The bones inside your body make a hard frame called the skeleton, and of the 206 bones in an adult skeleton, 80 are axial (skull, hyoid, ear bones, spine, ribs and breastbone) and 126 are appendicular (arms, shoulders, hands, legs, hips and feet). The bones of your skull protect your brain, your ribs form a cage that protects your heart and lungs, and your backbone protects your spinal cord. A skull has 8 bones in the part that holds the brain and 14 bones in the face, and the femur, or thighbone, is the longest and strongest bone in the human body, while the stapes, one of three tiny bones in the middle ear, is the smallest. A baby is born with 12 pairs of ribs, and the top seven connect to the breastbone with cartilage, and the spine has seven neck vertebrae, 12 chest vertebrae that connect to the rib cage, and five larger lower-back vertebrae.
By the end of the eighth week after conception, the skeleton's pattern is laid out in cartilage and soft membranes, and bone formation begins. Most bones form when a cartilage model is replaced by bone, a process called endochondral ossification, but some bones form inside sheet-like membranes instead of from a cartilage model, a process called intramembranous ossification. A baby's skull bones are not firmly joined at birth, which leaves soft spots on the head, and the fontanelle at the back of a baby's head usually closes by age 1 to 2 months, and the one at the top within 7 to 19 months. Baby skull bones are joined by sutures that gradually gain minerals and harden, a process called ossification, and a skull's cranial seams close at different times, with the first (the metopic suture) closing at about 3 to 9 months and others not until adulthood.
A long bone has a hollow shaft (diaphysis), flared ends called metaphyses below the growth plates, and rounded ends called epiphyses above them. Cartilage in the growth plate keeps growing until it stops in the early twenties, when the plate turns to bone and the bone can no longer grow longer. Yellow marrow is found in the large cavities of large bones and is mostly fat cells, while in infants red marrow fills the bone cavities, but in adults it is mostly limited to the skull, ribs, sternum, collarbones, vertebrae and pelvis. Red blood cells are made in the red marrow inside bones, and bone marrow holds stem cells that can become bone, cartilage, fat or fibrous tissue, separate from the stem cells that make blood.
Bone remodeling begins before birth and continues until death, and it is a four-step cycle: activation, resorption, reversal, then formation. Osteoclasts are the only cells known to be able to break down (resorb) bone, and bone-building cells (osteoblasts) and bone-removing cells (osteoclasts) balance each other to keep bone healthy. In each remodeling cycle, osteoclasts take only about 2 to 4 weeks to remove old bone, but forming new bone takes about 4 to 6 months. In adults, dense cortical bone turns over at only about 2 to 3% a year, which is enough to keep it mechanically strong. Old bone is constantly broken down and replaced by new bone, a process called remodeling.
Osteoblasts that become buried in bone matrix turn into osteocytes, which are linked to each other by a network of tiny channels. Osteocytes sense bending and stretching of bone and turn those stress signals into biological activity, a job called mechanosensation, and osteocytes may live for decades in human bone that is not turned over. Compact bone is built from osteons: a central canal with blood vessels surrounded by rings of matrix, with bone cells in small spaces between them, and lamellar bone is strong because collagen fibrils are laid down in alternating directions, similar to plywood.
Bone strength depends on bone mass, geometry and composition, material properties and microstructure, and bone mass accounts for about 50 to 70% of bone strength. As a bone widens, its strength rises with the fourth power of its radius, so larger bones are stronger than smaller ones even at the same bone mineral density. Bone mineral gives bone rigidity and load-bearing strength, while the organic matrix gives it elasticity and flexibility, and bone is about 50 to 70% mineral, 20 to 40% organic matrix, 5 to 10% water and less than 3% fat. Spongy bone's trabeculae follow the lines of stress and can realign if the direction of stress changes, and Wolff's law describes the observation that long bones change shape to accommodate the stresses placed on them. Even after bones stop growing longer, they can keep getting thicker throughout life when muscle activity or weight puts more stress on them.
Calcium is released from bone and stored in it almost continuously to keep the level in the blood steady. Vitamin D plays an indirect role in helping new bone matrix mineralize by keeping blood calcium and phosphorus high enough, and vitamin D helps the body absorb the calcium it gets from food. Children who do not get enough vitamin D can develop rickets, in which the bones are soft and weak. A DXA scan uses low-dose X-rays to measure how much calcium and other minerals are in a bone, usually at the hip and spine, and each step down in a bone density T-score raises the risk of broken bones by 1.5 to 2 times.
Not everything stiff in your body is bone. The part of your ear you can see is made of cartilage, a tough bendy tissue, not bone, and a shark has a skeleton made of cartilage instead of bone. Giraffe necks have seven big bones, the same as almost all mammals, while the necks of sauropod dinosaurs had up to 19. A bird's bones are hollow and light, which helps it fly, and bird bones have thin walls and are filled with extensions of the lungs' air sacs, yet resist the bending stresses of flight.
How can doctors see bones inside a living body? Wilhelm Röntgen found X-rays by accident in 1895 while experimenting with vacuum tubes, and his X-ray picture of his wife's hand showed the bones of her hand and her ring. Doctors use X-rays to see whether a bone is broken, and bones show up white on an X-ray because they absorb the most radiation. A cast or splint holds a broken bone still while it heals, and children's broken bones can heal well and heal quickly. Broken bones heal in three stages: inflammatory, reparative and remodeling.
Old bones can answer questions about the past. The femur is one of the best-preserved bones in skeletal remains, so it contributes a great deal to archaeology and forensic science. Archaeologists can tell something about a person's work from bones, because the pull of a laborer's muscles reshapes the bone where the muscles attach. Lucy, a famous early human relative found in Ethiopia, has a skeleton that is about 40% complete, and she is about 3.2 million years old. She got her nickname from a Beatles song that was playing in the team's camp. Lucy's bones had no duplicates, which supports the conclusion that they came from one individual, and her small skull, long arms and cone-shaped rib cage resemble an ape's, but her spine, pelvis and knee are more human-like because she walked upright.
Spaceflight shows what happens when stress is removed from bone. Early studies of Mir crews found that space travelers can lose 1 to 2% of bone mass each month. On the International Space Station, crew lost spongy bone in the hip at 2.2 to 2.7% and dense outer bone at 1.6 to 1.7% for each month in space, on average, and hip bone strength fell by an average of 2.5% for each month of flight. In prolonged weightlessness, bone mass appears to fall because the lack of stress slows the formation of osteoblast cells, and researchers think the lack of stress on bones in weightlessness may cause the steady bone loss seen in people who live in space for a long time. No one knows whether bone loss in space continues at the same rate on very long trips or levels off, so ISS results should not be used to predict much longer missions.
Words to know
- osteocytes: buried bone cells
- mechanosensation: sensing force
- resorption: bone breakdown
- trabeculae: spongy bone struts
- metaphyses: flared bone ends
