How growth works
Growth is a complicated process. Most growth or height increases are due to an increase in bone length. This happens when the bones in the body are stimulated by substances (such as hormones), causing them to get longer. This section will explain how and when this occurs.
When babies are born, most of their bones are cartilage. This is soft and flexible bone. In adults, cartilage is still present in the nose and ears, but in the rest of the body it has been replaced by bone.
As babies grow up, their bones become longer and change from cartilage to proper bone, which is much harder and durable. Bone growth occurs from the end of the bone, at a place called the growth plate.
Growth plates are on 'long' bones, e.g. thigh bone (femur) or shin bone (tibia). They are disc-shaped pieces of cartilage. As cartilage matures, cells called osteoblasts develop with it. The osteoblasts help to produce new bone so the bone gets longer.
After a child stops growing (usually in the late teens), the growth plates become calcified, which means they get thinner and eventually close. After this point, growth of the bones is no longer naturally possible.
Factors that affect growth
There are a number of substances that affect the activity of the growth plates. These include:
Growth factors are naturally occurring substances. In particular, the liver produces large amounts of growth factors, which are carried by the blood flow to where they are needed.
Growth factors help cells to grow and mature, these processes are known as cell proliferation and differentiation. They are released by the connective tissue and the bones, where they help the bones to grow.
Hormones are chemical messengers that generally produce slow, gradual effects in the body. They control many different bodily functions, one of which is growth. They are produced by organs in the body called glands.
Some hormones act directly on the growth plates themselves, while others stimulate the production of growth factors, which in turn act on the growth plates. Examples include:
- Thyroxine – Produced by the thyroid gland, these hormones regulate protein, fat, and carbohydrate metabolism as well as the activity of growth hormone. The less thyroxine in the blood, the slower growth is.
- Growth hormone (also known as somatropin) - Very important for growth. From the sixth month of life until puberty, growth hormone has the greatest impact on growth. It is produced by a gland in the brain known as the pituitary gland. Growth hormone stimulates cells to produce growth factors, which causes cartilage cells to multiply and older cartilage to be transformed into bone. It also helps to burn fat, stimulate the development of muscles and strengthen the heart function. It may also play a role in emotional wellbeing.
- Sex hormones – These hormones play a really important role during puberty and can cause growth spurts. Sex hormones promote the production of additional growth hormone and so increase activity at the growth plates. They also help the cartilage to change into bone faster and promote bone maturation.
Growth is a complex process that involves the interplay of several growth factors and hormones. These work together, first by promoting growth of the bones, then by promoting bone maturation and eventually closing the growth plates once growth is completed.
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