The skeletal system is made up of four primary tissues: bones, cartilage, ligaments, and tendons. Together, these connective tissues form the structural framework that supports your body, protects your organs, stores minerals, and produces blood cells. Adults have 206 bones, but the system extends well beyond bone itself to include the softer tissues that cushion, connect, and stabilize everything.
Bones: The Structural Core
Bone is the most recognizable component of the skeletal system, but it’s far from a simple solid material. By weight, bone is about 60% mineral (primarily a crystalline form of calcium and phosphate), 30% protein (over 90% of which is collagen), and 10% water. The mineral content gives bone its hardness and rigidity, while the collagen fibers provide flexibility and resistance to fracturing. Without the minerals, bone would bend like rubber. Without the collagen, it would shatter like chalk.
Two distinct types of bone tissue make up every bone in your body. Dense, solid bone forms the outer shell and makes up roughly 80% of total bone mass. The remaining 20% is spongy bone, a honeycomb-like mesh of tiny plates and rods found mainly at the ends of long bones and inside the vertebrae. The ratio between these two types varies dramatically depending on location. The shaft of your forearm bone is about 95% dense bone, the head of your thighbone is a 50/50 split, and your vertebrae are roughly 75% spongy bone. This variation reflects the different mechanical demands placed on each area.
Inside the spaces of spongy bone sits bone marrow. Red marrow produces blood cells, generating the red blood cells, white blood cells, and platelets your body needs. Yellow marrow is mostly fat and contains stem cells that can develop into cartilage, fat, or new bone cells. In children, red marrow fills most bones. In adults, red marrow concentrates in the spine, pelvis, ribs, skull, and the ends of long bones, while yellow marrow takes over the central shafts.
The Cells That Build and Break Down Bone
Bone is living tissue, constantly being remodeled by three types of specialized cells. Bone-building cells line the surface of bones, making up about 4 to 6% of all bone cells. They lay down new bone in two stages: first depositing a soft protein framework, then mineralizing it into hard tissue. Bone-dissolving cells work in the opposite direction, secreting acids and enzymes that break down old or damaged bone so it can be replaced.
The third type, and by far the most numerous, are the sensor cells embedded deep within the bone matrix. These make up 90 to 95% of all bone cells and can live up to 25 years. They’re connected to each other through a vast network of tiny channels, forming a communication system that detects mechanical stress. When you walk, run, or lift something heavy, these cells sense the forces and signal the building and dissolving cells to strengthen bone where it’s needed most. When sensor cells die, the chemical signals they release attract bone-dissolving cells to that area, triggering a localized repair cycle.
The Two Divisions of the Skeleton
The 206 bones in an adult skeleton are divided into two groups. The axial skeleton runs along your body’s central axis and contains 80 bones: the skull, vertebral column, ribs, and breastbone. This division primarily protects vital organs like the brain, spinal cord, heart, and lungs.
The appendicular skeleton contains 126 bones and includes everything else: your arms, legs, hands, feet, and the girdles that attach them to the axial skeleton (the shoulder blades and collarbones at the top, the pelvis at the bottom). This division is built for movement and manipulation. Babies are born with significantly more bones than adults because many separate pieces of bone gradually fuse together during childhood and adolescence.
Cartilage: The Body’s Shock Absorber
Cartilage is a firm but flexible tissue that cushions joints, maintains the shape of structures like your nose and ears, and provides a smooth surface for bones to glide against each other. Three types serve different purposes throughout the body.
- Hyaline cartilage is the most common type, with a glassy, smooth appearance. It covers the ends of bones inside joints and forms the structural framework of your nose, windpipe, and the connections between your ribs and breastbone. Despite being the most widespread, it’s actually the weakest of the three types. During fetal development, much of the skeleton starts as hyaline cartilage before gradually converting to bone.
- Fibrocartilage is the strongest type. It contains thick layers of dense collagen fibers oriented along the lines of stress, making it exceptionally tough. You’ll find it in the discs between your vertebrae, in joint capsules, and in certain ligaments. It handles compression and heavy loads better than any other cartilage.
- Elastic cartilage is flexible and springy, allowing it to maintain shape while bending repeatedly. It forms the outer ear, the epiglottis (the flap that covers your windpipe when you swallow), and parts of the voice box.
Ligaments and Tendons
Ligaments are bands of fibrous tissue that connect bones to other bones. Their primary job is stabilization: holding joints together and preventing bones from moving in directions they shouldn’t. The ligaments in your knee, for example, keep the joint from bending sideways or rotating too far.
Tendons connect muscles to bones and serve a fundamentally different purpose. Where ligaments stabilize, tendons transmit force. When a muscle contracts, the tendon pulls on the bone to create movement. Both are made of dense connective tissue, but tendons are structured more like cords or cables, designed to handle the pulling forces of muscular contraction.
How Joints Connect It All
Joints are the points where two or more bones meet, and they come in three functional types based on how much movement they allow.
Fixed joints permit no movement at all. The bones of your skull, for instance, are locked together by fibrous connective tissue. Slightly movable joints, like the ones between your vertebrae or at the pubic symphysis, are connected by cartilage and allow limited motion. The small amount of flex in each spinal joint adds up to meaningful range of motion across the whole spine.
Freely movable joints are the ones most people think of when they picture a joint. Your knees, hips, shoulders, elbows, and finger joints all fall into this category. These synovial joints are enclosed in a fluid-filled capsule that lubricates the joint surfaces, with cartilage covering the bone ends and ligaments reinforcing the connection. Even within this category, the degree of motion varies widely. A ball-and-socket joint like the shoulder allows rotation in nearly every direction, while a hinge joint like the elbow moves primarily along one plane, limited by the ligaments holding it in place.