Skeleton: structure, types, functions and the human skeletal system

Types of skeletons

Biological skeletons fall into several broad categories that evolved independently in different lineages:

• Endoskeleton — an internal bony or cartilaginous framework, typical of vertebrates such as mammals. See general vertebrate notes at mammals overview and human specifics at human skeleton.
• Exoskeleton — a hard outer shell or cuticle that supports and protects the body of many arthropods and some molluscs.
• Hydrostatic skeleton — a fluid-filled internal cavity that provides support through pressure, used by worms, jellyfish and other soft-bodied animals.

Structure and development

In vertebrates the skeleton is primarily composed of bone and cartilage. Bone is a living tissue made of cells embedded in a mineralized matrix; cartilage is a more flexible connective tissue that often precedes bone during development or remains in joints and certain structures. Individual bones have regions such as the shaft (diaphysis), ends (epiphyses), and the marrow cavity where blood cells are produced. The cartilage, ligaments and tendons join bones and enable movement. For a basic component list see bone components and the relation to skin and soft tissues at skin and support.

Primary functions

The skeleton performs several essential roles for an animal's survival and mobility. Major functions include:

1. Support: maintaining body shape and posture by providing a firm framework.
2. Protection: shielding delicate organs — for example the skull encloses the brain and the rib cage protects the heart and lungs; see related notes on organ sheltering at internal organ protection, brain and heart.
3. Movement: bones act as levers against which muscles pull, enabling locomotion and manipulation.
4. Mineral storage and metabolic roles: bone stores calcium and phosphorus and participates in mineral homeostasis.
5. Blood cell formation: bone marrow produces red and white blood cells and platelets.

The human skeleton and health

An adult human typically has 206 named bones arranged into the axial skeleton (skull, vertebral column, ribs) and the appendicular skeleton (limbs and girdles). Bones develop through ossification, a process that transforms cartilage or fibrous tissue into mineralized bone during growth. Common clinical issues affecting the skeleton include fractures, degenerative joint disease (osteoarthritis), inflammatory conditions (arthritis), and metabolic bone disorders such as osteoporosis. Proper nutrition, exercise and medical care maintain skeletal health.

Notable distinctions and facts

Different animal groups demonstrate the diversity of skeletal solutions: insects rely on exoskeletons for muscle attachment and defense, while many marine animals depend on buoyancy and hydrostatic pressure instead of rigid bones. The evolution of internal skeletons was a key step in vertebrate diversification, enabling larger body sizes and more complex locomotion. For further background and introductory resources see muscle interactions, human overview, and additional references at general resource.