Bone is a composite of collagen (flexibility) and mineral salts like calcium phosphate (hardness). Osteocytes maintain the matrix; osteoblasts build new bone; osteoclasts resorb old bone. Bone continuously remodels in response to stress and hormonal signals, maintaining strength and providing mineral storage.
From your study of connective tissue, you know that bone is a specialized connective tissue — but it accomplishes something unique among connective tissues by mineralizing its extracellular matrix. Bone's mechanical properties emerge from a two-component composite: collagen fibers provide tensile strength and flexibility (bone that is demineralized — soaked in acid until the mineral dissolves — becomes rubbery and bendable), while hydroxyapatite, a calcium phosphate mineral, provides compressive strength and hardness (bone with the collagen destroyed becomes brittle and crumbles). Neither alone would make a useful structural material; the combination creates a tissue that can absorb both tension and compression without failing.
Three specialized cell types maintain this tissue throughout life. Osteoblasts synthesize and secrete the organic components of the bone matrix (osteoid), then mineralize it; they originate from mesenchymal stem cells. Osteoclasts are large multinucleated cells derived from monocyte precursors; they resorb bone by secreting acids and enzymes that dissolve mineral and digest collagen. Osteocytes are osteoblasts that became surrounded by and embedded within the mineralized matrix they made; they form a communication network through tiny canals (canaliculi) and act as mechanosensors, detecting physical load and signaling whether more or less bone is needed.
Bone is not inert once formed — it undergoes continuous remodeling throughout life, removing old or damaged matrix and replacing it with new. This process follows a tight coupling rule: osteoclast resorption at a site is normally followed by osteoblast formation in the same location. The balance point is controlled by hormones (parathyroid hormone stimulates resorption to raise blood calcium; calcitonin suppresses resorption; estrogen and testosterone favor formation) and by mechanical loading. Wolff's Law captures the mechanical side: bone adapts its density and architecture to the stresses placed upon it. Trabecular bone (the spongy inner lattice) aligns along principal stress lines; loaded bone becomes denser; unloaded bone is resorbed. Astronauts lose bone in weightlessness; weight-bearing exercise builds it.
The clinical consequence of this balance is stark. Osteoporosis is not a failure of bone to form — it is a tipping of the remodeling balance toward net resorption, so that bone is broken down faster than it is replaced. Estrogen loss at menopause removes a key brake on osteoclast activity, which explains why postmenopausal women are disproportionately affected. Calcium storage (the skeleton holds ~99% of the body's calcium) means that whatever disrupts calcium homeostasis — vitamin D deficiency, hyperparathyroidism, prolonged glucocorticoid therapy — ultimately threatens bone integrity through the remodeling machinery.