Structure and Function: The Developing Body

A child’s body grows and changes continuously from birth to adulthood through processes that are largely controlled by hormones. The most dramatic changes take place during infancy, when rapid growth occurs, and during puberty, when the body is approaching sexual maturity. At certain times, some parts of the body develop faster than others, which is why body proportions change throughout childhood. A child’s brain is almost fully grown by the age of 6, while the rest of the body remains relatively undeveloped.

Changing body proportions

At birth, a baby’s head is as wide as the shoulders and appears large in relation to the rest of the body. The head continues to grow quickly, and by the age of 2, the brain is four-fifths of its adult size. As the child gets older, the rate of growth of the head decreases compared with the rest of the body. Eventually, usually by the age of 18, the proportion of the head with the rest of the body stays the same and growth stops.

How proportions change

A newborn baby’s head makes up about a quarter of total body length, and the legs three-eighths. By 18 years, proportions have changed; the head is an eighth of total body length and the legs a half.

How bones develop

Hard-bone formation (ossification) commences before birth at sites in the bone shafts known as primary ossification centres. In a newborn baby, only the shafts (diaphyses) are ossified. The ends of these bones (epiphyses) consist of tissue called cartilage, which is gradually replaced by bone that develops from secondary ossification centres. Between the shaft and the ends is a zone called the growth plate, which produces more cartilage to elongate the bones.

Hand growth

At the age of 2, ossified hand bone shafts look opaque on an X-ray. The growing cartilage looks transparent. By 18 years, all bone is ossified.

Long bone of a newborn baby

The shaft (diaphysis) is mostly bone, while the ends (epiphyses) are made of cartilage that gradually hardens.

Long bone of a child

The epiphyses contain secondary ossification centres from which bone forms. A growth plate near the ends produces new cartilage.

Long bone of an adult

By the age of 18, the shafts, growth plates, and epiphyses have all ossified and fused into continuous bone.

How the skull and brain develop

At birth, a newborn’s brain has its full complement of billions of neurons (nerve cells) that transmit and receive messages along axons (nerve fibres). At first, these neural networks are partially developed. In the first 6 years, the networks expand, and the brain grows rapidly to allow new skills to be learned. The skull expands to accommodate this growth. Between the ages of 6 and 18, neural pathways develop at a slower rate.

How the brain and skull grow

During infancy, the part of the skull around the brain (cranium) grows rapidly at the seams (sutures) and soft gaps (fontanelles) between the bone plates. By the age of 6, the brain is almost full size.

At birth

At 6 years

At 18 years

Neural network

This highly magnified view of adult brain tissue shows neurons within a complex network of neural pathways.

How nerves develop

Early in life, most nerve fibres (axons) become wrapped in insulating sheaths made of a fatty substance known as myelin. This insulation speeds up nerve transmission by as much as 100 times and is essential for the body to grow and function normally.


Supportive cells, composed mainly of fatty material known as myelin, wind around an axonto form a protective and insulating sheath.

The axon is now wrapped in sections of myelin sheath separated by gaps (nodes of Ranvier). Nerve signals jump from one gap to the next.

Factors affecting growth

A child’s potential maximum growth is determined by genes, but achieving the maximum depends on a number of other factors, such as nutrition and general health. Girls and boys have similar patterns of yearly growth in childhood (here). A growth spurt at puberty is triggered by sex hormones, which also speed up the fusion of growth plates in the bone. Growth is complete by about the age of 18.

Why boys grow taller than girls

In boys, the growth spurt occurs later in puberty than in girls, giving them longer to grow in childhood before their full growth is complete.

From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.

The subjects, conditions and treatments covered in this encyclopaedia are for information only and may not be covered by your insurance product should you make a claim.

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