Bones Are Living Tissue: How Your Skeleton Rebuilds Itself
Plastic skeletons are useful for learning landmarks, but they can accidentally teach the wrong lesson about bone. They make it look finished: hard, dry and essentially permanent.
Inside a living person, bone is nothing like that.
Bone has a blood supply, nerves, specialised cells and a constantly changing extracellular matrix. It responds to mechanical loading, helps regulate mineral balance, repairs microscopic damage and can rebuild after fracture. The marrow protected within the skeleton is also the site of blood-cell production.
So when we describe bone as living tissue, that is not a metaphor. The skeleton is being maintained, sensed, resorbed and rebuilt throughout life.
Inside living bone
Take a long bone such as the femur. The dense outer shell is cortical bone, also called compact bone. It is particularly thick through the shaft and provides much of the bone's rigidity and resistance to bending. Deeper inside is trabecular bone, a lighter lattice of interconnected plates and struts. Trabecular bone has a much larger surface area relative to its volume, so it is particularly active in bone turnover.
The outside of most bone is covered by the periosteum, except where articular cartilage covers joint surfaces. The periosteum is vascular and richly innervated. Internal surfaces, including the medullary cavity and the surfaces of trabeculae, are lined by the much thinner endosteum. These surfaces matter because much of bone formation and resorption takes place along them.
Bone is also well supplied with blood. Periosteal vessels help supply the outer cortex, while nutrient arteries enter through nutrient foramina and contribute to the deeper circulation. In long bones, metaphyseal and epiphyseal vessels add further supply. These vessels deliver oxygen and nutrients and provide routes for hormones, signalling molecules and cells involved in repair.
Within the spaces created by this architecture sits bone marrow. Red marrow contains haematopoietic tissue, where developing blood-cell lineages produce erythrocytes, leukocytes and platelets. In adults it is concentrated mainly in the axial skeleton, including the vertebrae, ribs, sternum and pelvis, with additional red marrow at the proximal ends of some long bones. Marrow is not mineralised bone tissue, but it is protected by bone and shares its vascular environment. Bleepbook has a separate article on how bone marrow produces blood cells.
The hard tissue surrounding those spaces is built from an extracellular matrix with two main components. The organic part is mainly type I collagen, which contributes tensile strength and some flexibility. The mineral component consists largely of hydroxyapatite crystals containing calcium and phosphate, which give bone much of its stiffness and resistance to compression.
Bone works because those two components are combined. Mineral alone would be hard but brittle. Collagen alone would not provide the rigidity needed to support body weight. The matrix gives bone its strength, but the cells within and around it are what keep that matrix alive.
The cells that maintain the skeleton
Three cell types sit at the centre of bone physiology: osteoblasts, osteocytes and osteoclasts. They are often taught as separate definitions, but they make more sense as parts of one continuous process.
Osteoblasts form new bone. They arise from mesenchymal osteoprogenitor cells and produce osteoid, the unmineralised organic matrix that is rich in type I collagen. Calcium and phosphate are then deposited into this matrix during mineralisation. Some osteoblasts remain on the surface as bone-lining cells, some undergo programmed cell death and some become surrounded by the matrix they helped create.
Those embedded cells become osteocytes.
Osteocytes sit within tiny spaces called lacunae. Long cell processes extend through microscopic channels called canaliculi, creating a communication network through the mineralised matrix. This lacunocanalicular network allows osteocytes to sense changes in mechanical strain and fluid movement within bone and to signal to other bone cells.
Osteoclasts resorb bone. They come from the monocyte/macrophage cell lineage and fuse to form large multinucleated cells. Once attached to a bone surface, an osteoclast seals off a small resorption area, acidifies it to dissolve mineral and releases enzymes that break down the organic matrix.
Resorption sounds destructive, but healthy bone needs it. Old or microscopically damaged tissue has to be removed before it can be replaced. That removal and replacement is bone remodelling.
How bone remodelling works
Remodelling happens in many tiny localised areas rather than across the whole skeleton at once. In each area, a group of cooperating cells removes a small amount of old bone and then replaces it with new tissue.
The sequence is usually described as activation → resorption → reversal → formation → mineralisation. During activation, local signals recruit osteoclast precursors. Osteoclasts then resorb a small amount of bone. During the reversal phase the resorbed surface is prepared for new formation. Osteoblasts lay down osteoid, and that osteoid subsequently mineralises.
The cells do not work independently. One important signalling system linking them is the RANKL-RANK-OPG pathway. Receptor activator of nuclear factor kappa-B ligand (RANKL), produced mainly by osteoblast-lineage cells and osteocytes, binds to RANK on osteoclast precursors and promotes osteoclast formation and activity. Osteoprotegerin (OPG) acts as a decoy receptor, binding RANKL and reducing that signal.
The result is a tightly regulated conversation between cells that remove bone and cells that replace it. In healthy adult bone, resorption and formation remain closely coupled over time. They do not happen at exactly the same moment, but if the amount removed is repeatedly greater than the amount replaced, bone mass and microarchitecture begin to deteriorate.
This is why the familiar claim that the skeleton is replaced every seven to ten years can be misleading. Bone turnover is continuous, but it varies between bones, between cortical and trabecular tissue and across the lifespan. There is no single moment when an old skeleton is exchanged for a new one.
There is also a related process called bone modelling. In remodelling, resorption and formation are linked at the same site. In modelling, they can occur on different surfaces, allowing bone to alter its size and shape. Modelling is especially important during growth, while remodelling continues throughout adult life to renew tissue and respond to damage.
The important question, then, is what tells bone when to change. Part of that answer comes from the forces passing through it every day.
Bone responds to load, hormones and minerals
Every step, stair climb and muscle contraction places force through the skeleton. Bone bends by microscopic amounts under that load. Osteocytes detect those changes and alter the signals they send to osteoblasts and osteoclasts.
One of the molecules involved is sclerostin, which is produced mainly by osteocytes and suppresses pathways that support osteoblast activity. Mechanical loading tends to reduce sclerostin signalling in areas experiencing strain, helping favour bone formation. Reduced loading has the opposite effect.
That gives a physiological explanation for what happens during prolonged immobility. When the skeleton is loaded less, the signal supporting bone formation falls. At the same time, muscle mass and strength decline, reducing the forces normally transmitted through bone even further. Over time, resorption can begin to exceed formation.
Bone also responds to the body's need to maintain calcium and phosphate balance. The skeleton contains the great majority of the body's calcium and phosphate, but those minerals are not locked away permanently. Calcium is required for nerve transmission, muscle contraction, intracellular signalling, blood coagulation and normal cardiac function, so the concentration of calcium in extracellular fluid is tightly regulated.
When ionised calcium falls, parathyroid hormone (PTH) helps restore it through effects on the kidneys, vitamin D metabolism and bone. PTH increases renal calcium reabsorption, influences phosphate handling, stimulates formation of active vitamin D and can indirectly increase osteoclast-mediated bone resorption. The active form of vitamin D, calcitriol, increases intestinal absorption of calcium and phosphate. Calcitonin can inhibit osteoclast activity, although its role in day-to-day calcium regulation in adults is comparatively limited.
This means bone is responding to two kinds of demand at once. It adapts to the mechanical forces placed through it, but it also participates in wider metabolic regulation. Both processes depend on the same living cells and mineralised matrix.
That distinction helps explain why osteomalacia and osteoporosis are different. Osteomalacia involves defective mineralisation of newly formed bone matrix, often associated with vitamin D deficiency. Osteoporosis involves reduced bone strength because bone quantity and microarchitecture have deteriorated. Both can increase fracture risk, but they arise through different problems in bone physiology.
How this physiology appears in patients
Once you follow the process from matrix to cells to remodelling, common clinical problems make more sense.
After the menopause, falling oestrogen changes the signalling that regulates osteoclasts and osteoblasts. Osteoclast activity can increase, so resorption begins to outpace formation and bone is lost more quickly. Long-term systemic glucocorticoid treatment can also increase fracture risk, partly by suppressing osteoblast formation and function and altering calcium handling. Prolonged immobility reduces mechanical stimulation. Vitamin D deficiency can impair mineralisation. Ageing changes bone turnover and the ability to replace what has been removed.
These mechanisms are different, but they all act on the same tissue that has been described throughout this article. In osteoporosis, the effect may remain silent until a fragility fracture occurs. Bleepbook's osteoporosis article takes that pathology further.
A fragility fracture also brings the vascular side of bone biology into view. When a bone breaks, vessels are disrupted as well as mineralised tissue. Bleeding contributes to formation of a fracture haematoma, and the local inflammatory response begins the repair process. In the common pattern of secondary fracture healing, a soft callus forms and is gradually replaced by mineralised callus before longer-term remodelling reshapes the bone. Direct or primary fracture healing is different and can occur when a fracture is held in very rigid stability, so callus formation should not be treated as universal.
Consider an older adult admitted after a fall from standing height with a fractured neck of femur. The fracture is obvious, but the physiology asks a second question: what was happening in the bone before the fall?
Previous fragility fractures, long-term systemic glucocorticoid use, menopause, reduced mobility, low body weight, poor nutritional intake, smoking, alcohol use and conditions associated with secondary osteoporosis may all contribute to the picture. Current UK guidance recommends fracture-risk assessment in appropriate patients, using tools such as FRAX or QFracture within their validated age ranges alongside clinical assessment. The exact pathway and treatment threshold will depend on current national guidance and local policy.
After surgery, the same biology continues. Pain control affects whether the patient can mobilise. Nutrition provides the substrates needed for recovery. Safe mobilisation helps preserve muscle and restore mechanical loading. Falls prevention matters because one fragility fracture predicts future risk. Physiotherapy and occupational therapy plans matter because recovery is not simply about waiting for a fracture to unite.
You will never see an osteoclast working at the bedside. You will see the result of years of osteoclast, osteoblast and osteocyte activity in the strength of a patient's bone, the way it responds to immobility and the way it repairs after injury.
References
Florencio-Silva, R., Sasso, G.R.S., Sasso-Cerri, E., Simões, M.J. and Cerri, P.S. (2015) ‘Biology of bone tissue: structure, function, and factors that influence bone cells’, BioMed Research International, 2015, 421746. Available at: PubMed Central.
Langdahl, B., Ferrari, S. and Dempster, D.W. (2016) ‘Bone modeling and remodeling: potential as therapeutic targets for the treatment of osteoporosis’, Therapeutic Advances in Musculoskeletal Disease, 8(6), pp. 225–235. Available at: PubMed Central.
Ono, T., Hayashi, M., Sasaki, F. and Nakashima, T. (2020) ‘RANKL biology: bone metabolism, the immune system, and beyond’, Inflammation and Regeneration, 40, 2. Available at: PubMed Central.
National Institute for Health and Care Excellence (NICE) (2012, updated 2017) Osteoporosis: assessing the risk of fragility fracture (CG146). Available at: NICE.
NHS (n.d.) Osteoporosis. Available at: NHS. Accessed 5 October 2026.
NHS (n.d.) Osteoporosis: causes. Available at: NHS. Accessed 5 October 2026.
NHS (n.d.) Rickets and osteomalacia. Available at: NHS. Accessed 5 October 2026.
Buck, H.V. and Stains, J.P. (2024) ‘Osteocyte-mediated mechanical response controls osteoblast differentiation and function’, Frontiers in Physiology, 15, 1364694. Available at: PubMed Central.
Benulič, Č., Canton, G., Rasio, N., Murena, L. and Kristan, A. (2022) ‘Mechanobiology of indirect bone fracture healing under conditions of relative stability: a narrative review for the practicing clinician’, Acta Biomedica, 92(Suppl 3), e2021582. Available at: PubMed Central.