CorpusHuman Anatomy

Long Bone — Structure

Coronal section of a femur with magnified compact bone (osteons)

The left half shows a long bone (modelled on the femur) cut lengthwise. A thick tube of compact bone forms the diaphysis around the medullary cavity, which in adults is filled with fatty yellow marrow; the expanded ends (epiphyses and metaphyses) are a thin cortical shell filled with spongy (trabecular) bone, whose spaces hold haemopoietic red marrow. Hyaline articular cartilage caps the joint surfaces, periosteum covers every other external surface, and endosteum lines the cavity. For comparison, the proximal end shows an active epiphyseal (growth) plate as in a child, and the distal end the epiphyseal line left after the plate closes. The nutrient artery enters obliquely through the nutrient foramen and divides into ascending and descending medullary branches.

The magnified inset is a transverse section through the cortex: cylindrical osteons (Haversian systems) of concentric lamellae surround central canals carrying an arteriole, venule and nerve; osteocytes sit in lacunae linked by canaliculi; perforating (Volkmann) canals join neighbouring central canals and the periosteum; interstitial and circumferential lamellae fill the remaining space. Use the steps to walk from the outside of the bone inwards.

Step-by-step

  1. Regions of a long bone

    A long bone has a shaft (diaphysis), two expanded ends (epiphyses) and, between them, the flared metaphyses where the growth plate lies during development. The metaphysis is the most vascular, fastest-remodelling region — the commonest site of haematogenous osteomyelitis and primary bone tumours in children.

  2. Coverings: articular cartilage and periosteum

    Joint surfaces are capped by avascular hyaline articular cartilage (no perichondrium, nourished by synovial fluid). Every other external surface is covered by periosteum: an outer fibrous layer anchored by Sharpey fibres and an inner osteogenic (cambium) layer that widens bone and forms callus after fracture. Periosteum is richly innervated — the source of fracture pain.

  3. Compact (cortical) bone

    The dense outer wall, thickest in the diaphysis where bending stresses are greatest, and only a thin shell over the epiphyses. Compact bone makes up ≈ 80% of skeletal mass. Its lamellae are arranged as circumferential lamellae under the periosteum and endosteum, osteons, and interstitial lamellae between them.

  4. The osteon (Haversian system)

    The structural unit of compact bone: a cylinder of concentric lamellae around a central (Haversian) canal containing an arteriole, venule and nerve, bounded by a cement line. Osteons run parallel to the long axis, so the shaft resists compression along its length.

  5. Osteocytes, canaliculi and perforating canals

    Osteocytes — trapped former osteoblasts — sit in lacunae between lamellae and communicate through canaliculi via gap junctions, sensing strain and secreting sclerostin and RANKL to steer remodelling. Perforating (Volkmann) canals cross the lamellae to link central canals with periosteal and medullary vessels.

  6. Spongy bone and red marrow

    Inside the epiphyses and metaphyses, a lattice of trabeculae aligned along lines of stress (Wolff law) lightens the bone. Trabeculae have no osteons — their osteocytes are nourished directly from the marrow spaces, which hold haemopoietic red marrow (in adults mainly in the proximal femur and humerus, vertebrae, sternum, ribs and pelvis).

  7. Medullary cavity and endosteum

    The hollow centre of the diaphysis, filled in adults with fatty yellow marrow (which can revert to red marrow in severe anaemia). It is lined by the thin endosteum, containing osteoprogenitor cells, osteoblasts and osteoclasts that remodel the inner surface — the cavity enlarges as the bone grows.

  8. Epiphyseal plate and epiphyseal line

    The hyaline cartilage growth plate (proximal end here) lengthens the bone by endochondral ossification: chondrocytes proliferate on the epiphyseal side and are replaced by bone on the metaphyseal side. After puberty it closes (femur ≈ 14–19 years) and leaves a bony epiphyseal line (distal end here). Plate injuries are graded by the Salter–Harris classification.

  9. Blood supply

    The nutrient artery enters through the nutrient foramen and splits into ascending and descending medullary branches supplying the marrow and inner cortex, then reaches the osteons via perforating and central canals. Periosteal, metaphyseal and epiphyseal arteries complete the supply; loss of epiphyseal vessels causes avascular necrosis (e.g. femoral head after an intracapsular neck fracture).

Labelled parts