Most bones start as a cartilage model in the fetus that is gradually replaced by bone. Growth continues through childhood until the growth plates close in late adolescence.
Growth plates
Near each end of a long bone lies a growth plate (physis), a disc of cartilage. Cartilage cells multiply on one side and are turned into bone on the other, so the bone gets longer.
Growing wider
Bones grow in width by adding new bone under the periosteum, the tough outer covering. At the same time, bone is removed from the inside so the marrow cavity widens and the bone does not get too heavy.
Remodeling
Throughout life, osteoclasts break down old bone and osteoblasts build new bone. This constant turnover repairs small damage and lets bone adapt to the loads placed on it.
Hormones
Growth hormone, thyroid hormone and sex hormones drive growth. The rise of sex hormones at puberty causes a growth spurt and then closes the growth plates.
At the bedside
In a child, an injury near a joint may involve the growth plate and affect future growth, so compare both sides and look carefully at the X-ray. Plot height on a growth chart to spot growth problems early.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).