Why can our bodies grow without end, yet stop at a set height?

I began as one cell, a fertilized egg. That one became two, then four, doubling endlessly into the body I have now. A cell can keep dividing in two. And yet it is strange. If it can multiply without limit, why did I not grow on and on to two or three meters, but stop at a certain height? What is more, the pace of growth is uneven. As a newborn I doubled my weight and shot up in a single year. Then for some years I grew slowly. Then in puberty I surged again. What sets this rhythm, and what, in the end, gives the command to stop?

We often suppose it grows up to a set height and then stops, or that the height is written in the genes. It is true that genes matter a great deal for height. But that alone does not explain how a cell knows to stop, or why the pace of growth differs from stage to stage. The real picture is this. Growth is not mere swelling but a program in which the body's signals switch on and off stage by stage. When to grow, when to stop, and when to grow again are all set by signals.

Let us return to the first question. If it can multiply without limit, why does it stop? Because there is a signal to stop. Each single cell is braked by contact inhibition, and height by the closing of the growth plate. Why does it grow differently by stage? Because it is a program in which the conductor changes at each stage. The difference between inside the womb and outside is explained here too. Inside, the placenta and insulin and growth factors lead the growing; outside, the conductor passes from nutrition to growth hormone and then to sex hormones. Though it is the same body, the surroundings and the signals at work differ, so the way of growing differs. Growth, then, is not just getting bigger and then stopping. It is a precise process in which signals conduct when and how much to grow. Even the stopping is not a running out of fuel, but an active command.

Step through the stop signal (normal/cancer toggle), the growth plate opening then closing (slider), the conductor that changes by stage (fetus/infant/childhood/puberty toggle), and the lifetime curve of growth speed (tap a phase).

One. Why cells do not multiply without end

A cell has the ability to divide in two. But a normal cell also carries a signal to stop. One such signal is contact inhibition. As a cell grows and presses tightly against its neighbors, it receives the signal that the space is full and halts its division. So when the skin is wounded, cells grow until they fill the gap, and once it is filled they grow no more and stop. Here lies an important contrast. When this stop signal fails, so that cells keep dividing even though the space is packed, that is cancer. In other words, our stopping at a fitting point is because a device that restrains endlessly multiplying cells is at work. Stopping happens not from a lack of ability, but because there is a command to stop.

Two. Where height grows, and how it ends

The growing of height happens in a cartilage at the ends of the bones called the growth plate. The cells of this cartilage line up and lengthen, and as hard bone fills in behind them, the bone grows longer. Height increases only while the growth plate is open. Then, around the end of puberty, the growth plate closes. The cartilage has all turned to bone, leaving no more room to lengthen. Once it closes, height growth is over for good. Here is a curious paradox. The sex hormones that surge height in puberty are also the very signal that closes that growth plate. The hand that switches on and the hand that switches off are the same. So if puberty arrives early, the plate closes early and height falls short; if puberty comes late, one grows that much more.

Three. Why the pace of growth differs by stage

Growth is not one engine running from start to finish. It is more like a relay in which a different signal leads at each stage. As a fetus in the womb, insulin and growth factors lead the growing; this is the fastest-growing time of life. The newborn grows mainly on nutrition: it grows as much as it feeds, which is why the first year's storm of growth is possible. Entering childhood, growth hormone and thyroid hormone take charge, lengthening height slowly but steadily. Then with puberty, sex hormones raise the output of growth hormone and a surge of growth comes once more. And soon, those same sex hormones close the growth plate. So the pace of growth is not erratic. Each time the conductor changes by stage, the rhythm of growing changes with it.

So whether one grows poorly or grows early is not simply a matter of eating well or not. The different signals must mesh at the right time, and the timing of puberty in particular weighs heavily on final height. That among the same-aged some grow first and some grow later is because the moment each one's conductor appears differs. More broadly, our body is not one that merely grows but one that knows how to stop. Because there is a signal to stop, we come to a fitting size, and when that signal breaks down, problems like cancer arise. Growing and stopping are two sides of one coin. A fully grown body is not one whose growth has run out, but one in which the signals that know when to grow and when to stop have finished their work. Our body knows how to stop as precisely as it knows how to grow.

• NIH (NCBI Bookshelf, Endotext)Endocrine control of growth, the growth plate, and the stages of stature
• Oxford Endocrine ReviewsThe growth plate: estrogen-driven closure and the end of height growth
• Karolinska InstitutetGrowth plate biology and the regulation of longitudinal bone growth
• NIH (National Cancer Institute)Control of cell growth and contact inhibition