Cell differentiation and how organisms grow

Could a cell taken from a carrot root grow into a whole new carrot plant, leaves and all? Most people would say no, because a root cell is a root cell. Yet in nineteen fifty-eight, the botanist Frederick Steward grew whole carrot plants from root cells kept in a flask. Cells from your skin could never build you a new finger. So what had those carrot cells kept that yours have lost? Steward's carrot gets its answer when a plant and an animal are set side by side.

A new cell made by mitosis is not ready for any particular job yet. Differentiation is the process by which it changes into a specialised cell. As a cell differentiates, it gains the sub-cellular structures its job needs. A cell becoming muscle gains many more mitochondria, and fibres that can contract. A cell becoming a root hair cell grows a long, thin extension out into the soil. A specialised cell is the result: a cell whose structure fits it to one particular job. Differentiation is the process, and a specialised cell is the product. A cell differentiates, and then it is a specialised cell. Cells in a six-week-old human embryo are becoming heart muscle cells. Four sentences describe them. Which two sentences describe the process, and which two describe the product? The process: it is differentiating, and it is gaining mitochondria and contracting fibres. The product: it is a specialised cell, and it can contract to pump blood. Here is one line from an examiner's report on a Higher paper: "Most did not understand the difference between differentiation and specialised cells." The fix: use differentiation for the process and specialised for the result, each in its own place, every time.

In animals, most types of cell differentiate at an early stage, while the animal is still an embryo, and then they stay that type. That is why an animal's shape and adult size are settled early. That does not mean an adult stops making cells. An adult animal divides cells every day, mainly for repair and replacement: new skin over a graze, new blood cells, a new gut lining. An animal grows in two steps. Cells divide by mitosis, then the new cells differentiate. That is how the embryo's heart muscle cells were made. Now, which can an adult human do by cell division: mend a broken bone, regrow a lost toe, or both? Mend a broken bone. That is repair and replacement. A toe needs new parts built, and the cells that could have built them differentiated long ago.

Plants work differently. Many types of plant cell keep the ability to differentiate for the whole life of the plant. A plant keeps growing at the growing tips of its shoots and roots. Cells there divide by mitosis, all through the plant's life. Then comes a step animals do not have. Just behind each tip, the new cells get longer. That is elongation, and it is where most of a shoot's or a root's new length comes from. Last, the new cells differentiate into whatever specialised cells that part of the plant needs. Because many plant cells can still differentiate, a piece of a plant can grow the parts it is missing, and a tree can grow new branches all its life. Over two days, an oak seedling's root grows two centimetres longer. Over two months, a puppy's leg bone grows two centimetres longer too. Which process does the oak use that the puppy does not? Elongation: the new cells just behind the tip stretch out. A puppy has division and differentiation, and nothing else. Now look ten years ahead: whose cells are still building new parts, and why? The oak's. Many of its cells keep the ability to differentiate, so it keeps growing at its tips. The dog's cells differentiated early, and its cell division is now mainly for repair and replacement.

Doctors check that a child is growing as expected with a growth chart. Each curve on the chart is a percentile. A baby on the twenty-fifth percentile curve is heavier than twenty-five per cent of babies of the same age and sex, and lighter than the other seventy-five per cent. A child who stays on one curve is growing steadily, whichever curve it is. Crossing curves, especially downwards, is what gets checked. A chart is read in three moves, in order: read the value with its unit, describe the trend, then explain it. This chart is for baby boys, and Ali's mass is plotted at two, five and eight months. He starts on the fiftieth curve and ends on the twenty-fifth. Did Ali's mass go down between two months and eight months? No. Read the mass axis: five point six, then seven point two, then eight point one kilograms. His mass went up every time, just more slowly than most babies', so he moved down across the curves. Now a reason: why might a doctor keep watching Ali, but not a baby who stays on the ninth? Ali is drifting from the fiftieth towards the twenty-fifth, and that drift is the warning sign. A baby who stays on the ninth is growing steadily. One reading alone proves nothing. One examiner's report on a Higher paper says: "A very common misconception seen was that children above the ninety-fifth percentile or below the fifth percentile are not growing properly which limited many higher ability candidates to three marks." The fix: judge the line a child follows over time, never the curve the child happens to sit on.

To finish, three quick questions on how living things grow. A sunflower grows thirty centimetres taller in a week. Which three processes make that happen? The three processes: cell division at the shoot tip, elongation just behind it, then differentiation. Try this one: Sam has stayed on the ninety-first percentile for three years. Is he too big? No. Sam has kept to one curve, so he is growing steadily, and a high curve is not a problem in itself. One more, and it is quick: which word names the process, differentiation or specialised? Differentiation. A specialised cell is what that process produces. And Steward's carrot: its root cells had kept the ability to differentiate, so they could become leaf and stem. Most of your cells lost that before you were born.

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