Ask Dr. Gary: Genes dictate as embryo develops at warp speed

Women should follow a healthy regime if they think they are likely to conceive.

A reader has written in, amazed at the speed and complexity of her baby's heart development in utero. How do we go from a single cell to a foetus with a heart, brain, kidneys and lungs in just eight weeks, all of it packed into a human embryo the size of a lentil?

The short answer is "genes", so let's expand on that for a better understanding.

Genes provide the blueprint for how we should develop. We start as a single cell the size of a grain of sand, but because of the key concept of "exponential growth" we don't stay that small for long. One cell splits into two, two into four, four into eight ... The first couple of cell divisions are slow but this soon ramps up to warp speed.

In adults, we're producing about two million new red blood cells each minute. Cell division in the embryo also happens at astronomically fast rates. Exponential growth explains the remarkable speed at which an embryo grows.

But trillions of cells reproducing furiously require a traffic cop to co-ordinate them. It is genes that co-ordinate this explosive growth, dictating what proteins are made, which cells are formed, and which cells go where.

One surprising finding of evolutionary biologists is that a relatively small number of genes can make lots of different tissues depending on when the genes are turned on and off and in what order. Just like our alphabet, where just 26 letters can cope with the entire span of English literature, depending on the order the letters are in and where you put the spaces.

For instance, a Hox gene from a fruit fly can be put into the wrong place on another fruit fly and "turned on", causing legs to sprout where antennae normally grow, or wings to sprout from the mouthpiece. A Pax6 gene from a mouse can "switch on" eye development in a fly. And because Pax6 genes are identical in mice and humans, a turned-off Pax6 gene inserted into a human's DNA could theoretically result in an eyeless human.

The genes are there in the cells, like a software program loaded into a 3D printer, ready to crank out a lot of specialised tissues using the same basic building blocks; the whole rig is just waiting for the right on-off signals in the form of other genes such as Hox and Pax6.

These genes, or transcription factors, are in turn influenced by environmental signals and stresses such as vitamin deficiencies, high blood sugar levels or alcohol use. So it's not all genes. Your environment, health, and other conditions matter.

One important concept is that the essential time for a woman to take pre-natal vitamins or quit smoking or drinking is actually before she finds out that she is pregnant. Even before the first missed period organs are being formed.

The time to be at your healthiest, for the sake of your baby, is any time you are having sex without contraception, and might become pregnant.

• Gary Payinda is an emergency doctor who would like to hear your medical questions. Email him at drpayinda@gmail.com. This column provides general information, and is not a substitute for the medical advice of your own doctor.