skeleton

A skeleton is the internal framework that supports and shapes an organism’s body. In vertebrates it is an endoskeleton composed mainly of bone and cartilage, whereas some invertebrates rely on exoskeletons or hydrostatic skeletons.

Components and functions: The skeletal system includes bones, joints, cartilage, ligaments, and associated tissues such as bone marrow. Bones provide structural support, protect internal organs, and act as levers for movement when muscles contract. They store minerals, particularly calcium and phosphate, and house bone marrow, which produces blood cells. Joints and cartilage enable flexibility and mobility, while ligaments stabilize joints.

Anatomical organization and structure: In humans and many other vertebrates, the skeleton is divided into the axial skeleton (skull, vertebral column, rib cage) and the appendicular skeleton (limbs and girdles). Adults typically have about 206 bones, though this number can vary with anatomical variations. Bone tissue includes compact bone, which is dense, and spongy bone, which contains marrow. Long bones have growth plates (epiphyseal plates) that allow lengthwise growth during development, and bones are continually remodeled by osteoblasts and osteoclasts.

Development and clinical relevance: Bone formation proceeds via intramembranous or endochondral ossification. The skeletal system can be affected by conditions such as osteoporosis (reduced bone density), osteoarthritis (joint degenerative disease), rickets (bone softening due to vitamin D deficiency), and osteogenesis imperfecta (fragile bones). Evolutionarily, many invertebrates rely on exoskeletons, while vertebrates use internal endoskeletons to support larger body plans.