DisclosureWe earn commission on partner links; ranking is set by our evidence-based methodology — not advertisers. Read policy
A clear, evidence-based guide to the human skeletal system: its 206 bones, how bone tissue works, the organs and structures involved, common conditions like osteoporosis, and proven ways to keep your bones strong.
Reviewed & updated
The skeletal system is the body's framework of bones, cartilage, ligaments, and tendons. In adults it contains 206 bones (newborns start with roughly 270, some of which fuse during growth, with ossification typically complete around age 25). Far from being a static scaffold, bone is living, metabolically active tissue that constantly rebuilds itself throughout life.
The skeletal system performs five core jobs:
Anatomists divide the skeleton into two parts: the axial skeleton and the appendicular skeleton.
The axial skeleton (about 80 bones) forms the central axis: the skull (cranium), the hyoid bone, the vertebral column, and the thoracic cage (sternum and ribs). The spine has 33 vertebrae — 7 cervical, 12 thoracic, 5 lumbar, plus the fused sacrum (5 fused vertebrae) and coccyx (4 fused vertebrae). The thoracic cage includes the sternum and 12 pairs of ribs.
The appendicular skeleton (126 bones) includes the upper limbs, lower limbs, and the shoulder and pelvic girdles that anchor them to the axial skeleton. Six tiny auditory ossicles in the middle ears are counted separately.
Bone is built from two architectures. Compact (cortical) bone is the dense outer shell that resists bending and provides strength. Spongy (trabecular) bone is the lighter, honeycomb-like interior found at the ends of long bones and inside flat bones; it houses marrow and helps absorb stress.
The bone matrix — the extracellular material that makes up roughly 90% of bone volume — has two key ingredients:
Four cell types maintain this matrix:
Bone remodeling is a tightly coupled, lifelong cycle in which osteoclasts break down old bone and osteoblasts rebuild it. This repairs micro-damage, adapts bone to mechanical stress, and releases or stores minerals. Bone mass typically peaks in the 20s (somewhat earlier in women, into the late 20s in men), then gradually declines.
Bone also helps regulate blood calcium, which is essential for nerves, muscles, and the heart. Parathyroid hormone (PTH) raises blood calcium partly by stimulating bone breakdown, while vitamin D boosts calcium absorption from food. Calcitonin can lower blood calcium. This hormonal interplay keeps calcium within a narrow, life-sustaining range.
The information below is general and educational; it describes conditions but is not a diagnosis or treatment plan.
> This article is educational and is not a substitute for professional medical advice, diagnosis, or treatment. Talk to a qualified clinician about screening (such as a DXA bone-density scan) and about any bone or joint symptoms before making changes to your care.
Several evidence-based habits help build and protect bone across the lifespan:
Building strong bones early and protecting them through adulthood gives the skeletal system the best chance of supporting you for life. Individual needs vary, so discuss supplements, screening, and exercise with your healthcare provider.
In-depth, independently reviewed guides to the major organs and structures of this system.
This guide draws on 13 peer-reviewed and authoritative sources (StatPearls, NIH/NHLBI, MedlinePlus, and primary literature).