How does one fertilised egg turn into trillions of cells?

Chris Smith spoke to Adrian Pini to hear how embryos develop...

Adrian - As the question suggests, it's incredibly complicated and not that well understood at all. Essentially what happens is that the first cell that gets fertilised by a sperm divides in two, and the daughter cells themselves divide. So the first thing that happens is that you generate a whole load of cells. As the division goes on, you then begin to get sub-specialisation of different cells into different regions. For example, you get cells that are going to become the skin, cells that are going to become you bones, and your blood and guts and so on. All these differentiation events, as they're called, are beginning to occur as the embryo is beginning to get bigger. In the nervous system especially, you get a layer of flat cells, and within that thing you get changes which first of all are at a genetic level so you can't really see much of what's going on. Then different genetic processes seem to kick in in different orders throughout the nervous system that give rise to further specialisations that will eventually become the brain and other parts of the nervous system.

Chris - It's amazing to think that the brain, which has a million million nerve cells in it, can work out exactly where to connect all the different nerve cells together. How does a nerve cell in my brain that's supposed to control my finger know where to connect to and get it right? Adrian - It is a very complicated process. Nerve cells can communicate with one another and read signals. There are two sorts of signals that nerve cells can read. One is a signal that diffuses in from elsewhere, like a dog sniffing a scent. The other way that two nerve cells can talk is by simply touching it. Here there is an exchange of chemical signals, which usually involves proteins but can be other molecules as well. These are the signalling things that do the talking. For example, if a nerve cell is sitting up in the brain somewhere, it has to navigate quite along distance. One of the signals it can read are signals from the area to which it is going to be directed. The target zone will secrete an attractant, so that the nerve cell can extend a very thin process called an axon. This can go from where the cell is in the brain, down the spinal cord and to wherever it's going, which may be a metre away. The thing to remember is that embryos are very small, and so the distances the nerve fibres have to grow over are also very manageable.

Kat - I think I remember hearing once that the brain is still making these connections once you're born. What's the key time in life for that, and can you improve a baby's intelligence by stimulating it in the right ways?

艾德里安-当然,很多t的建设he brain is activity dependent. This means that, for example, as your visual system is developing, it's useful if you can actually see. The old doctrine is that cells that fire together, wire together. That means that you get functional connections between cells that are going to be useful later on. One of the important things about the way the nervous system develops is that you actually make a lot more nerve cells than you're actually ever going to need. What happens is that there's a control process in which you over-produce cells and they undergo an almost Darwinian selection amongst themselves, so that the right connections get established. What happens then is that the cells that are not properly connected die.

Kat - I've heard that that's the Darwinian theory of drinking. You kill off some brain cells and hope that the strongest ones hang on in there!

Adrian - But the problem is that it kills off all sorts of other cells as well!