Summary
Highlights
The skeletal system is often thought of as just bones, but it's a living tissue made of collagen and minerals. It includes bones and connective tissues like ligaments (bone to bone), tendons (muscle to bone), and cartilage. Different organisms have varying skeletal systems, such as hydrostatic skeletons in earthworms, exoskeletons in insects, and endoskeletons in humans.
The skeletal system supports the body, protects organs, stores minerals (especially calcium), produces red and white blood cells, and aids in movement with muscles. An adult human has around 206 bones. It's divided into the axial skeleton (skull, ears, throat bone, vertebral column, ribcage) and the appendicular skeleton (arms, legs, shoulder and pelvic girdles).
Bones are classified by shape: long bones (longer than wide, like the femur and humerus), short bones (cube-shaped, like carpals and tarsals), sesamoid bones (roundish, like the patella), flat bones (often curved and thin, like skull bones and scapulae), and irregular bones (complex shapes, like vertebrae).
Bones consist of compact bone tissue (hard outer layer) and spongy bone tissue, which contains bone marrow. Yellow marrow stores fat for energy, while red marrow produces blood cells (red and white) and platelets. Bones are vascular, meaning they have a rich blood supply.
Osteoblasts make bone and mature into osteocytes, which maintain bone structure. Osteoclasts break down bone using lysosomes. This process, called bone remodeling, removes old or damaged bone and rebuilds new bone, with 5-10% of the skeleton being remodeled annually. This also releases stored minerals like calcium, regulated by hormones.
Chondroblasts make cartilage, which supports bones in joints and serves as a template for bone development, especially during fetal development. Osteoblasts contribute to bone growth in length until early adulthood and in diameter even after length growth ceases. This involves complex processes and growth plates.
When a bone breaks, a fracture hematoma forms. Chondrocytes and osteoblasts create internal and external calluses to stabilize the break. Osteoclasts remove damaged bone, and osteoblasts rebuild new bone over 6-8 weeks, though this varies. Severe fractures can cause internal bleeding or fat embolism syndrome. Conditions like osteogenesis imperfecta, a genetic disorder affecting collagen production, lead to brittle bones.