Height is a polygenic trait, which means that many genes work together to shape it, not just one gene acting alone. Studies suggest that about 60% to 70% of differences in height between people are decided by the heights of their parents. Children in a family often grow in similar patterns, although each child grows at their own pace. Kids cannot change the genes that decide how tall they will be, but healthy eating habits and being active help them make the most of it. Eating well supports growth, but eating extra food or extra vitamins does not make a child grow taller.

Where does the growing happen? Long bones are the bones that are longer than they are wide, such as the thighbone, the forearm bones and the bones of the hands. A long bone does not grow from its middle outward, because it grows at each end, next to a growth plate. Most long bones have two growth plates, one near each end, and growth plates are the places where children's bones make new bone, so the bones can grow longer. During childhood, much more new bone is put down than is taken away, so the skeleton grows and the bones get denser.

A growth plate is made of cartilage, a rubbery, flexible material, and the tip of your nose is made of cartilage too. Growth plates sit between two parts of a bone, which are the wide end of the shaft, called the metaphysis, and the very end of the bone, called the epiphysis. A second area of bone, the secondary ossification center, appears in the epiphysis, and when it has turned its cartilage into bone, the old cartilage is divided into the growth plate and the cartilage at the joint surface. The growth plate helps decide the future length and shape of the grown-up bone.

The growth plate has layers. Cells in the top resting zone are the farthest from the bone-forming front, and as these cells start to divide, they form the proliferating zone, where the cells organize into columns and that stacking helps the bone get longer. Inside a growth plate, cartilage cells multiply and grow in length, and then they change into hard, mineralized bone. Big hypertrophic cells either die or turn into bone-making cells called osteoblasts, which form a bony matrix, so cartilage is replaced by bone as the bone lengthens. Bone elongation is dictated by the rate at which proliferating chondrocytes undergo hypertrophy, so how fast a bone lengthens depends on how fast growth-plate cells swell into their large, hypertrophic form.

Bone is living tissue, so old bits of bone are removed and replaced with new bone all the time. Bones are made of a framework of a protein called collagen, made hard by a mineral called calcium phosphate. Three kinds of bone cells do the work: osteoblasts make new bone, osteocytes are mature bone cells, and osteoclasts break bone down so it can be shaped. Bones also hold calcium and release some into the blood when other parts of the body need it.

A growth plate is weaker than solid bone, so it is easier to injure, but most growth plate fractures heal without changing how the bone grows later. When a person is fully grown, the growth plates harden into solid bone, and after a growth plate has closed, that bone is no longer growing longer. The bone you build as a kid helps your bones for the rest of your life, because the most bone you will ever have is built when you are young. Strong bones in childhood are a good start for bone health throughout life.

Words to know

• polygenic: from many genes
• cartilage: rubbery tissue
• hypertrophic: swollen large
• epiphysis: bone end