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. Bone is a framework of the protein collagen made hard by the mineral calcium phosphate, and the mineral that hardens bone is mostly hydroxyapatite, which gives bone much of its strength and density. Bones are living tissue, and old bone is constantly broken down and replaced by new bone, a process called remodeling.
A baby's skull bones are not firmly joined at birth, which leaves soft spots on the head. 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 as a child grows, the skull bones that cover the brain fuse together at joints called sutures. In adults these skull joints do not move. A skull has 8 bones in the part that holds the brain and 14 bones in the face.
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). A baby is born with 12 pairs of ribs, and the top seven connect to the breastbone with cartilage. The ribs form a cage that protects your heart and lungs. The spine has seven neck vertebrae, 12 chest vertebrae that connect to the rib cage, and five larger lower-back vertebrae. The femur, or thighbone, is the longest and strongest bone in the human body, and the stapes, one of three tiny bones in the middle ear, is the smallest.
Long bones grow longer at growth plates near their ends, and 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. Bone mass, the amount of bone tissue in the skeleton, peaks by the mid- to late 20s. Bones store calcium, a mineral, and give some back to the blood when the body needs it. Foods with calcium and vitamin D help build healthy bones, and vitamin D helps the body absorb the calcium it gets from food. Bones get stronger when you use them by being active, while astronauts on long space missions lose some bone.
Not every skeleton is the same. 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. A giraffe has seven big bones in its neck, 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.
How do doctors see a bone inside a 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. Children's bones are softer and more likely to bend than to break all the way through, and children's broken bones can heal well and heal quickly. Broken bones heal in three stages: inflammatory, reparative and remodeling.
Some skeletons are very old. Lucy, a famous early human relative found in Ethiopia, has a skeleton that is about 40 percent 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. 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.
Words to know
- ossification: bone hardening
- hydroxyapatite: bone mineral
- fontanelle: baby's soft spot
- vertebrae: spine bones
- sutures: skull joints
