Bones are made of a combination of minerals, proteins, water, and living cells. About 65% of bone tissue is hard mineral, mainly calcium and phosphorus locked together in a crystalline salt called hydroxyapatite. The remaining 35% is a flexible protein framework, roughly 90% of which is collagen. Bones also contain about 31% water by weight, making them far less dry than most people assume.
The Mineral Component: What Makes Bones Hard
The hardness you associate with bone comes from its mineral content. Calcium and phosphorus combine into hydroxyapatite crystals, which pack tightly throughout the bone tissue. These crystals give bone its rigidity and compressive strength, meaning bones can bear heavy loads without cracking. Without this mineral layer, your skeleton would be rubbery and unable to support your body weight.
Bone also serves as the body’s main storage vault for calcium. When blood calcium levels drop, the body pulls calcium from bone to keep muscles, nerves, and the heart functioning properly. This is one reason long-term calcium deficiency gradually weakens bones: the body prioritizes immediate needs over skeletal strength.
The Protein Framework: What Makes Bones Flexible
If minerals were the whole story, bones would be brittle like chalk. The organic matrix, that other 35%, prevents this. Collagen fibers form a scaffold throughout the bone, arranged in tightly cross-linked chains that give bone its tensile strength and slight flexibility. This is why a healthy bone can absorb impact without shattering. Think of it like rebar inside concrete: the mineral crystals resist compression while the collagen resists pulling and bending forces.
The remaining 10% of the organic matrix consists of non-collagenous proteins that play supporting roles. Some regulate how minerals deposit onto the collagen scaffold. Others control collagen fiber diameter and organization. One protein, osteonectin, binds both collagen and hydroxyapatite crystals, essentially acting as glue between the hard and soft components of bone. These proteins are small in quantity but critical for maintaining bone quality.
Living Cells Inside Bone
Bones are living tissue, constantly maintained and reshaped by specialized cells. Three main cell types do the work.
Osteocytes are by far the most abundant, accounting for 90% of all cells in the mature skeleton. These are former bone-building cells that became embedded in the matrix they created. They sit in tiny pockets within the bone, connected to each other through microscopic channels, and act as sensors. They monitor mechanical stress and regulate calcium and phosphorus levels in the surrounding tissue.
Osteoblasts build new bone. They produce the collagen matrix and help deposit minerals into it. Once they finish building, some become osteocytes, while others flatten out along the bone surface.
Osteoclasts do the opposite: they break bone down. These large cells dissolve both the mineral and protein components, releasing calcium back into the bloodstream. This sounds destructive, but it’s essential. Old or damaged bone needs to be removed so new bone can replace it.
How Bone Constantly Rebuilds Itself
Your skeleton isn’t a fixed structure. It goes through a continuous remodeling cycle with five stages: activation, resorption, reversal, formation, and rest. At any given spot in your skeleton, osteoclasts spend about 3 to 6 weeks dissolving old bone. Then osteoblasts move in to lay down new bone, a process that takes 4 to 5 times longer than the breakdown phase.
Mineralization itself happens in two waves. The initial round takes 2 to 3 weeks and deposits roughly 70% of the final mineral content into the fresh collagen framework. The second wave, where crystals mature and pack more densely, can continue for a year or more. This is why newly formed bone is softer than fully mature bone, and why fracture sites need months to regain full strength.
Compact Bone vs. Spongy Bone
Not all bone tissue is structured the same way. The outer shell of most bones is compact (cortical) bone, a dense, solid layer that handles most of the mechanical load. Under a microscope, compact bone is organized into cylindrical units called osteons. Each osteon has a central canal carrying blood vessels, surrounded by concentric rings of mineralized matrix. Bone cells sit in small pockets between these rings, connected by tiny channels that deliver nutrients and allow cell-to-cell communication.
Inside the dense outer shell, many bones contain spongy (cancellous) bone. This looks like a honeycomb or lattice, with thin struts of bone tissue separated by open spaces. Spongy bone is lighter and less dense, but its architecture is remarkably efficient at distributing stress. You’ll find it concentrated at the ends of long bones, inside vertebrae, and in the pelvis, all places where the skeleton needs to absorb force from multiple directions.
Bone Marrow: What Fills the Spaces
The hollow centers and spongy regions of bone are filled with marrow, which comes in two types. Red bone marrow is a blood cell factory. Stem cells inside it produce red blood cells, white blood cells, and platelets. In adults, red marrow is concentrated in the spine, pelvis, ribs, and the ends of large bones like the femur.
Yellow bone marrow is mostly fat. It fills the central cavities of long bones and serves as an energy reserve. Yellow marrow also contains stem cells that can produce bone, cartilage, muscle, and fat cells. If the body faces severe blood loss or certain diseases, yellow marrow can convert back to red marrow to ramp up blood cell production.
Water in Bone
Bones contain about 31% water, according to data from the U.S. Geological Survey. Water fills the tiny channels between bone cells, occupies spaces within the collagen framework, and is bound within the hydroxyapatite crystals themselves. This water content contributes to bone’s ability to absorb shock. Dehydrated bone, like a specimen sitting in a lab, is noticeably more brittle than living bone, partly because it has lost this built-in cushioning.