Bone

Core Concepts

The cells of bone: builders, residents, and demolition crew

Three cell types run the tissue. Osteoprogenitor cells are stem-like cells that can divide and become osteoblasts. Osteoblasts are the builders: they secrete new bone matrix and, once surrounded by it, mature into osteocytes. Osteocytes are the resident managers — the most numerous bone cells — living inside small chambers called lacunae and communicating through tiny channels (canaliculi) that carry nutrients and signals. Osteoclasts are the demolition crew: large, multinucleated cells derived from the same lineage as white blood cells (monocytes) that dissolve bone matrix by secreting acid and enzymes. In healthy bone, build and demolition are balanced; the balance is what remodeling is about.

The matrix: rebar and concrete

Bone's matrix is like reinforced concrete. The mineral component — mostly hydroxyapatite (calcium phosphate crystals) — gives bone its hardness and resistance to compression, like concrete. The organic component — chiefly collagen fibers — gives bone its flexibility and resistance to tension, like steel rebar. Remove the minerals (as in rickets or osteomalacia, where mineralization is poor) and bone becomes soft and bendable; destroy the collagen (as prolonged boiling does to a bone in soup) and the mineral scaffold crumbles. This two-part design lets bone be both stiff and slightly springy, so it can absorb impact without shattering.

Compact bone and spongy bone

Compact (cortical) bone forms the dense outer shell of bones. It is organized into cylindrical units called osteons (Haversian systems): rings of matrix (lamellae) around a central canal that carries blood vessels and nerves, with osteocytes in lacunae connected by canaliculi — a plumbing system that keeps deep bone cells alive. Spongy (cancellous) bone lies inside, built of a lattice of thin struts called trabeculae, oriented along lines of stress. The open spaces are filled with red bone marrow (in adults, mainly in the sternum, ribs, vertebrae, pelvis, and skull) where blood cells are made; the central cavities of long bones hold yellow marrow, which stores fat. The periosteum (outer fibrous membrane) and endosteum (lining the inner cavities) contain the osteoprogenitor cells needed for growth and repair.

How bones form and grow

Bones arise by two routes. Intramembranous ossification builds bone directly within a connective-tissue membrane — this is how the flat bones of the skull and part of the clavicle form. Endochondral ossification first lays down a cartilage model of the bone, then replaces the cartilage with bone tissue — the route for most of the skeleton, including the long bones of the limbs. Long bones grow in length at the epiphyseal plates (growth plates), zones of dividing cartilage between the shaft and the bone ends; as the cartilage cells divide, they are replaced by bone, lengthening the bone. When growth plates close in late adolescence (at different ages in different bones), length growth stops — the reason skeletal age can be estimated from X-rays of growth plates. A bone also grows in thickness as periosteal cells deposit new compact bone on the outside while osteoclasts widen the marrow cavity inside.

Remodeling: lifelong construction

Bone is never finished. Throughout life, osteoclasts and osteoblasts work in coordinated cycles — osteoclasts carve a small tunnel, then osteoblasts fill it in — replacing roughly the whole skeleton over a period of years (a commonly taught figure is that the adult skeleton turns over about every decade, though rates vary by bone and age). Remodeling serves three purposes: repairing micro-damage from everyday stress, allowing bone to thicken in response to mechanical load (Wolff's law: bone adapts to the forces placed on it, which is why weight-bearing exercise strengthens bone and bed rest or spaceflight weakens it), and releasing stored calcium into the blood when needed. Blood calcium is regulated mainly by two hormones: parathyroid hormone (PTH), which raises blood calcium by activating osteoclasts (and acting on the kidneys), and calcitonin, which lowers blood calcium mainly by inhibiting osteoclasts — a feedback loop covered in more detail in the endocrine chapter.

Common Confusions