the rate of new bone formation (as it does after menopause or during prolonged bed rest), the result is a gradual loss of bone density.
Bone Marrow and Blood Cell Production
The cavities inside spongy bone and the central shafts of long bones are filled with bone marrow, a soft tissue that comes in two varieties. Red bone marrow is a blood cell factory. It contains stem cells that produce red blood cells (which carry oxygen), white blood cells (which fight infection), and platelets (which help blood clot). In children, red marrow is found throughout the skeleton. In adults, it retreats mostly to flat bones like the pelvis, ribs, and skull, along with the ends of long bones.
Yellow bone marrow is composed mostly of fat. It fills the central shafts of adult long bones and contains stem cells that can become cartilage, fat, or bone cells. In cases of severe blood loss, yellow marrow can convert back to red marrow to ramp up blood cell production.
The Membranes That Protect and Repair Bone
Two thin membranes cover every bone in your body. The periosteum wraps the outer surface. It’s rich in stem cells and blood vessels, and it plays a central role in bone growth during childhood by adding new layers to the bone’s exterior. When a fracture occurs, blood vessels in the periosteum bleed and form clots around the broken ends. Within about two days, stem cells in the periosteum begin multiplying rapidly and differentiating into bone-building cells, forming a bridge of new tissue called a callus that gradually hardens into solid bone.
The endosteum lines the inner surfaces, including the walls of the central marrow cavity. It works in the opposite direction from the periosteum, gradually resorbing bone from the inside to widen the marrow cavity as the bone grows in diameter. During fracture healing, endosteal cells also multiply quickly and help rebuild bone from the interior. Together, these two membranes give bone a remarkable capacity for self-repair that few other tissues in the body can match.