The adult skeleton has 206 bones (≈270–300 ossification elements are present at birth; many fuse during growth). It is divided into the axial skeleton (80 bones: skull 22, hyoid 1, auditory ossicles 6, vertebral column 26, sternum 1, ribs 24), which forms the central axis and protects the brain, spinal cord and thoracic viscera, and the appendicular skeleton (126 bones: pectoral girdles and upper limbs 64, pelvic girdle and lower limbs 62), which is specialised for locomotion and manipulation. Bones are classified by shape as long (femur, phalanges), short (carpals, tarsals), flat (cranial vault, sternum, ribs, scapula), irregular (vertebrae, sphenoid) and sesamoid (patella, pisiform).
Bone is a living, highly vascular connective tissue. Its extracellular matrix combines type I collagen (tensile strength and flexibility) with crystals of hydroxyapatite [Ca10(PO4)6(OH)2] (compressive strength and stiffness). Two architectural forms are present in every bone: dense compact (cortical) bone, organised into cylindrical osteons, forms the outer shell and the shafts of long bones; spongy (trabecular, cancellous) bone, a lattice of trabeculae aligned along lines of stress, fills the ends of long bones and the interior of flat and irregular bones and holds red marrow. Bone is covered externally by periosteum and lined internally by endosteum, both of which contain osteoprogenitor cells.
Four cell types maintain bone: osteoprogenitor cells (mesenchymal stem-cell derived), osteoblasts (secrete osteoid and mineralise it), osteocytes (osteoblasts entombed in lacunae, connected by canaliculi, which sense mechanical load and orchestrate remodelling) and osteoclasts (multinucleated cells of the monocyte–macrophage lineage that resorb bone). Coupled resorption and formation — remodelling — renews ≈5–10% of the adult skeleton each year, repairs microdamage, adapts architecture to load (Wolff's law) and releases or stores calcium under the control of parathyroid hormone, calcitriol and calcitonin.
Bones develop by two routes. Intramembranous ossification converts mesenchyme directly into bone (flat bones of the cranial vault, most facial bones, much of the clavicle and mandible). Endochondral ossification replaces a hyaline-cartilage model (most of the skeleton, including the limb bones, vertebrae and skull base). Longitudinal growth continues at the epiphyseal (growth) plates until they fuse after puberty; growth in width is appositional, beneath the periosteum.
Joints (articulations) are classified structurally as fibrous (sutures, syndesmoses, gomphoses), cartilaginous (primary synchondroses of hyaline cartilage and secondary symphyses of fibrocartilage) or synovial, and functionally as synarthroses (immovable), amphiarthroses (slightly movable) or diarthroses (freely movable). Synovial joints — the joints of the limbs — have articular cartilage, a fibrous capsule lined by synovial membrane and a cavity of synovial fluid; they are subclassified by shape into plane, hinge, pivot, condylar (ellipsoid), saddle and ball-and-socket joints. Their stability depends on bony congruity, ligaments and the muscles that cross them, a trade-off seen clearly between the mobile, frequently dislocated shoulder and the deep, stable hip.
Development
The skeleton is mesodermal (with a major neural-crest contribution to the head). Paraxial mesoderm forms somites, whose sclerotomes migrate around the notochord and neural tube to form the vertebrae and ribs; each vertebral body forms from the caudal half of one sclerotome fused with the cranial half of the next ("resegmentation"), which is why spinal nerves exit between vertebrae and the intervertebral discs form between them. The notochord persists only as the nucleus pulposus. Lateral plate (somatic) mesoderm forms the limb skeleton, the sternum and the pelvic and pectoral girdles; limb buds appear at ≈4 weeks (upper limb slightly before lower) and their cartilage models form by week 6. The neurocranium comprises a membranous part (vault, intramembranous) and a cartilaginous chondrocranium (skull base, endochondral); the viscerocranium (face) arises largely from neural crest of the first two pharyngeal arches (maxilla, mandible, zygomatic, malleus, incus, stapes, styloid process, hyoid). The first bone to begin ossifying is the clavicle (week 5–6); primary centres of most long bones appear in weeks 7–12; most secondary centres appear after birth and the last growth plates (e.g. medial clavicle, iliac crest) fuse in the early-to-mid 20s.