SciWhys: How do organisms develop?

Every month OUP editor and author Jonathan Crowe answers your science questions in the monthly SciWhys column. Got a burning question about science that you’d like answered? Just email it to us, and Jonathan will answer what he can.

Today: how do organisms develop?

By Jonathan Crowe

Each of our bodies is a mass of cells of varying types — from the brain cells that give us the power of thought, to the cardiac cells that form our heart and keep our blood circulating; from the lung cells that take in oxygen from the air around us, to the skin cells that envelop the organs and tissues that lie within. Regardless of their ultimate function, however, each of these cells has come from a single source — the fertilised egg. But how can the complexity and intricacy of a fully-functioning organism stem from such humble beginnings?

At heart, the growth of any organism relies on the repeated growth and division of cells. A cell grows, then splits into two. Each of those cells grows, then splits into two… and so the cycle continues. Before long, we’ve gone from having one cell to two, from two to four, and then to eight, to sixteen, etc. In fact, after ten ‘cycles’ we already have over 1000 cells. (We still have some way to go to generate the millions of cells that form an embryo, but you get the idea.)

Initially, the egg divides to from a hollow ball of cells. However, living creatures aren’t hollow. Instead, they have a clear inside and outside, with the inside usually comprising some kind of gut, which passes the length of the body, from mouth to anus. So how do we go from a hollow ball to something with a clear internal structure? Well, imagine holding a sponge ball between the fingers of two hands, and then pushing in the bottom of the ball with your thumbs. The bottom of the ball folds up and in, almost forming a ‘tunnel’ into the ball. Our hollow ball of cells does the same thing: the cells at the bottom of the hollow ball move up and inside to form a tunnel. These cells will go on to form the digestive tract, which (as our experience tells us) runs right through the inside of our bodies.

Shortly after, a strip of cells along the back of the ball of cells roll up to form a furrow. The cells forming this furrow will go on to form the nervous system, with the furrow itself becoming our spinal cord. And, again, this fits with our experience: our spinal cord does indeed run up and along our back.

The previous paragraphs reveal an important feature of the development of a living organism. It’s not just a question of having lots of cells: to have a fully-functioning organism, we need different cells to do different things – to have different functions. After all, our bodies would be quite useless (not to mention odd-looking) if we were composed entirely of lung cells. Instead, as a population of cells grows, it also clusters into groups with common functions, forming different tissues and different organs.

So how does a cell know what kind of cell it should become? At the simplest level, it depends on the cell’s location – its position in the embryo. But how can cells tell where they are? Do they have some kind of cellular GPS system? Actually, in a way they do. Just as the GPS feature of a mobile phone can tell us our location by picking up a signal from a satellite, cells can also receive signals from their surroundings, which vary according to their location. And, because cells at different positions in the embryo — top or bottom, front or back, left or right — receive different signals, they behave in different ways.

Our everyday experience tells us that our behaviour is modified by signals in the world around us – the most obvious example being the traffic lights that tell us when to stop or go when driving. In a cellular world, the signals tend to be different chemicals. (For example, when we’re placed in a frightening situation, our bodies release adrenaline, which is a naturally-produced chemical. Adrenaline affects how our cells behave — by making our heart pump faster, for example.)

But how does a chemical signal actually have its effect on the cell? I’ve mentioned in previous posts how all cells in an organism have the same library of biological information in their copies of the genome. But no one cell uses all of this information. Instead, different cells use different combinations of information; that’s what makes them different. It’s a bit like preparing a meal from a cupboard full of cooking ingredients: different chefs could select different ingredients from the same overall selection to create quite different dishes. In the case of our cells, different cells ‘read’ different sets of genes – and it is the chemical signals they are exposed to that determine which genes are read and which are ignored.

So, a particular cell receives signals from its surroundings, which influences its choice of genetic ‘ingredients’. And the specific mix of genetic ingredients selected will determine what type of cell it is.

Proper development relies on the right genes being switched on and off in the right cells at the right time.  If the wrong signals are received (or signals are received at the wrong time) things can go awry – as demonstrated by this three-legged frog, whose development didn’t proceed quite as it should have. Humans aren’t immune, either: a breakdown in signalling during the development of our limbs can lead to polydactyly — the growth of extra fingers or toes, as this image shows:

Ultimately, then, cell growth needs to be carefully controlled – and, needless to say, problems can arise if this control is lost, as I’ll explore in my next post.