Bones are often perceived as dry, brittle, and lifeless structures, like those seen in museum displays. This common perception misrepresents the true nature of living bone. Far from being arid, living bone tissue is a complex, dynamic, and hydrated biological material.
The Hydrated Nature of Living Bone
Living bone is a sophisticated composite material, integrating organic and inorganic components with significant water content. Water constitutes a substantial portion of bone volume, typically around 25% by volume. This water is distributed as free water in microscopic pores and bound water associated with collagen and mineral crystals.
The organic matrix, primarily type I collagen, accounts for 30-35% of bone volume, providing tensile strength and flexibility. Inorganic mineral crystals, predominantly hydroxyapatite, make up 40-60% of bone volume, imparting rigidity and compressive strength. Water is important for integrating these components, contributing to bone’s elasticity and its ability to absorb and dissipate mechanical stress, enhancing its fracture resistance.
Bone as a Dynamic Living Tissue
Beyond its chemical composition, living bone is a metabolically active tissue, constantly undergoing change. It contains specialized cells that continuously build, resorb, and maintain the bone matrix. Osteoblasts form new bone tissue, secreting the organic matrix that later mineralizes. Conversely, osteoclasts break down old or damaged bone through resorption.
Osteocytes, mature bone cells embedded within the mineralized matrix, maintain the tissue and sense mechanical stresses. These cells are supported by a rich network of blood vessels and nerves that permeate the bone. This blood supply delivers nutrients, oxygen, and hormones, and removes waste products, all essential for continuous bone remodeling. This ongoing remodeling ensures bones remain strong, adapt to mechanical demands, and repair micro-damage throughout life.
Why Bones Appear Dry
The common perception of bones as dry stems from what happens to them after an organism dies. Post-mortem, decomposition begins, leading to the gradual loss of bone’s water content. As soft tissues decay, the water within the bone evaporates, and the organic matrix, primarily collagen, degrades over time.
This loss of water and organic material significantly reduces the bone’s weight and changes its physical properties, leaving behind mostly the rigid mineral framework. The “bone dry” appearance of skeletal specimens, such as those found in museums or archaeological sites, is a direct result of these post-mortem changes. These specimens are often fossilized, where the original organic material has been replaced by minerals from the surrounding environment, or they have simply dried out, contrasting sharply with the hydrated, living bones within a functioning body.