Bones Explained
What bone is made of, how it constantly rebuilds itself, and how a fracture heals. The surprisingly active life of your skeleton.
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Bone Is Alive
When people think about bones, they often picture the dry specimens in a museum or biology classroom. Living bone is something entirely different: it is warm, richly supplied with blood vessels, threaded with nerves, and in a state of constant activity. It responds to your movements, repairs itself and stores vital minerals — all simultaneously.
Understanding what bone is made of, how it maintains itself, and how it heals when broken gives you a much more accurate picture of your skeleton than the static framework it is often imagined to be.
What Bone Is Made Of
Bone tissue is a composite material, meaning it combines two very different substances to get the best properties of each.
The organic component — about one-third of bone by weight — is mainly collagen, a strong, flexible protein. Collagen gives bone its ability to bend slightly under load without fracturing, like a green stick that bends before it snaps.
The inorganic (mineral) component — roughly two-thirds of bone by weight — is made largely of a calcium-phosphate compound called hydroxyapatite. These mineral crystals are deposited within and between the collagen fibres. They provide the rigidity and compressive strength that allow bone to bear heavy loads.
Together, the collagen and mineral make bone more effective than either component alone. Pure mineral would be brittle; pure collagen would be too flexible. The combination creates a material that is both strong and somewhat impact-resistant. This is the same principle used in reinforced concrete — steel rods (like collagen) embedded in concrete (like the mineral).
The Cells That Build and Maintain Bone
Three main types of cell are responsible for creating, maintaining and remodelling bone:
| Cell type | Job | Where found |
|---|---|---|
| Osteoblasts | Produce and deposit new bone matrix; gradually mineralise it | Bone surfaces — periosteum and endosteum |
| Osteoclasts | Break down (resorb) old or damaged bone tissue, releasing minerals into the blood | Bone surfaces, especially remodelling sites |
| Osteocytes | Sense mechanical stress; coordinate the activity of osteoblasts and osteoclasts | Embedded within the bone matrix itself |
Osteoblasts and osteoclasts work in coordinated teams called basic multicellular units. Osteoclasts go first, dissolving a section of old bone. Osteoblasts follow, filling the cavity with fresh bone. In healthy adults, the amount removed and replaced is roughly balanced, so bone density stays relatively stable. When the balance tips — for example, due to low oestrogen after menopause — more is removed than replaced and bone density falls.
Inside a Bone: Two Types of Tissue
A cross-section through a long bone reveals two distinct zones of bone tissue, each suited to a different demand.
Compact (cortical) bone forms the dense outer shell. It is organised into tightly packed cylindrical units called osteons (or Haversian systems), each with a central canal carrying blood vessels and nerves. Compact bone gives bone its rigidity and resistance to bending. It makes up about 80% of the skeleton's total mass.
Spongy (trabecular or cancellous) bone forms the interior lattice. Rather than solid mass, it consists of thin struts and plates of bone called trabeculae arranged along the lines of habitual mechanical stress — like a cleverly engineered internal scaffolding. This arrangement provides good strength relative to mass while leaving space for bone marrow. Spongy bone makes up about 20% of skeletal mass but has a much larger surface area than compact bone — which is why it is also a major site of metabolic activity.
Red and Yellow Marrow
The spaces within bone tissue are not empty. They are filled with bone marrow, of which there are two types.
Red bone marrow is the manufacturing site of blood cells. This process is called haematopoiesis. Red marrow produces red blood cells (which carry oxygen), white blood cells (which fight infection) and platelets (which help blood clot). In children, most bone marrow is red. With age, much of this is gradually replaced by yellow marrow.
Yellow bone marrow is mainly adipose (fat) tissue and acts as an energy reserve. In adults, it fills the medullary cavity of long bones. Under extreme demand — such as severe blood loss — yellow marrow can convert back to red marrow and resume blood cell production.
In adults, red marrow is mainly found in the flat bones (skull, sternum, scapulae, ribs) and the epiphyses (ends) of long bones such as the humerus and femur, as well as throughout the pelvis and vertebrae.
How a Broken Bone Heals
A fracture triggers an impressive sequence of repair. The process can be divided into four overlapping stages:
- Haematoma formation — blood vessels in the broken bone bleed, forming a clot (haematoma) at the fracture site within hours. This clot acts as a scaffold and releases chemical signals that initiate repair.
- Fibrocartilaginous callus — over one to two weeks, new cartilage-producing cells invade the haematoma and form a soft, flexible "soft callus" bridging the gap. This is why a fracture is still somewhat flexible during this phase.
- Bony callus — over the following weeks, osteoblasts replace the cartilage with woven bone (a less organised form of bone tissue). This "hard callus" gradually stiffens the fracture site and restores some load-bearing capacity.
- Remodelling — over months to years, osteoclasts and osteoblasts remodel the woven bone into organised compact and spongy bone. The bulge of callus is gradually smoothed away. In children especially, the repaired site can return to almost its original appearance.
Whether a fracture heals well depends on adequate blood supply, proper alignment of the bone ends, adequate nutrition (particularly calcium, protein and vitamin D) and the absence of factors that impair healing. Concerns about fractures or bone health should be discussed with a healthcare professional.
Bone, Minerals and Nutrition
Bone tissue is the body's main mineral reservoir. About 99% of the body's calcium is stored in bones and teeth. Calcium is essential not only for structural purposes but also for muscle contraction, nerve signalling and blood clotting. The body guards blood calcium levels carefully — if dietary intake is inadequate, calcium is withdrawn from bone to maintain those levels in the short term.
Vitamin D is essential because without it, the intestine cannot absorb calcium effectively from food. Vitamin D is mainly produced in the skin on exposure to sunlight, with relatively small amounts available from food.
Phosphorus, along with calcium, forms the mineral crystals in bone. Phosphorus is widespread in food and deficiency is uncommon in most diets.
For a broader view of the skeleton these bones compose, see our skeletal system guide. For how bones connect to each other, visit joints explained. You can browse skeleton anatomy interactively with the skeleton explorer or look up terms in the anatomy glossary.